Classification of Mechanical Ventilators.

Full text

A Taxonomy for Mechanical Ventilation: 10 Fundamental Maxims
Robert L Chatburn MHHS RRT-NPS FAARC, Mohamad El-Khatib PhD MD RRT FAARC,
and Eduardo Mireles-Cabodevila MD
Introduction
What Is a Mode of Mechanical Ventilation?
The 10 Maxims
Application of the Taxonomy
Discussion
The Problem of Growing Complexity
The Problem of Identifying Unique Modes
The Problem of Teaching Mechanical Ventilation
The Problem of Implementation
Conclusions
The American Association for Respiratory Care has declared a benchmark for competency in
mechanical ventilation that includes the ability to “apply to practice all ventilation modes currently
available on all invasive and noninvasive mechanical ventilators.” This level of competency pre-
supposes the ability to identify, classify, compare, and contrast all modes of ventilation. Unfortu-
nately, current educational paradigms do not supply the tools to achieve such goals. To fill this gap,
we expand and refine a previously described taxonomy for classifying modes of ventilation and
explain how it can be understood in terms of 10 fundamental constructs of ventilator technology:
(1) defining a breath, (2) defining an assisted breath, (3) specifying the means of assisting breaths
based on control variables specified by the equation of motion, (4) classifying breaths in terms of
how inspiration is started and stopped, (5) identifying ventilator-initiated versus patient-initiated
start and stop events, (6) defining spontaneous and mandatory breaths, (7) defining breath se-
quences (8), combining control variables and breath sequences into ventilatory patterns, (9) de-
scribing targeting schemes, and (10) constructing a formal taxonomy for modes of ventilation
composed of control variable, breath sequence, and targeting schemes. Having established the
theoretical basis of the taxonomy, we demonstrate a step-by-step procedure to classify any mode on
any mechanical ventilator. Key words: taxonomy; ontology; mechanical ventilation; mechanical ven-
tilator; modes of ventilation; classification; ventilator; survey; standardized nomenclature; controlled
vocabulary. [Respir Care 2014;59(11):1747–1763. © 2014 Daedalus Enterprises]
Introduction
TheAmericanAssociationforRespiratoryCare(AARC)
has sponsored a number of conferences to outline the com-
petencies of the registered respiratory therapist (RRT) of
the future.
1-3
One of the competencies in the area of crit-
ical care was declared as the ability to “apply to practice
Mr Chatburn and Dr Mireles-Cabodevila are affiliated with the Respira-
tory Institute, Cleveland Clinic, Cleveland, Ohio and the Lerner College
of Medicine of Case Western Reserve University, Cleveland, Ohio.
Dr El-Khatib is affiliated with the Department of Anesthesiology, Amer-
ican University of Beirut Medical Center, Beirut, Lebanon.
Supplementary material related to this paper is available at http://
www.rcjournal.com.
RESPIRATORY CARE •NOVEMBER 2014 VOL 59 NO11 1747

all ventilation modes currently available on all invasive
and noninvasive mechanical ventilators, as well as all ad-
juncts to the operation of modes.”
2
Kacmarek
4
recently
published a paper discussing the expectations of this future
RRT regarding mechanical ventilation competencies. (Of
course, these competencies apply to any clinician respon-
sible for managing ventilated patients, as many countries
do not have RRTs.) He states that:
The RT of 2015 and beyond must be a technical
expert on every aspect of the mechanical ventilator.
They should be able to discuss all of the technical
nuances of the mechanical ventilator. They should
be able to compare the capabilities of one ventilator
to the other. They should be able to discuss in detail
the mechanism of action of all of the modes and
adjuncts that exist on the mechanical ventilator.
He further says that “The RT of 2015 and beyond should
be capable of defining the operational differences between
each of these modes.” These statements seem reasonable
at first glance, but further consideration reveals some ma-
jor challenges.
The number of modes of ventilation has grown expo-
nentially in the last 3 decades. Consider just one popular
textbook on respiratory care equipment
5
that includes 174
unique names of modes on 34 different ventilators. The
level of complexity in terms of the real number of unique
modes is much greater: most ICU ventilators allow the
operator to activate various features that modify a given
mode and actually transform it into anther mode without
any naming convention to signify the transition. The result
is that there are many more unique modes (in terms of
different patterns of patient-ventilator interaction) than
there are names indicated on the ventilators, in operators’
manuals, or in textbooks. This growing complexity has
generated an urgent need for a classification system (tax-
onomy) for modes of mechanical ventilation to facilitate
the identification and comparison of the technical capabil-
ities of ventilators.
The purpose of this article is to describe a formal tax-
onomy for modes of mechanical ventilation (ie, a classi-
fication of modes into groups based on similar character-
istics) using a simple structured approach to teaching and
learning the fundamental principles of ventilator opera-
tion. This taxonomy has recently been adopted by the
ECRI (formerly the Emergency Care Research Institute)
for describing and comparing ventilators.
6
We do not dis-
cuss clinical application, but rather the technology that is
the foundation for clinical application. This is a topic that
we believe is not sufficiently discussed in current text-
books. We have developed this system over many years of
clinical experience and instruction of medical students,
physicians, and respiratory therapists in both the hospital
and university environments. It is based on what we con-
sider to be 10 fundamental theoretical constructs or max-
ims (Table 1) that are recognizable to most people familiar
with mechanical ventilation.
7
We demonstrate how these
10 maxims form the basis of the taxonomy. We also show
how the taxonomy is a practical tool for dealing with the
complexity represented by the many mode names men-
tioned above. Figure 1 illustrates a hierarchy of skills we
believe must be mastered before one is fully able to use
mechanical ventilation technology as suggested by the
AARC competency statements. Note that this hierarchy is
consistent with Bloom’s revised taxonomy of learning ob-
jectives (a classification of levels of intellectual behavior
important in learning).
8
MrChatburnis apaidconsultantfor Philips Respironics,Covidien, Dra¨ger,
Hamilton Medical, and ResMed. The other authors have disclosed no
conflicts of interest.
Correspondence: Robert L Chatburn MHHS RRT-NPS FAARC, Cleve-
land Clinic, M-56, 9500 Euclid Avenue, Cleveland, OH 44195. E-mail:
chatbur@ccf.org.
DOI: 10.4187/respcare.03057
Table 1. Ten Basic Maxims for Understanding Ventilator Operation
(1) A breath is one cycle of positive flow (inspiration) and negative
flow (expiration) defined in terms of the flow vs time curve.
(2) A breath is assisted if the ventilator provides some or all of the
work of breathing.
(3) A ventilator assists breathing using either pressure control or
volume control based on the equation of motion for the
respiratory system.
(4) Breaths are classified according to the criteria that trigger (start)
and cycle (stop) inspiration.
(5) Trigger and cycle events can be either patient-initiated or
ventilator-initiated.
(6) Breaths are classified as spontaneous or mandatory based on both
the trigger and cycle events.
(7) Ventilators deliver 3 basic breath sequences: CMV, IMV, and
CSV.
(8) Ventilators deliver 5 basic ventilatory patterns: VC-CMV, VC-
IMV, PC-CMV, PC-IMV, and PC-CSV.
(9) Within each ventilatory pattern, there are several types that can
be distinguished by their targeting schemes (set-point, dual, bio-
variable, servo, adaptive, optimal, and intelligent).
(10) A mode of ventilation is classified according to its control
variable, breath sequence, and targeting schemes.
CMV ⫽continuous mandatory ventilation
IMV ⫽intermittent mandatory ventilation
CSV ⫽continuous spontaneous ventilation
VC ⫽volume control
PC ⫽pressure control
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What Is a Mode of Mechanical Ventilation?
A mode of mechanical ventilation may be defined, in gen-
eral, as a predetermined pattern of patient-ventilator interac-
tion. It is constructed using 3 basic components: (1) the
ventilator breath control variable, (2) the breath sequence,
and (3) the targeting scheme (Fig. 2) To understand each
of these components, we use the maxims that form the
basis for the taxonomy of mechanical ventilation. These
10 maxims describe, in a progressive manner, the rationale
of how we classify modes by understanding what a mode
does. Maxims 1–3 explain the ventilator breath control
variable. Maxims 4– 8 explain the breath sequence. Maxim
9 explains the targeting schemes. Maxim 10 pulls together
the previous maxims to formulate the complete taxonomy.
The 10 Maxims
The following sections describe the 10 theoretical con-
structs that we believe form the basis of a practical sylla-
bus for learning mechanical ventilation technology. They
also provide the context for some basic definitions of terms
used to construct a standardized vocabulary (see the sup-
plementary materials at http://www.rcjournal.com). We
start with very simple, intuitively obvious ideas and then
build on these concepts to form a theoretical framework
for understanding and using ventilators.
(1) A breath is one cycle of positive flow (inspiration)
and negative flow (expiration) defined in terms of the flow-
time curve. A breath is defined in terms of the flow-time
curve (Fig. 3). By convention, positive flow (ie, values of
flow above zero) is designated as inspiration. Negative
flow (values below zero) indicates expiration. Inspiratory
time is defined as the period from the start of positive flow
to the start of negative flow. Expiratory time is defined as
the period from the start of negative flow to the start of
positive flow. Total cycle time (also called the ventilatory
period) is the sum of inspiratory and expiratory times. It is
also equal to the inverse of breathing frequency (total cy-
cle time ⫽1/frequency, usually expressed as 60 s/breaths/
min). The inspiratory-expiratory ratio is defined as the
ratio of inspiratory time to expiratory time. The duty cycle
(or percent inspiration) is defined as the ratio of inspira-
tory time to total cycle time. The tidal volume (V
T
)isthe
integral of flow with respect to time. For constant flow
inspiration, this simply reduces to the product of flow and
inspiratory time.
(2) A breath is assisted if the ventilator provides some
or all of the work of breathing. An assisted breath is one
for which the ventilator does some portion of the work of
breathing. This work may be defined, for example, as the
integral of inspiratory transrespiratory pressure with respect
to inspired volume. Graphically, this corresponds to airway
pressure increasing above baseline during inspiration. In-
creased work of breathing per breath, as a result of increased
resistive and/or elastic work, is characterized by increased
transrespiratory pressure (for a definition of transrespiratory
pressure, see the supplementary materials at http://www.
rcjournal.com). In contrast, a loaded breath is one for which
transrespiratory pressure decreases below baseline during in-
spiration
9
and is interpreted as the patient doing work on the
ventilator (eg, to start inspiration).
A ventilator provides all of the mechanical work of
inspiration (ie, full support) only if the patient’s inspira-
tory muscles are inactive (eg, drug-induced neuromuscular
blockade). An unassisted breath is one for which the ven-
tilator simply provides flow at the rate required by the
patient’s inspiratory effort, and transrespiratory system
pressure stays constant throughout the breath. An example
of this would be CPAP delivered with a demand valve. A
ventilator can assist expiration by making the transrespi-
ratory pressure fall below baseline during expiration. An
example of this is automatic tube compensation on the
Evita XL ventilator (Dra¨ger, Lu¨beck, Germany). When
tube compensation is activated, the ventilation pressure in
the breathing circuit is increased during inspiration or de-
creased during expiration. The airway pressure is adjusted
to the tracheal level if 100% compensation of the tube
resistance has been selected. Another example is the use of
a cough-assist device (eg, CoughAssist mechanical insuf-
flator-exsufflator, Philips Respironics, Murrysville, Penn-
sylvania). In this case, transrespiratory pressure goes neg-
ativeduring expiration becausepressure on thebodysurface
is increased while pressure at the mouth remains at atmo-
spheric pressure.
(3) A ventilator assists breathing using either pressure
control or volume control based on the equation of motion
for the respiratory system. The theoretical framework for
understanding control variables is the equation of motion
Fig. 1. Pyramid of skills required to master ventilator technology.
The terms in green are from Bloom’s revised taxonomy of learning
objectives.
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for the passive respiratory system: P(t) ⫽EV(t) ⫹RV
˙(t).
This equation relates pressure (P), volume (V), and flow
(V
˙) as continuous functions of time (t) with the parameters
of elastance (E) and resistance (R). If any one of the func-
tions (P, V, or V
˙) is predetermined, the other two may be
derived. The control variable refers to the function that is
controlled (predetermined) during a breath (inspiration).
This form of the equation assumes that the patient makes
no inspiratory effort and that expiration is complete (no
auto-PEEP).
Volume control (VC) means that both volume and flow
are pre-set prior to inspiration. Setting the V
T
is a neces-
sary but not sufficient criterion for declaring volume con-
trol because some modes of pressure control allow the
operator to set a target V
T
but allow the ventilator to
determine the flow (see adaptive targeting scheme below).
Similarly, setting flow is also a necessary but not sufficient
criterion. Some pressure control modes allow the operator
to set the maximum inspiratory flow, but the V
T
depends
on the inspiratory pressure target and respiratory system
mechanics.
Pressure control (PC) means that inspiratory pressure as
a function of time is predetermined. In practice, this cur-
rently means pre-setting a particular pressure waveform
(eg, P(t) ⫽constant), or inspiratory pressure is set to be
proportional to patient inspiratory effort, measured by var-
ious means. For example, P(t) ⫽NAVA level ⫻EAdi(t),
where NAVA stands for neurally adjusted ventilatory as-
sist, and EAdi stands for electrical activity of the dia-
phragm (see servo targeting scheme below). In a passive
patient, after setting the form of the pressure function (ie,
the waveform), volume and flow depend on elastance and
resistance.
10
Time control is a general category of ventilator modes
for which inspiratory flow, inspiratory volume, and in-
spiratory pressure are all dependent on respiratory system
mechanics. As no parameters of the pressure, volume, or
flow waveforms are pre-set, the only control of the breath
is the timing (ie, inspiratory and expiratory times). Exam-
ples of this are high-frequency oscillatory ventilation (3100
ventilator, CareFusion, San Diego, California) and volu-
metric diffusive respiration (Percussionaire, Sagle, Idaho).
(4) Breaths are classified according to the criteria that
trigger(start) and cycle (stop) inspiration. Inspiration starts
(or is triggered) when a monitored variable (trigger vari-
able) achieves a pre-set threshold (the trigger event). The
simplest trigger variable is time, as in the case of a pre-set
breathing frequency (recall that the period between breaths
is 1/frequency). Other trigger variables include a minimum
level of minute ventilation, a pre-set apnea interval, or
various indicators of inspiratory effort (eg, changes in base-
line pressure or flow or electrical signals derived from
diaphragm movement).
Inspiration stops (or is cycled off) when a monitored
variable (cycle variable) achieves a pre-set threshold (cy-
cle event). The simplest cycle variable is a pre-set inspira-
tory time. Other cycle variables include pressure (eg, peak
airway pressure), volume (eg, V
T
), flow (eg, percent of
peak inspiratory flow), and electrical signals derived from
diaphragm movement.
Fig. 2. Building blocks for constructing a mode. CMV ⫽continuous mandatory ventilation; IMV ⫽intermittent mandatory ventilation;
CSV ⫽continuous spontaneous ventilation.
Fig. 3. A breath is defined in terms of the flow-time curve. Impor-
tant timing parameters related to ventilator settings are labeled.
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(5) Trigger and cycle events can be either patient-ini-
tiated or ventilator-initiated. Inspiration can be patient-
triggered or patient-cycled by a signal representing in-
spiratory effort (eg, changes in baseline airway pressure,
changes in baseline bias flow, or electrical signals derived
from diaphragm activity, as with neurally adjusted venti-
latory assist
11
or a calculated estimate of muscle pres-
sure
12
). Furthermore, the ventilator can be triggered and
cycled solely by the patient’s passive respiratory system
mechanics (elastance and resistance).
13
For example, an
increase in elastance or resistance in some modes will
increase airway pressure beyond the alarm threshold and
cycle inspiration. Inspiration may be ventilator-triggered
or ventilator-cycled by pre-set thresholds.
Patient triggering means starting inspiration based on a
patient signal, independent of a ventilator-generated trig-
ger signal. Ventilator triggering means starting inspiratory
flow based on a signal (usually time) from the ventilator,
independent of a patient-triggered signal. Patient cycling
means ending inspiratory time based on signals represent-
ing the patient-determined components of the equation of
motion (ie, elastance or resistance and including effects
due to inspiratory effort). Flow cycling is a form of patient
cycling because the rate of flow decay to the cycle thresh-
old (and hence, the inspiratory time) is determined by
patient mechanics (ie, the time constant and effort). Ven-
tilator cycling means ending inspiratory time independent
of signals representing the patient-determined components
of the equation of motion.
As a further refinement, patient triggering can be de-
fined as starting inspiration based on a patient signal oc-
curring in a trigger window, independent of a ventilator-
generated trigger signal. A trigger window is the period
composed of the entire expiratory time minus a short re-
fractory period required to reduce the risk of triggering a
breath before exhalation is complete (Fig. 4). If a signal
from the patient (ie, some measured variable indicating an
inspiratory effort) occurs within this trigger window, in-
spiration starts and is defined as a patient-triggered event.
A synchronization window is a short period, at the end
of a pre-set expiratory or inspiratory time, during which a
patient signal may be used to synchronize the beginning or
ending of inspiration to the patient’s actions. If the patient
signal occurs during an expiratory time synchronization
window, inspiration starts and is defined as a ventilator-
triggered event initiating a mandatory breath. This is be-
cause the mandatory breath would have been time-trig-
gered regardless of whether the patient signal had appeared
or not and because the distinction is necessary to avoid
logical inconsistencies in defining mandatory and sponta-
neous breaths (see below), which are the foundation of the
mode taxonomy. Trigger and synchronization windows
are another way to distinguish between continuous man-
datory ventilation (CMV) and intermittent mandatory ven-
tilation (IMV) (see below). Sometimes a synchronization
window is used at the end of the inspiratory time of a
pressure control, time-cycled breath. If the patient signal
occurs during such an inspiratory time synchronization
window, expiration starts and is defined as a ventilator-
cycled event, ending a mandatory breath.
Some ventilators offer the mode called airway pressure
release ventilation (or something similar with a different
name), which may use both expiratory and inspiratory
synchronization windows. This mode is an example of the
importance of distinguishing between trigger/cycle win-
dows (allowing for patient-triggered breaths) and synchro-
nization windows (allowing for patient-synchronized,
ventilator-triggered breaths). Airway pressure release ven-
tilation is intended to provide a set number of so-called
releases or drops from a high-pressure level to a low-
pressure level. Spontaneous breaths are possible at the
high-pressure and low-pressure levels (although there may
not be enough time to accomplish this if the duration of the
low pressure is too short). Using the standardized vocab-
ulary we have been discussing, these releases (paired with
their respective rises) are actually mandatory breaths be-
cause they are time-triggered and time-cycled. On some
ventilators, synchronization windows were added to both
the expiratory time (to synchronize the transition to high
pressure with a patient inspiratory effort) and the inspira-
tory time (to synchronize cycling with the expiratory phase
of a spontaneous breath taken during the high-pressure
level). If both triggering and cycling occurred with patient
signals in the synchronization window, and if we called
Fig. 4. Trigger and synchronization windows. If a patient signal
occurs within the trigger window, inspiration is patient-triggered. If
a patient signal occurs within a synchronization window, inspira-
tion is ventilator-triggered (or cycled if at the end of inspiration)
and patient-synchronized. Note that, in general, a trigger window
is used with continuous mandatory ventilation, a synchronization
window is used with intermittent mandatory ventilation.
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these events patient-triggered and patient cycled, then we
would end up with the ambiguous possibility of having
spontaneous breaths (ie, synchronized) occurring during
spontaneous breaths (unsynchronized breaths during the
high-pressure level). Another example occurs with a ven-
tilator such as the CareFusion Avea, which allows the
operator to set a flow cycle criterion for pressure control
PC-IMV. Thus, every inspiration is patient-cycled, and if
we said that any synchronized breaths (synchronized IMV)
were patient-triggered, we would be implying that these
mandatory breaths were really spontaneous breaths. This
would be misleading because the pre-set mandatory breath-
ing frequency would then be larger than what we count as
mandatory breaths when observing the patient. On modes
that are classified as forms of IMV (such as airway pres-
sure release ventilation), we need to distinguish between
the mandatory minute ventilation and the spontaneous min-
ute ventilation (to gauge the level of mechanical support),
and we cannot do this if the definitions of mandatory and
spontaneous breaths are in any way ambiguous. Figure 5
shows the decision rubric for classifying trigger and cycle
events.
(6) Breaths are classified as spontaneous or mandatory
based on both the trigger and cycle events. A spontaneous
breath is a breath for which the patient retains control over
timing. This means that the start and end of inspiration are
Fig. 5. Rubric for classifying trigger and cycle events. Courtesy Mandu Press.
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determined by the patient, independent of any ventilator
settings for inspiratory and expiratory times. That is, the
patient both triggers and cycles the breath. A spontaneous
breath may occur during a mandatory breath (eg, airway
pressure release ventilation). A spontaneous breath may be
assisted or unassisted. Indeed, the definition of a sponta-
neous breath applies to normal breathing as well as me-
chanical ventilation. Some authors use the term spontane-
ous breath to refer only to unassisted breaths, but that is an
unnecessary limitation that prevents the word from being
used as a key term in the mode taxonomy.
A mandatory breath is a breath for which the patient has
lost control over timing (ie, frequency or inspiratory time).
This is a breath for which the start or end of inspiration (or
both) is determined by the ventilator, independent of the
patient: the ventilator triggers and/or cycles the breath. A
mandatory breath can occur during a spontaneous breath
(eg, high-frequency jet ventilation). A mandatory breath
is, by definition, assisted.
(7) Ventilators deliver 3 basic breath sequences: CMV,
IMV, and continuous spontaneous ventilation CSV.A
breathsequence is aparticular pattern ofspontaneousand/or
mandatory breaths. The 3 possible breath sequences are
CMV, IMV, and CSV. CMV, commonly known as assist
control, is a breath sequence for which spontaneous breaths
are not possible between mandatory breaths because every
patient-triggered signal in the trigger window produces a
ventilator-cycled inspiration (ie, a mandatory breath). IMV
is a breath sequence for which spontaneous breaths are
possible between mandatory breaths. Ventilator-triggered
mandatory breaths may be delivered at a pre-set frequency.
Themandatory breathingfrequency forCMVmay behigher
than the set frequency but never below it (ie, the set fre-
quency is a minimum value). In some pressure control
modes on ventilators with an active exhalation valve, spon-
taneous breaths may occur during mandatory breaths, but
the defining characteristic of CMV is that spontaneous
breaths are not permitted between mandatory breaths. In
contrast, the set frequency of mandatory breaths for IMV
is the maximum value because every patient signal be-
tween mandatory breaths initiates a spontaneous breath.
There are 3 variations of IMV. (1) Mandatory breaths
are always delivered at the set frequency (eg, SIMV vol-
ume control mode on the PB840 ventilator, Covidien,
Mansfield, Massachusetts). In general, if a synchroniza-
tion window is used, the actual ventilatory period for a
mandatory breath may be shorter than the set period. Some
ventilators will add the difference to the next mandatory
period to maintain the set mandatory breathing frequency
(eg, Dra¨ger Evita XL ventilator). (2) Mandatory breaths
are delivered only when the spontaneous breathing fre-
quency falls below the set frequency (eg, BiPAP [bi-level
positive airway pressure] S/T mode on the Philips Respi-
ronics V60 ventilator). In other words, spontaneous breaths
may suppress mandatory breaths. (3) Mandatory breaths
are delivered only when the measured minute ventilation
(ie, product of breathing frequency and V
T
) drops below a
pre-set threshold (examples include Dra¨ger’s mandatory
minute volume ventilation mode and Hamilton Medical’s
adaptive support ventilation mode). Again, in this form of
IMV, spontaneous breaths may suppress mandatory
breaths.
(8) Ventilators deliver 5 basic ventilatory patterns: vol-
ume control VC-CMV, VC-IMV, PC-CMV, PC-IMV, and
PC-CSV. A ventilatory pattern is a sequence of breaths
(CMV, IMV, or CSV) with a designated control variable
(volume or pressure) for the mandatory breaths (or the
spontaneous breaths for CSV). Thus, with 2 control vari-
ables and 3 breath sequences, there are 5 possible venti-
latory patterns: VC-CMV, VC-IMV, PC-CMV, PC-IMV,
and PC-CSV. The VC-CSV combination is not possible
because volume control implies ventilator cycling, and
ventilator cycling makes every breath mandatory, not spon-
taneous (maxim 6). For completeness, we should also in-
clude the possibility of a time control ventilatory pattern
such as time control IMV. Although this is uncommon and
nonconventional, it is possible, as demonstrated by modes
such as high-frequency oscillatory ventilation and intrapul-
monary percussive ventilation. Because any mode of ven-
tilation can be associated with one and only one ventila-
tory pattern, the ventilatory pattern serves as a simple
mode classification system.
(9) Within each ventilatory pattern, there are several
types that can be distinguished by their targeting schemes
(set-point, dual, bio-variable, servo, adaptive, optimal, and
intelligent). A targeting scheme is a model
14
of the rela-
tionship between operator inputs and ventilator outputs to
achieve a specific ventilatory pattern, usually in the form
of a feedback control system. A target is a predetermined
goal of ventilator output. Targets can be viewed as the
goals of the targeting scheme. Targets can be set for pa-
rameters during a breath (within-breath targets). These pa-
rameters relate to the pressure, volume, and flow wave-
forms. Examples of within-breath targets include peak
inspiratory flow and V
T
or inspiratory pressure and rise
time (set-point targeting); pressure, volume, and flow (dual
targeting); and constant of proportionality between inspira-
tory pressure and patient effort (servo targeting).
Targets can be set between breaths to modify the with-
in-breath targets and/or the overall ventilatory pattern (be-
tween-breath targets). These are used with more advanced
targeting schemes, where targets act over multiple breaths.
Examples of between-breath targets and targeting schemes
include average V
T
(for adaptive targeting using pressure
control); work rate of breathing and minute ventilation (for
optimal targeting); and combined end-tidal P
CO
2
, volume,
and frequency values describing a zone of comfort (for
intelligent targeting, eg, SmartCare/PS [Dra¨ger Evita In-
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finity V500] or IntelliVent-ASV [S1 ventilator, Hamilton
Medical, Reno, Nevada]).
The targeting scheme (or combination of targeting
schemes) is what distinguishes one ventilatory pattern from
another. There are currently 7 basic targeting schemes that
comprise the wide variety seen in different modes of ven-
tilation. (1) Set-point is a targeting scheme for which the
operator sets all of the parameters of the pressure wave-
form (pressure control modes) or volume and flow wave-
forms (volume control modes). (2) Dual is a targeting
scheme that allows the ventilator to switch between vol-
ume control and pressure control during a single inspira-
tion. (3) Bio-variable is a targeting scheme that allows the
ventilator to automatically set the inspiratory pressure (or
V
T
) randomly to mimic the variability observed during
normal breathing. (4) Servo is a targeting scheme for which
the output of the ventilator (eg, inspiratory pressure) au-
tomatically follows a varying input (eg, inspiratory effort).
(5) Adaptive is a targeting scheme that allows the venti-
lator to automatically set one target (eg, pressure within a
breath) to achieve another target (eg, average V
T
over
several breaths). (6) Optimal is a targeting scheme that
automatically adjusts the targets of the ventilatory pattern
to either minimize or maximize some overall performance
characteristic (eg, work rate of breathing). (7) Intelligent is
a targeting scheme that automatically adjusts the targets of
theventilatory pattern using artificial intelligence programs
such as fuzzy logic, rule-based expert systems, and artifi-
cial neural networks.
(10) A mode of ventilation is classified according to its
controlvariable, breath sequence, and targeting scheme(s).
The preceding 9 maxims create a theoretical foundation
for the taxonomy of mechanical ventilation. Taxonomy is
the science of classification. A full explanation of how
taxonomies are created, as it applies to mechanical venti-
lation, has been published previously.
15
In short, the first
step is to create a standardized set of definitions. We have
refined such a vocabulary over the last 20 years (see the
supplementary materials at http://www.rcjournal.com). Se-
lected terms in the vocabulary are used to create a hierar-
chical classification system (essentially an outline) that
forms the structure of the taxonomy.
The taxonomy has 4 hierarchical levels (analogous to
order, class, genus, and species used in biological taxon-
omies): (1) control variable (pressure or volume, for the
primary breath), (2) breath sequence (for CMV, IMV, or
CSV), (3) primary breath targeting scheme (for CMV,
IMV, or CSV) , and (4) secondary breath targeting scheme
(for IMV). The primary breath is either the only breath that
occurs(mandatory breathsin CMVand spontaneousbreaths
in CSV) or the mandatory breath in IMV. We consider it
primary because if the patient becomes apneic, it is the
only thing keeping the patient alive.
The targeting schemes can be represented by single low-
ercase letters: set-point ⫽s, dual ⫽d, servo ⫽r, bio-
variable ⫽b, adaptive ⫽a, optimal ⫽o, and intelligent ⫽
i. For example, on the Covidien PB840 ventilator, there is
a mode called A/C volume control (volume assist control).
This mode is classified as VC-CMV with set-point target-
ing, represented by VC-CMVs.
Minor differences in a species of modes (such as unique
operational algorithms) can be accommodated by adding a
fifth level we could call variety (as is done in biology). As
an example, there are 3 varieties of PC-CSV using servo
targeting. One makes inspiratory pressure proportional to
the square of inspiratory flow (automatic tube compensa-
tion), one makes it proportional to the electrical signal
from the diaphragm (neurally adjusted ventilatory assist),
and one makes it proportional to the patient-generated vol-
ume and flow (proportional assist ventilation). The first
can support only the resistive load of breathing, whereas
the other two can support both the elastic and resistive
loads.
Application of the Taxonomy
Translating a name of a mode into a mode classification
using the taxonomy is a simple 3-step procedure. In step 1,
the primary breath control variable is identified. Simply
put, if inspiratory pressure is set or if pressure is propor-
tional to inspiratory effort, then the control variable is
pressure. In contrast, if the V
T
and inspiratory flow are set,
then the control variable is volume. Figure 6 shows the
decision rubric with a few refinements to accommodate
dual targeting. In step 2, the breath sequence is identified.
Figure 7 shows the decision rubric. In step 3, the targeting
schemes for the primary and, if applicable, secondary
breaths are identified (Table 2).
Examples
To demonstrate these steps, we will classify some of the
most commonly used modes in ICUs, starting with A/C
volumecontrol (Covidien PB840 ventilator). For this mode,
both inspiratory volume and flow are pre-set, so the con-
trol variable is volume (see Fig. 6). Every breath is vol-
ume-cycled, which is a form of ventilator cycling. Any
breath for which inspiration is ventilator-cycled is classi-
fied as a mandatory breath. Hence, the breath sequence is
CMV (see Fig. 7). Finally, the operator sets all of the
parameters of the volume and flow waveforms, so the
targeting scheme is set-point (see Table 2). Thus, the mode
is classified as VC-CMV with set-point targeting (VC-
CMVs).
Another common mode is SIMV volume control plus
(Covidien PB840 ventilator). For this mode, the operator
sets the V
T
, but not inspiratory flow. Because setting vol-
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ume alone (like setting flow alone) is a necessary but not
sufficient criterion for volume control, the control variable
is pressure (see Fig. 6). Spontaneous breaths are allowed
between mandatory breaths, so the breath sequence is IMV
(see Fig. 7). The ventilator adjusts inspiratory pressure for
mandatory breaths to achieve an average pre-set V
T
,sothe
targeting scheme for the mandatory breaths is adaptive
(see Table 2). For spontaneous breaths, inspiratory pres-
sure is set by the operator (eg, pressure support), so the
targeting scheme for these breaths is set-point. The mode
tag is PC-IMVa,s.
A very common mode for spontaneous breathing trials
(or for assistance of spontaneous breaths in IMV modes) is
pressure support or pressure support ventilation (note that
although ubiquitous, pressure support is a name, not a
classification). For this mode, the operator sets an inspira-
torypressure, so the control variable is pressure.All breaths
are patient-triggered and patient-cycled (note that flow cy-
Fig. 6. Rubric for determining the control variable of a mode. Paw ⫽airway pressure, SIMV ⫽synchronized intermittent mandatory
ventilation; V
T
⫽tidal volume; P ⫽pressure; E ⫽elastance; V ⫽volume; R ⫽resistance; V
˙⫽inspiratory flow. Courtesy Mandu Press.
TAXONOMY FOR MECHANICAL VENTILATION
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Fig. 7. Rubric for determining the breath sequence of a mode. Courtesy Mandu Press.
TAXONOMY FOR MECHANICAL VENTILATION
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cling is a form of patient cycling, as discussed above), so
the breath sequence is CSV. Because the ventilator does
notautomatically adjust any of the parameters of the breath,
the targeting scheme is set-point, and the tag is PC-CSVs.
If carefully applied, the taxonomy has the power to
clarify and unmask hidden complexity in a mode that has
a cryptic name. Take, for example, the mode called
CMV ⫹AutoFlow on the Dra¨ger Evita XL ventilator. Al-
though CMV on this ventilator is a mode equivalent to
volume assist control (described above), adding the Auto-
Flow feature changes it to a completely different mode.
For CMV ⫹AutoFlow, the operator sets a target V
T
, but
not inspiratory flow. Indeed, inspiratory flow is highly
variable because the ventilator automatically sets the in-
spiratory pressure within a breath. Thus, according to the
equation of motion, the control variable is pressure. Every
inspiration is time-cycled and hence mandatory, and the
breath sequence is CMV. The ventilator adjusts the in-
spiratory pressure between breaths to achieve an average
V
T
equal to the pre-set value using an adaptive targeting
scheme. Thus, the mode is classified as PC-CMV with
adaptive targeting (PC-CMVa).
On the other hand, the taxonomy can unmask the com-
plexity in an apparently simple mode. The mode called
volume control (Servo-i, Maquet, Wayne, New Jersey)
allows setting the V
T
and inspiratory time. Setting both
volume and inspiratory time is equivalent to setting mean
inspiratory flow (flow ⫽volume/time); hence, the control
variable is volume. Every breath is normally time-cycled
and hence mandatory, so our initial thought is that the
breath sequence is CMV. The tricky part is the targeting
scheme. The operator’s manual states that “...ifapres-
sure drop of 3 cm H
2
O is detected during inspiration, the
ventilator (switches) to Pressure Support with a resulting
increase in inspiratory flow.” This indicates dual targeting
as described above (see Table 2). Noting that the breath
may switch to pressure support alerts us that the breath
sequence is not what it first seemed to be. A breath may be
Table 2. Targeting Schemes
Name
(Abbreviation) Description Advantage Disadvantage Example Mode Name Ventilator
(Manufacturer)
Set-point (s) The operator sets all
parameters of the pressure
waveform (pressure control
modes) or volume and flow
waveforms (volume control
modes).
Simplicity Changing patient conditions
may make settings
inappropriate.
Volume control CMV Evita Infinity V500
(Dräger)
Dual (d) The ventilator can
automatically switch
between volume control
and pressure control during
a single inspiration.
It can adjust to changing
patient conditions and
ensure either a pre-set
V
T
or peak inspiratory
pressure, whichever is
deemed most important.
It may be complicated to
set correctly and may
need constant
readjustment if not
automatically controlled
by the ventilator.
Volume control Servo-i (Maquet)
Servo (r) The output of the ventilator
(pressure/volume/flow)
automatically follows a
varying input.
Support by the ventilator is
proportional to
inspiratory effort.
It requires estimates of
artificial airway and/or
respiratory system
mechanical properties.
Proportional assist
ventilation plus PB840 (Covidien)
Adaptive (a) The ventilator automatically
sets target(s) between
breaths in response to
varying patient conditions.
It can maintain stable V
T
delivery with pressure
control for changing
lung mechanics or
patient inspiratory effort.
Automatic adjustment may
be inappropriate if
algorithm assumptions
are violated or if they do
not match physiology.
Pressure-regulated
volume control Servo-i
Bio-variable (b) The ventilator automatically
adjusts the inspiratory
pressure or V
T
randomly.
It simulates the variability
observed during normal
breathing and may
improve oxygenation or
mechanics.
Manually set range of
variability may be
inappropriate to achieve
goals.
Variable pressure
support Evita Infinity V500
Optimal (o) The ventilator automatically
adjusts the targets of the
ventilatory pattern to either
minimize or maximize
some overall performance
characteristic (eg, work rate
of breathing).
It can adjust to changing
lung mechanics or
patient inspiratory effort.
Automatic adjustment may
be inappropriate if
algorithm assumptions
are violated or if they do
not match physiology.
ASV G5 (Hamilton
Medical)
Intelligent (i) This is a targeting scheme
that uses artificial
intelligence programs such
as fuzzy logic, rule-based
expert systems, and
artificial neural networks.
It can adjust to changing
lung mechanics or
patient inspiratory effort.
Automatic adjustment may
be inappropriate if
algorithm assumptions
are violated or if they do
not match physiology.
SmartCare/PS Evita Infinity V500
IntelliVent-ASV S1 (Hamilton
Medical)
CMV ⫽continuous mandatory ventilation
ASV ⫽adaptive support ventilation
VT⫽tidal volume
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patient-triggered with a patient inspiratory effort, and if
the effort is large enough and long enough, inspiration is
flow-cycled, not time-cycled. Flow cycling (at a certain
percent of peak inspiratory pressure) is, as we described
above, a form of patient cycling because the time constant
of the patient’s respiratory system determines when the
cycle threshold is met for passive inhalation (hence, in-
spiratory time is determined by the patient). Alternatively,
the patient may make an expiratory effort that cycles in-
spiration off. Either way, a patient-triggered and patient-
cycled breath is a spontaneous breath. Thus, spontaneous
breaths may occur between mandatory breaths, and the
breath sequence is actually IMV. Finally, the tag for this
mode is VC-IMVd,d. Note that with dual targeting modes,
we need to identify which control variable is in effect at
the start of inspiration, and in this case, it is volume. In
contrast, the mode called pressure A/C with machine vol-
ume (CareFusion Avea) is also dual targeting but starts out
in pressure control and may switch to volume control. This
convention is used because if the criterion that causes the
switch between control variables is never met during a
breath, the original control variable remains in effect
throughout the inspiratory time.
Finally, some modes are composed of compound tar-
geting schemes. For example, some ventilators offer tube
compensation, a feature that increases inspiratory pressure
in proportion to flow to support the resistive load of breath-
ing through an artificial airway. This is a form of servo
targeting. On the Dra¨ger Evita XL ventilator, tube com-
pensation can be added to CMV ⫹AutoFlow to obtain a
mode classified as PC-CMVar, where ar represents the
compound targeting scheme composed of adaptive plus
servo (note the absence of a comma between a and r be-
cause we are referring only to the primary breaths, and no
secondary breaths exist). A mode classified as PC-IMV
with set-point control for both primary (mandatory) and
secondary (spontaneous) breaths would have the tag PC-
IMVs,s (note the comma indicating set-point for primary
breaths and set-point for secondary breaths). If tube com-
pensation is used for the spontaneous breaths (eg, Covi-
dien PB840 ventilator), the tag would change to PC-
IMVs,r. If it is added to both mandatory and spontaneous
breaths (eg, Dra¨ger Evita XL ventilator), the tag would
change to PC-IMVsr,sr. Another example is IntelliVent-
ASV (Hamilton Medical S1 ventilator), which uses opti-
mal targeting to minimize the work rate and intelligent
targetingto establishlung-protectivelimits andadjustPEEP
and F
IO
2
. The tag for this mode is PC-CMVoi,oi.
The utility of this taxonomy becomes evident when com-
paring modes on different ventilators (eg, for making a
purchase decision), as shown in a recent issue of Health
Devices.
6
We have extended this type of comparison to
include several common ICU ventilators (Table 3). Table
3 is sorted by manufacturer and model, control variables,
breath sequences, and targeting schemes (simple to com-
plex). Table 4 shows the most commonly used modes for
the ventilators described in Table 3, sorted by classifica-
tion (tag). Table 4 illustrates how modes that are essen-
tially the same or very similar are given very different
names. We have constructed a table of all of the modes on
30 different ventilators from 11 different manufacturers
(not shown). The table lists 290 unique names of modes
representing45 differentclassifications(tags). Dealing with
this level of complexity is not unlike the challenge facing
clinicians when applying clinical diagnostic reasoning. The
elements of the mode taxonomy can thus be seen as anal-
ogous to the discriminating and defining features of a set
of diagnostic hypotheses,
16
as shown in Figure 8.
Discussion
The Problem of Growing Complexity
We mentioned in the introduction how modes of me-
chanical ventilation have evolved to a high level of com-
plexity. If we agree that the goal is to be able to appro-
priately use all modes of ventilation (even if we restrict
this goal to a single type of ventilator that might be avail-
able), then this implies the ability to compare and contrast
their advantages and disadvantages. The urgency for deal-
ing with the complexity of mechanical ventilation is ulti-
mately based on the growing concern for patient safety.
The ECRI “has repeatedly stressed the need for users to
understandtheoperation and features of ventilators...The
fact that ventilators are such an established technology by
no means guarantees that these issues are clearly under-
stood...Wecontinue to receive reports of hospital staff
misusing ventilators because they’re unaware of the de-
vice’s particular operational considerations.”
17
To deal with this complexity, researchers are designing
even more complex systems that attempt to better serve
the 3 goals of mechanical ventilation (ie, safety, comfort,
and liberation).
18
For example, Tehrani
19
has recently de-
scribed a system designed to automatically control the
support level in proportional assist ventilation to guarantee
delivery of a patient’s required ventilation (serving the
goal of safety). This system can also be used to control
the proportional assist ventilation support level based on
the patient’s work of breathing (serving the goal of com-
fort). Blanch et al
20
have developed the Better Care sys-
tem, which reliably detects ineffective respiratory efforts
duringexpiration (ie, inability of a patienttotrigger breaths)
with accuracy similar to that of expert intensivists and the
EAdi signal (serving the comfort goal). According to Kilic
and Kilic,
21
conventional weaning predictors ignore im-
portant dimensions of weaning outcome. They describe a
fuzzy logic system that provides an approach that can
handle multi-attribute decision making as a tool to over-
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Table 3. Classification of Modes on Common ICU Ventilators
Mode Name Tag
Covidien PB840
A/C volume control VC-CMVs*
SIMV volume control with pressure support VC-IMVs,s
SIMV volume control with tube compensation VC-IMVs,r
A/C pressure control PC-CMVs
A/C volume control plus PC-CMVa
SIMV pressure control with pressure support PC-IMVs,s
SIMV pressure control with tube compensation PC-IMVs,r
Bi-level with pressure support PC-IMVs,s
Bi-level with tube compensation PC-IMVs,r
SIMV volume control plus with pressure support PC-IMVa,s
SIMV volume control plus with tube compensation PC-IMVa,r
Spontaneous pressure support PC-CSVs
Spontaneous tube compensation PC-CSVr
Spontaneous proportional assist PC-CSVr
Spontaneous volume support PC-CSVa
Dräger Evita XL
CMV VC-CMVs
CMV with pressure-limited ventilation VC-CMVd
SIMV VC-IMVs,s
SIMV with automatic tube compensation VC-IMVs,sr
SIMV with pressure-limited ventilation VC-IMVd,s
SIMV with pressure-limited ventilation and automatic tube compensation VC-IMVd,sr
Mandatory minute volume ventilation VC-IMVa,s
Mandatory minute volume ventilation with automatic tube compensation VC-IMVa,sr
Mandatory minute volume with pressure-limited ventilation VC-IMVda,s
Mandatory minute volume with pressure-limited ventilation and automatic tube compensation VC-IMVda,sr
Pressure control ventilation plus assisted PC-CMVs
CMV with AutoFlow PC-CMVa
CMV with AutoFlow and tube compensation PC-CMVar
Pressure control ventilation plus/pressure support PC-IMVs,s
APRV PC-IMVs,s
Mandatory minute volume with AutoFlow PC-IMVa,s
SIMV with AutoFlow PC-IMVa,s
Mandatory minute volume with AutoFlow and tube compensation PC-IMVar,sr
SIMV with AutoFlow and tube compensation PC-IMVar,sr
Pressure control ventilation plus/pressure support and tube compensation PC-IMVsr,sr
APRV with tube compensation PC-IMVsr,sr
CPAP/pressure support PC-CSVs
SmartCare PC-CSVi
CPAP/pressure support with tube compensation PC-CSVsr
Hamilton Medical G5
Synchronized controlled mandatory ventilation VC-CMVs
SIMV VC-IMVs,s
SIMV with tube-resistance compensation CV-IMVs,sr
Pressure control CMV PC-CMVs
Adaptive pressure ventilation CMV PC-CMVa
Adaptive pressure ventilation CMV with tube-resistance compensation PC-CMVar
Pressure control CMV with tube-resistance compensation PC-CMVsr
Pressure SIMV PC-IMVs,s
NIV-spontaneous timed PC-IMVs,s
Nasal CPAP-pressure support PC-IMVs,s
APRV PC-IMVs,s
DuoPAP PC-IMVs,s
(continued)
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come the weaknesses of currently used weaning predictors
(serving the goal of liberation). Wysocki et al
22
provide a
very good overview of technical and engineering consid-
erations regarding closed-loop controlled ventilation, as
well as tangible clinical evidence that such systems make
mechanical ventilation safer and more efficient.
The Problem of Identifying Unique Modes
Before we can compare modes, we must have a list of
available modes. To generate such a list, we cannot just
copy the names of modes found in ventilator operators’
manuals for 2 reasons. First, there is no consistency among
manufacturers regarding how modes are named: some
names are the same but the modes are different, and some
names are different but the modes are the same. Second,
complex ICU ventilators offer features that, when acti-
vated, result in modes that are not named by the manu-
facturer and, in many cases, not even recognized as dif-
ferent modes. Such ambiguity makes learning about how
ventilators work very difficult.
Thus, before we can generate a list of modes to com-
pare, we first have to find some way to deal with this
confusion.One way is to distinguish between amode name,
which is determined by the manufacturer, and a mode tag
or general classification. However, before we can do that,
we must have a practical taxonomy. And before a taxon-
omy can be constructed, there must be a standardized glos-
sary (or controlled vocabulary, as it is called by taxono-
mists) of relevant terms.
15,23
The need for a standardized
Table 3. Continued
Mode Name Tag
Adaptive pressure ventilation SIMV PC-IMVa,s
Adaptive pressure ventilation SIMV with tube-resistance compensation PC-IMVar,sr
ASV PC-IMVoi,oi
IntelliVent-ASV PC-IMVoi,oi
ASV with tube-resistance compensation PC-IMVoir,oir
IntelliVent-ASV with tube-resistance compensation PC-IMVoir,oir
Pressure SIMV with tube-resistance compensation PC-IMVsr,sr
APRV with tube-resistance compensation PC-IMVsr,sr
Spontaneous with tube-resistance compensation PC-CSVr
Spontaneous PC-CSVs
NIV PC-CSVs
Maquet Servo-i
Volume control VC-IMVd,d
SIMV (volume control) VC-IMVd,d
Automode (volume control to volume support) VC-IMVd,a
Pressure control PC-CMVs
Pressure-regulated volume control PC-CMVa
SIMV (pressure control) PC-IMVs,s
BiVent PC-IMVs,s
Automode (pressure control to pressure support) PC-IMVs,s
SIMV pressure-regulated volume control PC-IMVa,s
Automode (pressure-regulated volume control to volume support) PC-IMVa,a
Spontaneous/CPAP PC-CSVs
Pressure support PC-CSVs
Neurally adjusted ventilatory assist PC-CSVr
Volume support PC-CSVa
* Targeting schemes are represented by single lowercase letters: s⫽set-point, r⫽servo, a ⫽adaptive, d ⫽dual, i ⫽intelligent, and o ⫽optimal. Combinations include: sr ⫽set-point with servo,
da ⫽dual with adaptive, as ⫽adaptive with set-point, ar⫽adaptive with servo, oi ⫽optimal with intelligent, and ois⫽optimal with intelligent and servo.
A/C ⫽assist control
SIMV ⫽synchronized intermittent mandatory ventilation
CMV ⫽continuous mandatory ventilations
CSV ⫽continuous spontaneous ventilation
IMV ⫽intermittent mandatory ventilation
NIV ⫽noninvasive ventilation
APRV ⫽airway pressure release ventilation
DuoPAP ⫽dual positive airway pressure
ASV ⫽adaptive support ventilation
VC ⫽volume control
PC ⫽pressure control
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vocabulary is the reason we included one in the supple-
mentarymaterials (http://www.rcjournal.com). This vocab-
ulary has been carefully developed by the authors over the
last 20 years with the specific purpose of establishing ba-
sic concepts that are logically consistent across all appli-
cations.
The Problem of Teaching Mechanical Ventilation
We believe that there is a growing and under-recog-
nized problem regarding training in the art and science of
mechanical ventilation that frequently leads to operational
errors. The reason is that technology is expanding faster
than our educational resources. Not even a 4-year respi-
ratorycare program can afford the time to ensurethe above-
mentioned mechanical ventilation competencies for RRTs.
The challenge for physicians may be even greater because
they generally rely on their residency (rather than medical
school) to learn mechanical ventilation. But according to
at least one study, “. . . internal medicine residents are not
gaining important evidence-based knowledge needed to
provide effective care for patients who require mechanical
ventilation.”
24
So how do instruction programs deal with the chal-
lenge? We recently conducted an informal survey of mem-
bers of the AARC specialty section on education. We
asked program directors to send us their outlines for teach-
ing mechanical ventilation. On the basis of what we found,
we contend there are 3 categories of organizational struc-
ture: (1) simple lists of skills needed to operate specific
ventilators, (2) lists of topics ranging from indications for
ventilation to weaning (and everything in between) with
no apparent order, and (3) topic organization identical to
or closely following the table of contents in textbooks on
mechanical ventilation. Having written ventilator-related
content in such textbooks, we can safely say that such
material was never designed to be used as the basis for a
class syllabus. In most of our previous writings focusing
on modes of mechanical ventilation,
7,14,25,26
the emphasis
has been on descriptions of modes rather than the back-
ground technical knowledge required to understand them.
That knowledge was assumed on the part of the reader
Table 4. Most Common Modes in Table 3 Sorted by Tag to Show Which Mode Names Have Equivalent Mode Classifications
Tag Covidien PB840 Dräger Evita XL Hamilton G5 Maquet Servo-i
VC-CMVs* A/C volume control CMV Synchronized controlled
mandatory ventilation NA†
VC-IMVs,s SIMV volume control with
pressure support SIMV SIMV NA‡
PC-CMVs A/C pressure control Pressure control ventilation
plus assisted Pressure control CMV Pressure control
PC-CMVa A/C volume control plus CMV with AutoFlow Adaptive pressure ventilation
CMV Pressure-regulated volume
control
PC-IMVs,s SIMV-pressure control with
pressure support Pressure control ventilation
plus/pressure support Pressure SIMV SIMV (pressure control)
Bi-level with pressure
support APRV NIV-spontaneous timed BiVent
Nasal CPAP-pressure support Automode (pressure control to
pressure support)
APRV
DuoPAP
PC-IMVa,s SIMV volume control plus
with pressure support Mandatory minute volume
with AutoFlow Adaptive pressure ventilation
SIMV SIMV pressure-regulated
volume control
SIMV with AutoFlow
PC-CSVs Spontaneous pressure support CPAP/pressure support Spontaneous Spontaneous/CPAP
PC-CSVa Spontaneous volume support NA NA Volume support
* Targeting schemes are represented by single lowercase letters: s⫽set-point, and a ⫽adaptive.
† Volume control continuous mandatory ventilation (VC-CMV) is not available because the mode called volume control allows some breaths to be patient-triggered and patient-cycled; hence, they
are spontaneous, making the breath sequence intermittent mandatory ventilation (IMV) rather than CMV.
‡ VC-IMV is available only with dual targeting, called SIMV (volume control) with the tag VC-IMVd,d.
PC ⫽pressure control
CSV ⫽continuous spontaneous ventilation
A/C ⫽assist control
SIMV ⫽synchronized intermittent mandatory ventilation
NA ⫽not available
APRV ⫽airway pressure release ventilation
DuoPAP ⫽dual positive airway pressure
NIV ⫽noninvasive ventilation
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(and instructor). This seems to be a universal theme among
authors of both articles and book chapters on how venti-
lators work, but as the technology grows more complex,
the gap between assumed and actual knowledge becomes
wider. We trust that this article helps to narrow that gap.
The Problem of Implementation
The first problem regarding implementation of any tax-
onomy (and its underlying standardized vocabulary) is, of
course, reaching a tipping point in acceptance by stake-
holders. We believe that such acceptance is achievable
only if the tools for implementing the taxonomy are sim-
ple, practical, and efficient in permitting both the identi-
fication and comparison of modes. The acceptance of this
taxonomy by the ECRI is a step in the right direction. We
hope that the detailed definitions, descriptions, and algo-
rithms provided here will address the deficiencies in the
available textbook references. Indeed, such tools are ame-
nable to dissemination using mobile computing technol-
ogy such as tablet computers and smartphones.
The second problem is ongoing maintenance of the tax-
onomy. Terms and concepts necessarily change as tech-
nology evolves. Ideally, a professional organization should
take responsibility for this function. The International Or-
ganization for Standardization and the Integrating the
Healthcare Enterprise Rosetta Terminology Mapping proj-
ect are working toward such a goal. Alternatively, the
AARC might be an appropriate venue for maintaining the
taxonomy as they do for clinical practice guidelines.
Conclusions
The rapid increase in the number and complexity of
mechanical ventilators, and specifically the modes they
offer, has outpaced development of tools for describing
them. A key problem has been the lack of a practical
classification system or taxonomy. Partial solutions to that
problem have been offered in our previous publications. In
this paper, we developed a full taxonomy and standardized
vocabulary for modes of mechanical ventilation. We
showed how the taxonomy is based on 10 fundamental
constructs, or maxims, describing ventilator technology.
Finally, we showed how to use the taxonomy to classify
modes of ventilation found on common ICU ventilators.
Identifying and classifying modes are necessary steps be-
fore being able to compare their relative merits and ulti-
mately to choose the most appropriate mode to serve the
goals of care for a particular patient.
18
The tools offered
in this paper (including the standardized vocabulary and
Fig. 8. Venn diagram illustrating how the mode taxonomy can be viewed in terms of discriminating features and defining features.
VC ⫽volume control; PC ⫽pressure control; CMV ⫽continuous mandatory ventilation; IMV ⫽intermittent mandatory ventilation;
CSV ⫽continuous spontaneous ventilation; P
ETCO
2⫽end-tidal partial pressure of carbon dioxide; a ⫽adaptive targeting; s⫽set-point
targeting. Courtesy Mandu Press.
TAXONOMY FOR MECHANICAL VENTILATION
1762 RESPIRATORY CARE •NOVEMBER 2014 VOL 59 NO11

2 summary handouts in the supplementary materials at
http://www.rcjournal.com) serve as a means for achieving
the mechanical ventilation competencies of the respiratory
therapist in 2015 and beyond.
4
Indeed, they serve the needs
ofall stakeholders, including clinicians (tounderstandtreat-
ment options), researchers (to evaluate treatment options),
educators (to prepare the next generation of ventilator ex-
perts), administrators (to make purchase decisions), and,
perhaps most important of all, manufacturers (to explain
the technical capabilities of their products and serve the
needs of the other stakeholders).
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TAXONOMY FOR MECHANICAL VENTILATION
RESPIRATORY CARE •NOVEMBER 2014 VOL 59 NO11 1763

HANDOUT 1 – HOW TO IDENTIFY MODES
Robert L. Chatburn, MHHS, RRT-NPS, FAARC
6/5/2014
Background Issues
Discussions about mechanical ventilator technology are hampered by the lack of a standardized vocabulary related to ventilator
function, and in particular, to modes of ventilation. In general, a “mode” is a predetermined pattern of interaction between the
ventilator and the patient. Manufacturers give certain patterns names and ignore other patterns. The result is that many ventilators have
functionality that is not explicitly recognized as distinct modes. Some names of modes are so commonly used that they are virtually
default classifications, eg, “Assist/Control” or “Pressure Support” albeit without any consensus on their exact meanings. Others are so
rare that they are used as marketing devices, eg, “SmartCarePS” or “Neurally Adjusted Ventilatory Assist”. The most popular
textbook on equipment for respiratory therapy lists 174 unique names of modes but offers no way to classify them. I have personally
identified almost 300 unique mode names using just the terms created by manufacturers in operators’ manuals. Clearly, a classification
system (formal taxonomy) is required in order to recognize, compare, and contrast modes or to contemplate future mode designs.
What follows is a very brief overview of a standardize vocabulary and a taxonomy I have developed and described in the literature
over the last 20 years. I have recently created implementation tools; see Respir Care 2012;57(12):2138-2150.
A Taxonomy for Mechanical Ventilation
The taxonomy is a logical classification system based on 10 maxims of ventilator design:
1. A breath is one cycle of positive flow (inspiration) and negative flow (expiration) defined in terms of the flow-time curve.
Inspiratory time is defined as the period from the start of positive flow to the start of negative flow. Expiratory time is defined as
the period from the start of expiratory flow to the start of inspiratory flow. The flow-time curve is the basis for many variables related
to ventilator settings.
2. A breath is assisted if the ventilator does work on the patient. An assisted breath is one for which the ventilator does some portion
of the work of breathing. For constant flow inflation, work is defined as inspiratory pressure multiplied by tidal volume. Therefore, an
assisted breath is identified as a breath for which airway pressure (displayed on the ventilator) rises above baseline during inspiration.
An unassisted breath is one for which the ventilator simply provides the inspiratory flow demanded by the patient and pressure stays
constant throughout the breath.
3. A ventilator assists breathing using either pressure control or volume control based on the equation of motion for the respiratory
system. Providing assistance means doing work on the patient, which is accomplished by controlling either pressure or volume. A
simple mathematical model describing this fact is known as the equation of motion for the passive respiratory system.
Pressure = (Elastance × Volume) + (Resistance × Flow)
Volume control means that both volume and flow are preset prior to inspiration. In other words, the right hand side of the equation
of motion remains constant while pressure changes with changes in elastance and resistance.
Pressure control means that inspiratory pressure is preset as either a constant value or it is proportional to the patient’s inspiratory
effort. In other words, the left hand side of the equation of motion remains constant while volume and flow change with changes in
elastance and resistance.
4. Breaths are classified by the criteria that trigger (start) and cycle (stop) inspiration. The start of inspiration is called the trigger
event. The end of inspiration is called the cycle event.
5. Trigger and cycle events can be initiated by the patient or the machine. Inspiration can be patient triggered or patient cycled by a
signal representing inspiratory effort. Inspiration may also be machine triggered or machine cycled by preset ventilator thresholds.
Patient triggering means starting inspiration based on a patient signal independent of a machine trigger signal.
Machine triggering means starting inspiratory flow based on a signal from the ventilator, independent of a patient trigger signal.
Patient cycling means ending inspiratory time based on signals representing the patient determined components of the equation of
motion, (ie, elastance or resistance and including effects due to inspiratory effort). Flow cycling is a form of patient cycling because
the rate of flow decay to the cycle threshold is determined by patient mechanics.
Machine cycling means ending inspiratory time independent of signals representing the patient determined components of the
equation of motion.
6. Breaths are classified as spontaneous or mandatory based on both the trigger and cycle events. A spontaneous breath is a breath
for which the patient both triggers and cycles the breath. A spontaneous breath may occur during a mandatory breath (eg Airway
Pressure Release Ventilation). A spontaneous breath may be assisted or unassisted. A mandatory breath is a breath for which the
machine triggers and/or cycles the breath. A mandatory breath can occur during a spontaneous breath (eg, High Frequency Jet
Ventilation). A mandatory breath is, by definition, assisted.
7. There are 3 breath sequences: Continuous mandatory ventilation (CMV), Intermittent Mandatory Ventilation (IMV), and
Continuous Spontaneous Ventilation (CSV). A breath sequence is a particular pattern of spontaneous and/or mandatory breaths. The
3 possible breath sequences are: continuous mandatory ventilation, (CMV, spontaneous breaths are not allowed between mandatory
breaths), intermittent mandatory ventilation (IMV, spontaneous breaths may occur between mandatory breaths), and continuous
spontaneous ventilation (CSV, all breaths are spontaneous).
8. There are 5 basic ventilatory patterns: VC-CMV, VC-IMV, PC-CMV, PC-IMV, and PC-CSV. The combination VC-CSV is not
possible because volume control implies machine cycling and machine cycling makes every breath mandatory, not spontaneous.

9. Within each ventilatory pattern there are several variations that can be distinguished by their targeting scheme(s). A targeting
scheme is a description of how the ventilator achieves preset targets. A target is a predetermined goal of ventilator output. Examples
of within-breath targets include inspiratory flow or pressure and rise time (set-point targeting), tidal volume (dual targeting) and
constant of proportionality between inspiratory pressure and patient effort (servo targeting). Examples of between-breath targets and
targeting schemes include average tidal volume (for adaptive targeting), percent minute ventilation (for optimal targeting) and
combined PCO2, volume, and frequency values describing a “zone of comfort” (for intelligent targeting, eg, SmartCarePS or
IntelliVent-ASV). The targeting scheme (or combination of targeting schemes) is what distinguishes one ventilatory pattern from
another. There are 7 basic targeting schemes that comprise the wide variety seen in different modes of ventilation:
Set-point: A targeting scheme for which the operator sets all the parameters of the pressure waveform (pressure control modes) or
volume and flow waveforms (volume control modes).
Dual: A targeting scheme that allows the ventilator to switch between volume control and pressure control during a single
inspiration.
Bio-variable: A targeting scheme that allows the ventilator to automatically set the inspiratory pressure or tidal volume randomly to
mimic the variability observed during normal breathing.
Servo: A targeting scheme for which inspiratory pressure is proportional to inspiratory effort.
Adaptive: A targeting scheme that allows the ventilator to automatically set one target (eg, pressure within a breath) to achieve
another target (eg, average tidal volume over several breaths).
Optimal: A targeting scheme that automatically adjusts the targets of the ventilatory pattern to either minimize or maximize some
overall performance characteristic (eg, minimize the work rate done by the ventilatory pattern).
Intelligent: A targeting scheme that uses artificial intelligence programs such as fuzzy logic, rule based expert systems, and artificial
neural networks.
10. A mode of ventilation is classified according to its control variable, breath sequence, and targeting scheme(s). The preceding 9
maxims create a theoretical foundation for a taxonomy of mechanical ventilation. The taxonomy is based on these
theoretical constructs and has 4 hierarchical levels:
Control Variable (Pressure or Volume, for the primary breath)
Breath Sequence (CMV, IMV, or CSV)
Primary Breath Targeting Scheme (for CMV or CSV)
Secondary Breath Targeting Scheme (for IMV)
The “primary breath” is either the only breath there is (mandatory for CMV and spontaneous for CSV) or it is the mandatory breath in
IMV. The targeting schemes can be represented by single, lower case letters: set-point = s, dual = d, servo = r, bio-variable = b,
adaptive = a, optimal = o, intelligent = i.
A tag is an abbreviation for a mode classification, such as PC-IMVs,s. Compound tags are possible, eg, PC-IMVoi,oi.
How to Classify a Mode
Step 1: Identify the primary breath control variable. If inspiration starts with a preset inspiratory pressure, or if pressure is
proportional to inspiratory effort, then the control variable is pressure. If inspiration starts with a preset tidal volume and inspiratory
flow, then the control variable is volume. If neither is true, the control variable is time.
Step 2: Identify the breath sequence. Determine whether trigger and cycle events are patient or machine determined. Then, use this
information to determine the breath sequence.
Step 3: Identify the targeting schemes for the primary breaths and (if applicable) secondary breaths.
Example Mode Classification
Mode Name: A/C Volume Control (Covidien PB 840)
Step 1: Inspiratory volume and flow are preset, so the control variable is volume.
Step 2: Every breath is volume cycled, which is a form of machine cycling. Any breath for which inspiration is machine cycled is
classified as a mandatory breath. Hence, the breath sequence is continuous mandatory ventilation.
Step 3: The operator sets all the parameters of the volume and flow waveforms so the targeting scheme is set-point. Thus, the mode
is classified as volume control continuous mandatory ventilation with set-point targeting (VC-CMVs).
Mode Name: SIMV Volume Control Plus (Covidien PB 840)
Step 1: The operator sets the tidal volume but not the inspiratory flow. Because setting volume alone (like setting flow alone) is a
necessary but not sufficient criterion for volume control, the control variable is pressure.
Step 2: Spontaneous breaths are allowed between mandatory breaths so the breath sequence is IMV.
Step 3: The ventilator adjusts inspiratory pressure between breaths to achieve an average preset tidal volume, so the targeting
scheme is adaptive. The mode tag is PC-IMVa,s.
Mode Name: Pressure Support
Step 1: Inspiratory pressure is preset, so the control variable is pressure.
Step 2: All breaths are patient triggered and patient cycled (note what was said about flow cycling above in Maxim 5) so the breath
sequence is CSV.
Step 3: Because the ventilator does not adjust any of the parameters of the breath, the targeting scheme is set-point and the tag is
PC-CSVs.

Standardized Vocabulary
assisted breath A breath during which all or part of inspiratory (or expiratory) flow is generated by the ventilator doing work on
the patient. In simple terms, if the airway pressure rises above end expiratory pressure during inspiration, the breath is assisted (as in
the Pressure Support mode).
breath A positive change in airway flow (inspiration) paired with a negative change in airway flow (expiration), associated with
ventilation of the lungs. This definition allows the superimposition of, for example, a spontaneous breath on a mandatory breath or
vice versa. The flows are paired by size, not necessarily by timing.
breath sequence A particular pattern of spontaneous and/or mandatory breaths. The 3 possible breath sequences are: continuous
mandatory ventilation, (CMV), intermittent mandatory ventilation (IMV), and continuous spontaneous ventilation (CSV).
compliance A mechanical property of a structure such as the respiratory system; a parameter of a lung model, or setting of a lung
simulator; defined as the ratio of the change in volume to the associated change in the pressure difference across the system.
continuous mandatory ventilation Commonly known as “Assist/Control”; CMV is a breath sequence for which spontaneous
breaths are not possible between mandatory breaths because every patient trigger signal in the trigger window produces a machine
cycled inspiration (ie, a mandatory breath). Machine triggered mandatory breaths may be delivered at a preset rate. Therefore, in
contrast to IMV, the mandatory breath frequency may be higher than the set frequency but never below it. In some pressure controlled
modes on ventilators with an active exhalation valve, spontaneous breaths may occur during mandatory breaths, but the defining
characteristic of CMV is that spontaneous breaths are not permitted between mandatory breaths.
continuous spontaneous ventilation A breath sequence for which all breaths are spontaneous.
elastance A mechanical property of a structure such as the respiratory system; a parameter of a lung model, or setting of a lung
simulator; defined as the ratio of the change in the pressure difference across the system to the associated change in volume. Elastance
is the reciprocal of compliance.
control variable The variable (ie, pressure or volume in the equation of motion) that the ventilator uses as a feedback signal to
manipulate inspiration. For simple set-point targeting, the control variable can be identified as follows: If the peak inspiratory pressure
remains constant as the load experienced by the ventilator changes, then the control variable is pressure. If the peak pressure changes
as the load changes but tidal volume remains constant, then the control variable is volume. Some primitive ventilators cannot maintain
either constant peak pressure or tidal volume and thus control only inspiratory and expiratory times (ie, they are time controllers).
cycle variable The variable (usually pressure, volume, flow, or time) that is used to end the inspiratory time (and begin expiratory
flow). To “cycle” inspiration means to end the inspiratory time and start expiratory flow.
equation of motion for the respiratory system A relation among pressure difference, volume, and flow (as variable functions
of time) that describes the mechanics of the respiratory system. The simplest and most useful form is a differential equation with
constant coefficients describing the respiratory system as a single deformable compartment including the lungs and chest wall
connected in series to a single flow conducting tube:
autoPEEPtVRtVEtPtP musTR )()()()( 
where PTR(t) = the change in transrespiratory pressure difference (i.e., airway opening pressure minus body surface pressure) as a
function of time (t), measured relative to end expiratory airway pressure. This is the pressure generated by a
ventilator, Pvent(t), during an assisted breath.
Pmus(t) = ventilatory muscle pressure difference as a function of time (t); the theoretical chestwall transmural pressure
difference that would produce movements identical to those produced by the ventilatory muscles during breathing
maneuvers (positive during inspiratory effort, negative during expiratory effort)
V(t) = volume change relative to end expiratory volume as a function of time (t)
)(tV

= flow as a function of time (t), the first derivative of volume with respect to time
E = elastance (inverse of compliance; E = 1/C)
R = resistance
autoPEEP = end expiratory alveolar pressure above end expiratory airway pressure
For the purposes of classifying modes of mechanical ventilation the equation is often simplified to:
VRVEPvent 
where Pvent = the transrespiratory pressure difference (ie, “airway pressure”) generated by the ventilator during an assisted breath
expiratory time The period from the start of expiratory flow to the start of inspiratory flow.
inspiratory hold (pause) time The period from the cessation of inspiratory flow (into the airway opening) to the start of expiratory
flow during mechanical ventilation.
inspiratory time The period from the start of inspiratory flow to the start of expiratory flow. Inspiratory time equals inspiratory flow
time plus inspiratory pause time.
intermittent mandatory ventilation Breath sequence for which spontaneous breaths are permitted between mandatory breaths.
For most ventilators, a short “window” is opened before the scheduled machine triggering of mandatory breaths to allow
synchronization with any detected inspiratory effort on the part of the patient. This is referred to as synchronized IMV (or SIMV).
Three common variations of IMV are: (1) Mandatory breaths are always delivered at the set frequency; (2) Mandatory breaths are
delivered only when the spontaneous breath frequency falls below the set frequency; (3) Mandatory breaths are delivered only when

the spontaneous minute ventilation (ie, product of spontaneous breath frequency and spontaneous breath tidal volume) drops below a
preset or computed threshold (aka Mandatory Minute Ventilation). Therefore, in contrast to CMV, with IMV the mandatory breath
frequency can never be higher than the set rate but it may be lower. For some modes (eg, Airway Pressure Release Ventilation), a
short window is also opened at the end of the inspiratory time. Because spontaneous breaths are allowed during the mandatory
pressure controlled breath, this window synchronizes the end of the mandatory inspiratory time with the start of spontaneous
expiratory flow, if detected. With these technological developments, potential confusion arises as to whether inspiration that is
synchronized (either start or stop) is considered patient triggered/cycled or machine triggered/cycled. If we say synchronized breaths
are patient triggered and cycled, we have the awkward possibility of a spontaneous breath occurring during another spontaneous
breath. This is avoided by distinguishing between a trigger window and a synchronization window. There are some modes where the
idea of IMV may be vague: With Airway Pressure Release Ventilation, relatively high frequency spontaneous breaths are
superimposed on low frequency mandatory breaths. However, the expiratory time between mandatory breaths is often set so short that
a spontaneous breath is unlikely to occur between them. Other ambiguous modes are High Frequency Oscillation, High Frequency Jet
Ventilation, Intrapulmonary Percussive Ventilation and Volumetric Diffusive Respiration. With these modes, high frequency
mandatory breaths are superimposed on low frequency spontaneous breaths and again, there is no possibility of a spontaneous breath
actually occurring between mandatory breaths. Nevertheless, we classify all these modes as forms of IMV because spontaneous
breaths can occur along with mandatory breaths and because spontaneous efforts do not affect the mandatory breath frequency.
mandatory breath A breath for which the patient has lost control over timing. This means a breath for which the start or end of
inspiration (or both) is determined by the ventilator, independent of the patient. That is, the machine triggers and/or cycles the breath.
A mandatory breath can occur during a spontaneous breath (eg, High Frequency Jet Ventilation). A mandatory breath is, by definition,
assisted.
primary breaths Mandatory breaths during CMV or IMV or spontaneous breaths during CSV.
resistance A mechanical property of a structure such as the respiratory system; a parameter of a lung model, or setting of a lung
simulator; defined as the ratio of the change in the pressure difference across the system to the associated change in flow.
secondary breaths Spontaneous breaths during IMV.
spontaneous breath A breath for which the patient retains substantial control over timing. This means the start and end of
inspiration may be determined by the patient, independent of any machine settings for inspiratory time and expiratory time. That is,
the patient both triggers and cycles the breath. Note that use of this definition for determining the breath sequence (ie, CMV, IMV,
CSV) assumes normal ventilator operation. For example, coughing during VC-CMV may result in patient cycling for a patient
triggered breath due to the pressure alarm limit. While inspiration for that breath is both patient triggered and patient cycled, this is not
normal operation and the sequence does not turn into IMV. A spontaneous breath may occur during a mandatory breath (eg Airway
Pressure Release Ventilation). A spontaneous breath may be assisted or unassisted.
synchronization window A short period, at the end of a preset expiratory time or at the end of a preset inspiratory time, during
which a patient signal may be used to synchronize a mandatory breath trigger or cycle event to a spontaneous breath. If the patient
signal occurs during an expiratory time synchronization window, inspiration starts and is defined as a machine triggered event. This is
because the mandatory breath would have been time triggered regardless of whether the patient signal had appeared or not and
because the distinction is necessary to avoid logical inconsistencies in defining mandatory and spontaneous breaths which are the
foundation of the mode taxonomy. If inspiration is triggered in a synchronization window, the actual ventilatory period for the
previous breath will be shorter than the set ventilatory period (determined by the set mandatory breath frequency). Some ventilators
add the lost time to the next mandatory breath period to maintain the set frequency. Sometimes a synchronization window is used at
the end of the inspiratory time of a pressure controlled, time cycled breath. If the patient signal occurs during such an inspiratory time
synchronization window, expiration starts and is defined as a machine cycled event. Some ventilators offer the mode called Airway
Pressure Release Ventilation (or something similar with a different name) that makes use of both expiratory and inspiratory
synchronization windows.
target A predetermined goal of ventilator output. Targets can be viewed as the goals of the targeting scheme. Within-breath targets
are the parameters of the pressure, volume, or flow waveform. Examples of within-breath targets include inspiratory flow or pressure
and rise time (set-point targeting), tidal volume (dual targeting) and constant of proportionality between inspiratory pressure and
patient effort (servo targeting). Note that preset values within a breath that end inspiration, such as tidal volume, inspiratory time, or
percent of peak flow, are also cycle variables. Between-breath targets serve to modify the within-breath targets and/or the overall
ventilatory pattern. Between-breath targets are used with more advanced targeting schemes, where targets act over multiple breaths.
Examples of between-breath targets and targeting schemes include average tidal volume (for adaptive targeting), percent minute
ventilation (for optimal targeting) and combined PCO2, volume, and frequency values describing a “zone of comfort” (for intelligent
targeting).
targeting scheme A model of the relationship between operator inputs and ventilator outputs to achieve a specific ventilatory
pattern, usually in the form of a feedback control system. The targeting scheme is a key component of a mode description.
tidal volume The volume of gas, either inhaled or exhaled, during a breath. The maximum value of the volume vs time waveform.
trigger variable The variable that the ventilator uses to start or “trigger” the inspiratory time. Common variables are time (pressure
control modes) and tidal volume (volume control modes). To “trigger” inspiration means to start inspiratory flow.
trigger window The period comprised of the entire expiratory time minus a short “refractory” period required to reduce the risk of
triggering a breath before exhalation is complete. If a signal from the patient (indicating an inspiratory effort) occurs within this
trigger window, inspiration starts and is defined as a patient triggered event.

HANDOUT 2 – HOW TO COMPARE MODES
Robert L. Chatburn, MHHS, RRT-NPS, FAARC
6/5/2014
Background Issues
The appropriate use of current modes, or the development of new modes, relies on the ability to compare and contrast their relative
advantages (assuming that we can identify and understand the functionality of modes in the first place; see Handout 1). In the larger
context of medicine, patients are linked to their data by the process of assessing their needs (diagnosis). They are also linked to
treatment options (biomedical innovation). But the fundamental responsibility of caregivers is to appropriately match patient needs to
available treatments (planning). In the more restricted context of mechanical ventilation, patient needs can be expressed as three
fundamental goals of mechanical ventilation (safety, comfort, and liberation). Treatment options can be viewed as the technological
capabilities of various modes to serve these goals. Thus, appropriate matching of technology to needs reduces to identifying which of
the available modes best serves the immediate clinical goals; see Respir Care. 2013;58(2):348-66
Why Compare Modes?
We need to compare modes because there are so many of them and because they differ enough in technological capability that they
cannot possibly all offer the same benefits to the patient. Hence, there is a need for comparison and choice. The issue is whether the
comparisons are based on logic and information or on personal bias. Unfortunately, the amount of good animal and clinical data is
relatively small. Thus, we tend to use mechanical ventilation based on tradition and the available technology rather than on evidence-
based medicine. In fact, after decades of clinical research, the only thing we seem to know is that smaller tidal volumes are better than
larger ones.
Which Modes Should be Compared?
As with any technology of sufficient complexity, the ability to compare and contrast objects requires a shift of focus away from names
to tags, using a formal classification system, or taxonomy. To briefly recap our discussion of taxonomy, all modes can be divided into
two broad orders, volume control and pressure control. Within these orders are families based on the breath sequences (possible
combinations of mandatory and spontaneous breaths). There are only 3 possible sequences of breaths a mode can generate: all
spontaneous breaths, called continuous spontaneous ventilation (CSV), mandatory breaths with the possibility of spontaneous breaths
between them, called intermittent mandatory ventilation (IMV), and mandatory breaths with no possibility of spontaneous breaths
between them, called continuous mandatory ventilation (CMV). Within the families are genus and species, identified by the targeting
schemes used for primary breaths (for CMV and CSV) and secondary breaths (for IMV). Major benefits accrue from using this
classification system; It allows us to start with a relatively large set of unique mode names on common ventilators and greatly reduce
it to a more manageable set of mode tags (classifications). In that set, redundancies are easily recognized and eliminated, leaving only
unique mode tags (at least to four or five levels of discrimination) that are amenable to comparison.
How Can Modes be Compared?
Despite the availability of a wide variety of modes, only the simplest set-point targeting schemes (mainly volume control continuous
mandatory ventilation) are used most of the time in daily practice. Such practice may be justified by the uncomplicated reliability of
these modes and the lack of evidence that any other mode is better in terms of major clinical outcomes. Yet we could also argue that
“lack of evidence is not evidence of lack of differential effectiveness”. And it takes little effort to understand why there will never be
enough clinical evidence to appropriately compare modes. Consider, for example, randomized controlled trials of 50 modes
(approximately the number of unique modes currently available), would require 1,225 head-to-head comparisons (ie, combinations of
50 modes taken 2 at time). Using the ARDSnet experience to estimate the resource cost per study of about 4 years and 38 million
dollars (in 1999), gathering evidence would take 4,900 labor years and over 46 billion US dollars! Thus, a complete set of clinical
evidence required to compare all modes of ventilation does not exist, and never will. Thus, to rationally compare the relative merits of
various modes, we must resort to deductive reasoning from first principles. We posit that a mode of mechanical ventilation has certain
design features that implement a general technological capability. These capabilities (identified in the next section) are defined on the
basis of an extensive analysis of all modes such that they can be used as unique identifiers whose benefits are intuitively obvious
(again, we have no data to prove their merits). Each technological capability serves a clinical aim. Each clinical aim, in turn, serves
specific objectives and general goals of mechanical ventilation based on the clinician’s assessment of the patient (Figure 1). Using this
rubric, any current or proposed feature of a mode should have a direct and logical link to specific patient needs.
Figure 1. Hierarchy of priorities showing how specific features of modes ultimately serve the goals of ventilation for the patient.

The utility of this hierarchical approach is that we can start on familiar ground (the general goals of mechanical ventilation) and
progress deductively to a linkage with specific ventilator capabilities and features, some of which might seem questionable without
such a line of reasoning to justify their existence. More to the point, the capabilities form the basis for comparing the relative benefits
of modes to guide appropriate selection for a given patient at a given time. The capabilities as described here are, by definition,
beneficial (given that the underlying assumptions of the targeting schemes are not violated). It follows that the more capabilities a
mode has, the better it serves the specific goals of mechanical ventilation that are judged to be most important in any given clinical
situation. Note that this approach explicitly ignores the issue of how modes are used. This conceptual distinction is essential because
of the huge variation in outcomes that can be attributed to the different knowledge base and skill levels of clinicians. Few would argue
that given current technology, a highly skilled clinician using a technologically simple mode would likely achieve better results than,
for example, a naïve clinician using a complex mode.
The Three goals of Mechanical Ventilation
Any number of indications for mechanical ventilation may be found in the literature, but they can all be condensed into three goals
and their associated objectives:
1. Promote Safety
a. Optimize ventilation/perfusion of the lung (maximize ventilation and oxygenation)
b. Optimize pressure/volume curve (minimize risk of atelectrauma and volutrauma)
2. Promote Comfort
a. Optimize patient-ventilator synchrony (minimize occurrence of trigger, flow, and cycle asynchronies)
b. Optimize work demand versus work delivered (minimize inappropriate shifting of work from vent to patient)
3. Promote Liberation
a. Optimize the weaning experience (minimize duration of ventilation and risk of adverse events)
Technical Capabilities of Modes
The procedure for identifying the most appropriate mode for a particular clinical goal starts with a list of available modes (eg, on
ventilators owned by a particular institution) identified by applying the mode taxonomy. Next, we construct a matrix that allows the
identification of the presence or absence of the technological capabilities that fulfill a clinical goal as described above. Finally, we
simply tabulate the capabilities for each mode. Three of the most common modes used in the world for adults are Volume
Assist/Control (classified as VC-CMVs), Pressure Control SIMV (classified as PC-IMVs,s) and Pressure Support (classified as PC-
CSVs). We will contrast these modes to more sophisticated modes, AutoMode PRVC-VS (classified as PC-IMVa,a ) and IntelliVent,
(classified as PC-IMVoi,oi; not available in the US). Ideally, this type of analysis should be applied to all unique modes for a complete
comparison. Note that there are some capabilities that are not matched to the modes in this example but do match other modes.
Goal
Technical Capability
A/C
PC-SIMV
PS
AutoMode
IntelliVent
Safety
Automatic minute ventilation target adjustment
x
Automatic Support adjustment with changing lung mechanics
x
x
Automatic frequency and/or tidal volume adjustment
x
x
Manual frequency and tidal volume settings
x
x
Automatic FiO2 adjustment
x
Automatic PEEP adjustment
x
Automatic lung protection limits
x
Minimizes tidal volume
Comfort
All breaths can be spontaneous
x
x
x
Trigger and cycle on diaphragm movement
Coordination of mandatory and spontaneous breaths
x
x
x
Automatic limits to avoid autoPEEP
x
Unrestricted inspiratory flow
x
x
x
x
Automatic adjustment of flow based on frequency
Automatic adjustment of support based on breathing pattern
Automatic adjustment of support to meet inspiratory effort
Liberation
Ventilator initiated weaning
x
Ventilator initiated spontaneous breathing trial
x
Automatic reduction of support with increased inspiratory effort
x
x
From this type of analysis we can identify a logical reason for preferring one mode over others on the basis of how well it serves the
clinical goal of mechanical ventilation for a particular patient at a particular point in time.

1
Standardized Vocabulary for Mechanical Ventilation
Version 7.9.14
2012 by Mandu Press Ltd.
active exhalation valve A mechanism for holding pressure in the breathing circuit by
delivering the flow required to allow the patient breathe spontaneously. This feature is
especially prominent in modes like Airway Pressure Release Ventilation that are intended
to allow unrestricted spontaneous breathing during a prolonged mandatory (i.e., time
triggered and time cycled) pressure controlled breath.
asynchrony (dyssynchrony) Regarding the timing of a breath, asynchrony means
triggering or cycling of an assisted breath that either leads or lags the patient’s inspiratory
effort. Regarding the size of a breath, asynchrony means the inspiratory flow or tidal
volume does not match the patient’s demand. Also, some ventilators allow a patient to
inhale freely during a pressure controlled mandatory breath but not to exhale, thus
inducing asynchrony. Asynchrony may lead to increased work of breathing and
discomfort.
adaptive targeting scheme A control system that allows the ventilator to
automatically set some (or conceivably all) of the targets between breaths to achieve
other preset targets. One common example is adaptive pressure targeting (e.g., Pressure
Regulated Volume Control mode on the Maquet Servo-i ventilator) where a static
inspiratory pressure is targeted within a breath (i.e., pressure controlled inspiration) but
this target is automatically adjusted by the ventilator between breaths to achieve an
operator set tidal volume target (aka, volume-targeted pressure control).
airway pressure The pressure at the airway opening measured relative to atmospheric
pressure during mechanical ventilation.
airway pressure release ventilation (APRV) A form of pressure control
intermittent mandatory ventilation that is designed to allow unrestricted spontaneous
breathing throughout the breath cycle. APRV is applied using I:E ratios much greater
than 1:1 and usually relying on short expiratory times and gas trapping to maintain end
expiratory lung volume rather than a preset PEEP. This is in contrast to Bilevel Positive
Airway Pressure (BIPAP) which is also pressure control intermittent mandatory
ventilation but with I:E ratios closer to 1:1, expiratory times that do not create significant
gas tapping and preset PEEP levels above zero.
assisted breath A breath during which all or part of inspiratory (or expiratory) flow is
generated by the ventilator doing work on the patient. In simple terms, if the airway
pressure rises above end expiratory pressure during inspiration, the inspiration is assisted
(as in the Pressure Support mode). It is also possible to assist expiration by dropping
airway pressure below end expiratory pressure (such as Automatic Tube Compensation
on the Dräger Evita 4 ventilator). In contrast, spontaneous breaths during CPAP are

2
unassisted because the ventilator attempts to maintain a constant airway pressure during
inspiration.
autoPEEP The positive difference between end-expiratory alveolar pressure (total or
intrinsic PEEP) and the end-expiratory airway pressure (set or extrinsic PEEP; Am J
Respir Crit Care Med 2011;184:756-762). When autoPEEP exists, a positive pressure
difference drives flow throughout exhalation until the subsequent breath interrupts
deflation. AutoPEEP is caused when expiratory time (either set by the patient’s brain or a
ventilator) is short relative to the expiratory time constant of the respiratory system
(possibly including the expiratory resistance of the breathing circuit).
automatic tube compensation A feature that allows the operator to enter the size of
the patient’s endotracheal tube and have the ventilator calculate the tube’s resistance and
then generate just enough pressure (in proportion to inspiratory or expiratory flow) to
compensate for the added resistive load. (See servo control.)
autotrigger A condition in which the ventilator repeatedly triggers itself because the
sensitivity is set too high(sometimes called “autocycling”). For pressure triggering, the
ventilator may autotrigger due to a leak in the system dropping airway pressure below a
pressure trigger threshold. When sensitivity is set too high, even the heartbeat can cause
inadvertent triggering. Autotriggering is a form of patient-ventilator asynchrony.
bio-variable targeting scheme A control system that allows the ventilator to
automatically set the inspiratory pressure or tidal volume randomly to mimic the
variability observed during normal breathing. Currently this “biologically variable”
targeting scheme is only available in one mode, Variable Pressure Support, on the Dräger
V500 ventilator. The operator sets a target inspiratory pressure and a percent variability
from 0% to 100%. A setting 0% means the preset inspiratory pressure will be delivered
for every breath. A 100 % variability setting means that the actual inspiratory pressure
varies randomly from PEEP/CPAP level to double the preset pressure support level.
blower A blower is a machine for generating relatively large flows of gas as the direct
ventilator output with a relatively moderate increase of pressure (e.g., 2 psi). Blowers are
used on home care and transport ventilators. (see compressor)
breath A positive change in airway flow (inspiration) paired with a negative change in
airway flow (expiration), associated with ventilation of the lungs. This definition
excludes flow changes caused by hiccups or cardiogenic oscillations. However, it allows
the superimposition of, for example, a spontaneous breath on a mandatory breath or vice
versa. The flows are paired by size, not necessarily by timing. For example, in Airway
Pressure Release Ventilation there is a large inspiration (transition from low pressure to
high pressure) possibly followed by a few small inspirations and expirations, followed
finally by a large expiration (transition from high pressure to low pressure). These
comprise several small spontaneous breaths superimposed on one large mandatory
breath. In contrast, during High Frequency Oscillatory Ventilation, small mandatory
breaths are superimposed on larger spontaneous breaths.

3
breathing circuit System of tubing connecting the patient to the ventilator.
breath sequence A particular pattern of spontaneous and/or mandatory breaths. The 3
possible breath sequences are: continuous mandatory ventilation, (CMV), intermittent
mandatory ventilation (IMV), and continuous spontaneous ventilation (CSV).
compliance A mechanical property of a structure such as the respiratory system; a
parameter of a lung model, or setting of a lung simulator; defined as the ratio of the
change in volume to the associated change in the pressure difference across the system.
Compliance is the reciprocal of elastance.
compressor A compressor is a machine for moving a relatively low flow of gas to a
storage container at a higher level of pressure (e.g, 20 psi). Compressors are generally
found on intensive care ventilators whereas blowers are used on home care and transport
ventilators. Compressors are typically larger and consume more electrical power than
blowers, hence the use of the latter on small, portable devices. (see blower)
CMV See continuous mandatory ventilation
continuous mandatory ventilation Commonly known as “Assist/Control”; CMV is
a breath sequence for which spontaneous breaths are not permitted between mandatory
breaths because every patient trigger signal in the trigger window produces a machine
cycled inspiration (ie, a mandatory breath). Machine triggered mandatory breaths may be
delivered at a preset rate. Therefore, in contrast to IMV, the mandatory breath frequency
may be higher than the set frequency but never below it. In some pressure controlled
modes on ventilators with an active exhalation valve, spontaneous breaths may occur
during mandatory breaths, but the defining characteristic of CMV is that spontaneous
breaths are not permitted between mandatory breaths. See mandatory breath,
intermittent mandatory ventilation, trigger window
continuous spontaneous ventilation A breath sequence for which all breaths are
spontaneous.
control variable The variable (ie, pressure or volume in the equation of motion) that
the ventilator uses as a feedback signal to manipulate inspiration. For simple set-point
targeting, the control variable can be identified as follows: If the peak inspiratory
pressure remains constant as the load experienced by the ventilator changes, then the
control variable is pressure. If the peak pressure changes as the load changes but tidal
volume remains constant, then the control variable is volume. Volume control implies
flow control and vice versa, but it is possible to distinguish the two on the basis of which
signal is used for feedback control. Some modes (eg, High Frequency Oscillation and
Intrapulmonary Percussive Ventilation) do not maintain either constant peak pressure or
tidal volume and thus control only inspiratory and expiratory times (ie, they may be
called time controllers).
CPAP Continuous positive airway pressure; the set or measured mean value of
transrespiratory system pressure during unassisted breathing or between assisted breaths.
While this term is sometimes used synonymously for PEEP, historically, PEEP came

4
first. PEEP mechanisms originally required the patient to drop transrespiratory system
pressure to below atmospheric pressure to inhale, imposing a load and causing an
increased work of breathing. CPAP mechanisms were developed so that the patient only
had to drop pressure below the set CPAP level to inhale, thus decreasing the imposed
load. See PEEP.
CSV See continuous spontaneous ventilation; all breaths are spontaneous. See
spontaneous breath.
cycle (cycling) To end the inspiratory time (and begin expiratory flow)
cycle variable The variable (usually pressure, volume, flow, or time) that is used to end
inspiratory time (and begin expiratory flow).
driving pressure The pressure causing delivery of the tidal volume during pressure
control modes (ie the change in transrespiratory pressure associated with tidal volume
delivery). Driving pressure may be estimated either from ventilator settings (ie, driving
pressure = set inspiratory pressure above total PEEP) or from the airway pressure
waveform (ie, driving pressure = end inspiratory pressure above total PEEP). See
airway pressure, inspiratory pressure, peak inspiratory pressure
dual targeting scheme A control system that allows the ventilator to switch between
volume control and pressure control during a single inspiration. Dual targeting is a more
advanced version of set-point targeting. It gives the ventilator the decision of whether the
breath will be volume or pressure controlled according to the operator set priorities. The
breath may start out in pressure control and automatically switch to volume control, as in
the Bird “VAPS” mode or, the reverse, as in the Dräger “Pressure Limited” mode feature.
The Maquet Servo-i ventilator has a mode called “Volume Control” and the operator
presets both inspiratory time and tidal volume as would be expected with any
conventional volume control mode. However, if the patient makes an inspiratory effort
that decreases inspiratory pressure by 3 cm H2O, the ventilator switches to pressure
control and, if the effort lasts long enough, flow cycles the breath. Indeed, if the tidal
volume and inspiratory time are set relatively low and the inspiratory effort is relatively
large, the resultant breath delivery is indistinguishable from Pressure Support. As a result,
the tidal volume may be much larger than the expected, preset value. This highlights the
need to understand dual targeting. Because both pressure and volume are the control
variables during dual targeting, we identify the control variable as the one with which the
breath initiates. This is because the alternate control variable may never be implemented
during the breath, depending on the other factors in the targeting scheme.
duty cycle The ratio of inspiratory time to total cycle time, usually expressed as a
percent.
dynamic compliance The slope of the pressure-volume curve drawn between two
points of zero flow (eg, at the start and end of inspiration).

5
dynamic hyperinflation The increase in lung volume that occurs whenever
insufficient exhalation time prevents the respiratory system from returning to its normal
resting end-expiratory equilibrium volume between breath cycles. Inappropriate operator
set expiratory time may lead to dynamic hyperinflation, inability of the patient to trigger
breaths, and an increased work of breathing.
elastance A mechanical property of a structure such as the respiratory system; a
parameter of a lung model, or setting of a lung simulator; defined as the ratio of the
change in the pressure difference across the system to the associated change in volume.
Elastance is the reciprocal of compliance.
elastic load The pressure difference applied across a system (e.g., a container) that
sustains the system's volume relative to some reference volume, and/or the amount of its
compressible contents relative to some reference amount. (For a linear system: elastance
volume, or, volume/compliance; for a container, the overall effective elastance
(compliance) includes the elastances (compliances) of its structural components and the
compressibility of the fluid [gas or liquid] within it.)
equation of motion for the respiratory system A relation among pressure
difference, volume, and flow (as variable functions of time) that describes the mechanics
of the respiratory system. The simplest and most useful form is a differential equation
with constant coefficients describing the respiratory system as a single deformable
compartment including the lungs and chest wall connected in series to a single flow
conducting tube:
autoPEEPtVRtVEtPtP musTR )()()()( 
where
PTR(t) = the change in transrespiratory pressure difference (i.e., airway opening
pressure minus body surface pressure) as a function of time (t), measured
relative to end expiratory airway pressure. This is the pressure generated
by a ventilator, Pvent(t), during an assisted breath.
Pmus(t) = ventilatory muscle pressure difference as a function of time (t); the
theoretical chestwall transmural pressure difference that would produce
movements identical to those produced by the ventilatory muscles during
breathing maneuvers (positive during inspiratory effort, negative during
expiratory effort)
V(t) = volume change relative to end expiratory volume as a function of time (t)
)(tV

= flow as a function of time (t), the first derivative of volume with respect to
time
E = elastance (inverse of compliance; E = 1/C)

6
R = resistance
autoPEEP = end expiratory alveolar pressure above end expiratory airway pressure
For the purposes of classifying modes of mechanical ventilation the equation is often
simplified to:
VRVEPvent 
where
Pvent = the transrespiratory pressure difference (ie, “airway pressure”) generated
by the ventilator during an assisted breath
expiratory flow time The period from the start of expiratory flow to the instant when
expiratory flow stops, usually expressed in seconds. By convention, expiratory flow is in
the negative direction (below zero) in graphs.
expiratory pause time The period from cessation of expiratory flow to the start of
inspiratory flow.
expiratory time The period from the start of expiratory flow to the start of inspiratory
flow, usually expressed in seconds. Expiratory time equals expiratory flow time plus
expiratory pause time.
feedback control Closed loop control accomplished by using the output as a signal that
is fed back (compared) to the operator-set input. The difference between the two is used
to drive the system toward the desired output (ie, negative feedback control). For
example, pressure controlled modes use airway pressure as the feedback signal to
manipulate gas flow from the ventilator to maintain an inspiratory pressure setpoint.
flow control Maintenance of an invariant inspiratory flow waveform despite changing
respiratory system mechanics
flow triggering The starting of inspiratory flow due to a patient inspiratory effort that
generates inspiratory flow above a preset threshold (ie, the trigger sensitivity setting).
flow target Inspiratory flow reaches a preset value that may be maintained before
inspiration cycles off.
flow cycling The ending of inspiratory time due to inspiratory flow decay below a
preset threshold (aka, the cycle sensitivity).
IMV See intermittent mandatory ventilation.
I:E The ratio of inspiratory time to expiratory time, TI/TE.

7
inspiratory flow The flow into the airway opening during the inspiratory time. By
convention, inspiratory flow is in the positive direction (above zero) in graphs.
inspiratory flow time The period from the start of inspiratory flow (into the airway
opening) to the cessation of inspiratory flow, usually expressed in seconds.
inspiratory hold An intentional maneuver during mechanical ventilation whereby
exhalation is delayed for a preset time (inspiratory hold time) after an assisted breath.
This maneuver is used to assess static respiratory system mechanics and also to increase
mean airway pressure during volume control ventilation in an attempt to improve gas
exchange.
inspiratory hold (pause) time The period from the cessation of inspiratory flow (into
the airway opening) to the start of expiratory flow during mechanical ventilation, usually
expressed in seconds.
inspiratory pressure General term for the pressure at the patient connection during
the inspiratory phase.
inspiratory pressure change The change in transrespiratory system pressure
associated with delivery of the tidal volume as described in the equation of motion for the
respiratory system. For pressure control modes, if inspiratory pressure is set relative to
atmospheric pressure, the term “peak inspiratory pressure” is used to describe the setting.
If inspiratory pressure is set relative to PEEP, the term “inspiratory pressure change” is
used. See equation of motion for the respiratory system, peak inspiratory
pressure
inspiratory time The period from the start of inspiratory flow to the start of expiratory
flow, usually expressed in seconds. Inspiratory time equals inspiratory flow time plus
inspiratory pause time.
intelligent targeting scheme A ventilator control system that uses artificial
intelligence programs such as fuzzy logic, rule based expert systems, and artificial neural
networks. Examples include the rule based system used by SmartCare (Dräger Evita XL
ventilator) and IntelliVent-ASV (Hamilton S1 ventilator).
intermittent mandatory ventilation Breath sequence for which spontaneous breaths
are permitted between mandatory breaths. For most ventilators, a short “window” is
opened before the scheduled machine triggering of mandatory breaths to allow
synchronization with any detected inspiratory effort on the part of the patient. This is
referred to as synchronized IMV (or SIMV).
Three common variations of IMV are: (1) Mandatory breaths are always delivered at the
set frequency; (2) Mandatory breaths are delivered only when the spontaneous breath
frequency falls below the set frequency; (3) Mandatory breaths are delivered only when
the spontaneous minute ventilation (ie, product of spontaneous breath frequency and
spontaneous breath tidal volume) drops below a preset or computed threshold (aka

8
Mandatory Minute Ventilation). Therefore, in contrast to CMV, with IMV the mandatory
breath frequency can never be higher than the set rate but it may be lower.
For some modes (eg, Airway Pressure Release Ventilation), a short window is also
opened at the end of the inspiratory time. Because spontaneous breaths are allowed
during the mandatory pressure controlled breath, this window synchronizes the end of the
mandatory inspiratory time with the start of spontaneous expiratory flow, if detected.
With these technological developments, potential confusion arises as to whether
inspiration that is synchronized (either start or stop) is considered patient triggered/cycled
or machine triggered/cycled. If we say synchronized breaths are patient triggered and
cycled, we have the awkward possibility of a spontaneous breath occurring during
another spontaneous breath. This is avoided by distinguishing between a trigger window
and a synchronization window.
There are some modes where the idea of IMV may be vague: With Airway Pressure
Release Ventilation, relatively high frequency spontaneous breaths are superimposed on
low frequency mandatory breaths. However, the expiratory time between mandatory
breaths is often set so short that a spontaneous breath is unlikely to occur between them.
Other ambiguous modes are High Frequency Oscillation, High Frequency Jet Ventilation,
Intrapulmonary Percussive Ventilation and Volumetric Diffusive Respiration. With these
modes, high frequency mandatory breaths are superimposed on low frequency
spontaneous breaths and again, there is no possibility of a spontaneous breath actually
occurring between mandatory breaths. Nevertheless, we classify all these modes as forms
of IMV because spontaneous breaths can occur along with mandatory breaths and
because spontaneous efforts do not affect the mandatory breath frequency. See machine
triggering, patient triggering, synchronization window, trigger window,
continuous mandatory ventilation
load The pressure required to generate inspiration (see elastic load and resistive load).
loaded breath A breath during which all or part of inspiratory (or expiratory) flow is
generated by the patient doing work on the ventilator. In simple terms, if the airway
pressure falls below end expiratory pressure during inspiration, the inspiration is loaded.
If pressure rises above baseline on expiration, then expiration is loaded.
machine cycling Ending inspiratory time independent of signals representing the
patient determined components of the equation of motion (Pmus, elastance, or resistance).
Common examples are cycling due to a preset tidal volume or inspiratory time. If a
patient signal (indicating expiration) occurs during an inspiratory time synchronization
window, inspiration stops and is defined as a machine cycled event that ends a mandatory
breath. See machine triggering, patient triggering, synchronization window,
trigger window, continuous mandatory ventilation, intermittent mandatory
ventilation
machine triggering Starting inspiratory flow based on a signal (usually time) from the
ventilator, independent of a patient trigger signal. Examples include triggering based on a
preset frequency (which sets the ventilatory period), or based on a preset minimum

9
minute ventilation (determined by tidal volume divided by the ventilatory period). If a
signal from the patient (indicating an inspiratory effort) occurs within a synchronization
window, the start of inspiration is defined as a machine trigger event that begins a
mandatory breath. See machine cycling, patient triggering, synchronization
window, trigger window, continuous mandatory ventilation, intermittent
mandatory ventilation
mandatory breath A breath for which the patient has lost control over timing. This
means a breath for which the start or end of inspiration (or both) is determined by the
ventilator, independent of the patient. That is, the machine triggers and/or cycles the
breath. A mandatory breath can occur during a spontaneous breath (eg, High Frequency
Jet Ventilation). A mandatory breath is, by definition, assisted. See assisted breath,
spontaneous breath
mandatory minute ventilation A form of intermittent mandatory ventilation (IMV) in
which the ventilator monitors the exhaled minute ventilation as a target variable. If the
exhaled minute ventilation falls below the operator set value, the ventilator will trigger
mandatory breaths or increase the inspiratory pressure until the target is reached.
mechanical ventilator An automatic machine designed to provide all or part of the
work required to generate enough breaths to satisfy the body’s respiratory needs.
minute ventilation The product of tidal volume times ventilatory frequency, usually
expressed in L/min.
mode of ventilation A predetermined pattern of interaction between a patient and a
ventilator, specified as a particular combination of control variable, breath sequence, and
targeting schemes for primary and secondary breaths.
negative pressure ventilation A type of assisted breathing for which transrespiratory
pressure difference is generated by keeping airway pressure equal to atmospheric
pressure and making body surface pressure less than atmospheric pressure. Examples
would be ventilation with an “iron lung” or “chest cuirass”.
Neurally Adjusted Ventilatory Assist The name of a mode using a servo targeting
scheme in which the controller sets airway pressure to be proportional to patient effort
based on the voltage recorded from diaphragmatic activity from sensors embedded in an
orogastric tube:
)t(KEdi)t(P
where P(t) is inspiratory pressure relative to end expiratory pressure as a function of time,
t, K is the NAVA support level (an amplification factor), Edi (t) is the electrical signal
from the diaphragm as a function of time. The operator inputs the constant of
proportionality between voltage and pressure (gain). Then the controller sets airway
pressure to equal the product of gain and the Edi.

10
optimal targeting scheme A ventilator control system that automatically adjusts the
targets of the ventilatory pattern to either minimize or maximize some overall
performance characteristic. One example is Adaptive Support Ventilation (Hamilton
Medical G5 ventilator) in which the ventilator adjusts the mandatory tidal volume and
frequency (for a passive patient) is such a way as to minimize the work rate of
ventilation.
partial ventilatory support the ventilator and the respiratory muscles each provide
some of the work of breathing; muscle pressure adds to ventilator pressure in the equation
of motion.
patient cycling Ending inspiratory time based on signals representing the patient
determined components of the equation of motion, (Pmus , elastance, or resistance).
Common examples of cycling variables are peak inspiratory pressure and percent
inspiratory flow. See machine triggering, machine cycling, patient triggering,
synchronization window, trigger window, continuous mandatory
ventilation, intermittent mandatory ventilation
patient triggering Starting inspiration based on a patient signal occurring in a trigger
window, independent of a machine trigger signal. The signal is related to one of the
patient determined components of the equation of motion (Pmus, elastance, or resistance).
Common examples of patient trigger variables are airway pressure drop below baseline
and inspiratory flow due to patient effort. See machine triggering, machine
cycling, synchronization window, trigger window, continuous mandatory
ventilation, intermittent mandatory ventilation
PC-CMV Pressure controlled continuous mandatory ventilation.
PC-IMV Pressure controlled intermittent mandatory ventilation.
PC-CSV Pressure controlled continuous spontaneous ventilation.
peak airway pressure The maximum airway pressure during a mechanically assisted
inspiration, measured relative to atmospheric pressure.
peak inspiratory pressure The inspiratory pressure change that is set relative to
atmospheric pressure during pressure control modes. See inspiratory pressure
change
PEEP Positive end expiratory pressure; the value of transrespiratory system pressure at
end expiration. See CPAP
positive pressure ventilation A type of assisted breathing for which transrespiratory
pressure difference is generated by raising airway pressure above body surface pressure
(usually equal to atmospheric pressure). Examples would be ventilation with intensive
care or transport ventilators.
pressure A measure of force per unit of area at a particular point in space.

11
pressure change The difference between pressure (or pressure gradient) measured at
one point in time and the same pressure measured at a previous point in time.
pressure gradient The difference between pressure measured at one point in space and
another point in space. Examples include the pressure difference across a cell membrane
causing gas diffusion into the cell and the pressure difference across the respiratory
system causing flow into the lungs. See transairway pressure, transalveolar
pressure, transchestwall pressure, transpulmonary pressure,
transrespiratory pressure, transthoracic pressure
pressure control A general category of ventilator modes for which pressure delivery is
predetermined by a targeting scheme such that inspiratory pressure is either proportional
to patient effort or has a particular waveform regardless of respiratory system mechanics.
When inspiratory pressure is preset, we further specify that inspiration must start out with
the preset pressure to avoid confusion with dual targeting that may switch from a preset
flow to a preset pressure (eg, Pmax feature used with volume control modes on the
Dräger Evita Infinity V500 ventilator). See dual targeting scheme. According to the
equation of motion, pressure control means that inspiratory pressure is predetermined as
the independent variable so that volume and flow become the dependent variables. See
volume control and equation of motion.
pressure cycling Inspiration ends (ie, expiratory flow starts) when airway pressure
reaches a preset threshold.
Pressure Support: The name of a mode using a set-point targeting scheme in which
all breaths are pressure or flow triggered, pressure targeted, and flow cycled.
pressure triggering The starting of inspiratory flow due to a patient inspiratory effort
that generates an airway pressure drop below end expiratory pressure larger than a preset
threshold (ie, the trigger sensitivity setting).
pressure target Inspiratory pressure reaches a preset value before inspiration cycles
off.
primary breaths Mandatory breaths during CMV or IMV or spontaneous breaths
during CSV.
Proportional Assist Ventilation (PAV) The name of a mode using a servo targeting
scheme based on the equation of motion for the respiratory system in the form:
)()()( 21 tVKtVKtP 
where inspiratory pressure relative to end expiratory pressure as a function of time P(t) is
the sum of two components. The first is the “volume assist” or the amount of elastic load
supported, ie, K1 times volume as a function of time V(t). The second component is the
“flow assist” or the amount of resistive load supported, ie, K2, times flow as a function of
time,
).(tV

The values of K1 and K2 are preset by the operator and represent the

12
supported elastance and resistance, respectively, whereas volume and flow are generated
by the patient. Because volume and flow are initiated by the patient’s inspiratory effort
created by muscle pressure, Pmus, the pressure generated by PAV can be thought of as an
amplifier of Pmus.
ramp A mathematical function whose value rises or falls at a constant rate. Ascending
(rising) or descending (falling) functions are sometimes used for inspiratory flow in
volume control modes.
resistance A mechanical property of a structure such as the respiratory system; a
parameter of a lung model, or setting of a lung simulator; defined as the ratio of the
change in the pressure difference across the system to the associated change in flow.
resistive load The pressure difference applied across a system (e.g., a container) that is
related to a rate of change of the system's volume and/or the flow of fluid within or
through the system. (For a linear system: resistance x flow, or, resistance x rate of change
of volume; for a container, the effective resistance includes the mechanical (usually
viscous) resistances of its structural components and the flow resistance of the fluid [gas
or liquid] within it.)
secondary breaths Spontaneous breaths during IMV.
sensitivity The sensitivity setting of the ventilator is a threshold value for the trigger
variable which, when met, starts inspiration. In other words, the sensitivity is the amount
the trigger variable must change to start inspiratory flow. Sensitivity is sometimes used to
refer to the cycle threshold.
servo targeting A control system for which the output of the ventilator automatically
follows a varying input. In practice, this means that inspiratory pressure is proportional to
inspiratory effort. For example, the Automatic Tube Compensation feature on the Dräger
Evita 4 ventilator tracks flow and forces pressure to be equal to flow squared and
multiplied by a constant (representing endotracheal tube resistance). Other examples
include Proportional Assist Ventilation (Covidien PB 840 ventilator; pressure is
proportional to spontaneous volume and flow) and Neurally Adjusted Ventilatory Assist
(Maquet Servo-i ventilator; pressure is proportional to diaphragmatic electrical activity).
For all three of these example modes airway pressure is effectively proportional to the
patient’s inspiratory effort.
set-point targeting A control system for which the operator sets all the parameters of
the pressure waveform (pressure control modes) or volume and flow waveforms (volume
control modes). Advanced volume control modes actually allow the ventilator to make
small adjustments to the set inspiratory flow to compensate for such factors as patient
circuit compliance. From an engineering point of view, this is adaptive feedback control,
but from a ventilator mode taxonomy point of view, such adjustments are better seen as a
way of implementing operator preset values, and thus classified as set-point targeting.

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sinusoid A mathematical function having a magnitude that varies as the sine of an
independent variable (eg time). A sinusoidal function is sometimes used for inspiratory
flow in volume control modes.
spontaneous breath A breath for which the patient retains substantial control over
timing. This means the start and end of inspiration may be determined by the patient,
independent of any machine settings for inspiratory time and expiratory time. That is, the
patient both triggers and cycles the breath. Note that use of this definition for determining
the breath sequence (ie, CMV, IMV, CSV) assumes normal ventilator operation. For
example, coughing during VC-CMV may result in patient cycling for a patient triggered
breath due to the pressure alarm limit. While inspiration for that breath is both patient
triggered and patient cycled, this is not normal operation and the sequence does not turn
into IMV. A spontaneous breath may occur during a mandatory breath (eg Airway
Pressure Release Ventilation). A spontaneous breath may be assisted or unassisted. See
assisted breath, mandatory breath
synchronized IMV (SIMV) A form of IMV in which mandatory breath delivery is
coordinated with patient effort. A synchronized breath is considered to be machine
triggered. See intermittent mandatory ventilation
synchronization window A short period, at the end of a preset expiratory time or at
the end of a preset inspiratory time, during which a patient signal may be used to
synchronize a mandatory breath trigger or cycle event to a spontaneous breath. If the
patient signal occurs during an expiratory time synchronization window, inspiration starts
and is defined as a machine triggered event. This is because the mandatory breath would
have been time triggered regardless of whether the patient signal had appeared or not and
because the distinction is necessary to avoid logical inconsistencies in defining
mandatory and spontaneous breaths which are the foundation of the mode taxonomy. If
inspiration is triggered in a synchronization window, the actual ventilatory period for the
previous breath will be shorter than the set ventilatory period (determined by the set
mandatory breath frequency). Some ventilators add the lost time to the next mandatory
breath period to maintain the set frequency. Sometimes a synchronization window is used
at the end of the inspiratory time of a pressure controlled, time cycled breath. If the
patient signal occurs during such an inspiratory time synchronization window, expiration
starts and is defined as a machine cycled event. Some ventilators offer the mode called
Airway Pressure Release Ventilation (or something similar with a different name) that
makes use of both expiratory and inspiratory synchronization windows. See
intermittent mandatory ventilation, machine triggering, patient triggering,
trigger window.
tag A mode classification. A tag can be an acronym. For example the mode named
Volume A/C is classified as volume control (VC) continuous mandatory ventilation
(CMV) with set-point targeting (s) and can be represented as VC-CMVs. Another
example (using both primary and secondary breaths) would be PRVC SIMV classified as
PC-IMVa,s, where the primary breath uses adaptive targeting (a) and the secondary
breath uses set-point targeting (s). The mode named Adaptive Support Ventilation has

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multiple targeting for each type of breath (ie, both optimal, o, and intelligent, i). It is
classified as PC-IMVoi,oi.
target A predetermined goal of ventilator output. Targets can be viewed as the goals of
the targeting scheme. Within-breath targets are the parameters of the pressure, volume, or
flow waveform. Examples of within-breath targets include inspiratory flow or pressure
and rise time (set-point targeting), tidal volume (dual targeting) and constant of
proportionality between inspiratory pressure and patient effort (servo targeting). Note that
preset values within a breath that end inspiration, such as tidal volume, inspiratory time,
or percent of peak flow, are also cycle variables. Between-breath targets serve to modify
the within-breath targets and/or the overall ventilatory pattern. Between-breath targets are
used with more advanced targeting schemes, where targets act over multiple breaths.
Examples of between-breath targets and targeting schemes include average tidal volume
(for adaptive targeting), percent minute ventilation (for optimal targeting) and combined
PCO2, volume, and frequency values describing a “zone of comfort” (for intelligent
targeting).
targeting scheme A model of the relationship between operator inputs and ventilator
outputs to achieve a specific ventilatory pattern, usually in the form of a feedback control
system. The targeting scheme is a key component of a mode description.
taxonomy A hierarchical classification system. A taxonomy for modes of ventilation
has four levels: 1) the control variable, 2) the breath sequence; 3) the targeting scheme for
primary breaths and ; 4) the targeting scheme for secondary breaths. These levels
correspond to the levels of Family, Class, Genus, and Species of the Linnaean taxonomy
used in biology.
TC-IMV Time controlled intermittent mandatory ventilation (eg, High Frequency
Oscillatory Ventilation or Intrapulmonary Percussive Ventilation).
tidal volume The volume of gas, either inhaled or exhaled, during a breath. The
maximum value of the volume vs time waveform.
time cycling Inspiratory time ends after a preset time interval has elapsed. The most
common examples are a preset inspiratory time or a preset inspiratory pause time.
time constant The time at which an exponential function attains 63% of its steady
state value in response to a step input; the time necessary for inflated lungs to passively
empty by 63%; the time necessary for the lungs to passively fill 63% during pressure
control ventilation with a rectangular pressure waveform. The time constant for a passive
mechanical system is calculated as the product of resistance and compliance and has units
of time (usually expressed in seconds). Passive inhalation or exhalation is virtually
complete after 5 time constants.
time control A general category of ventilator modes for which inspiratory flow,
inspiratory volume, and inspiratory pressure are all dependent on respiratory system
mechanics. As no parameters of the pressure or flow waveform are preset, the only

15
control of the breath is the timing, ie, inspiratory and expiratory times. Examples of this
are high frequency oscillatory ventilation (CareFusion 3100 ventilator) and Volumetric
Diffusive Respiration (Percussionaire).
tidal pressure the change in trans-alveolar pressure (i.e., pressure in the alveolar region
minus pressure in the pleural space, equivalent to elastance times volume in the equation
of motion) associated with the inhalation or exhalation of a tidal volume.
total cycle time Same as ventilatory period, the sum of inspiratory time and expiratory
time, usually expressed in seconds.
total PEEP The sum of autoPEEP and intentionally applied PEEP or CPAP.
Synonymous with intrinsic PEEP.
time triggering The starting of inspiratory flow due to a preset time interval. The most
common example is a preset ventilatory frequency.
total ventilatory support The ventilator provides all the work of breathing; muscle
pressure in the equation of motion is zero. This is normally only possible if the patient is
paralyzed or heavily sedated.
transairway pressure Pressure at the airway opening minus pressure in the lungs (i.e.,
alveolar pressure).
transalveolar pressure Pressure in the lungs minus pressure in the pleural space.
Equal to transpulmonary pressure only under static conditions.
transchestwall pressure Pressure in the pleural space minus pressure on the body
surface.
transpulmonary pressure Pressure at the airway opening minus pressure in the
pleural space.
transrespiratory pressure Pressure at the airway opening minus pressure on the body
surface; equal to the sum of transairway pressure plus transalveolar pressure plus
transchestwall pressure.
transthoracic pressure Pressure in the lungs minus pressure on the body surface;
equal to the sum of transalveolar pressure plus transchestwall pressure
trigger (triggering) To start the inspiratory time. See machine triggering, patient
triggering
trigger variable The variable (usually pressure, volume, flow, or time) that is used to
start the inspiratory time.
trigger window The period comprised of the entire expiratory time minus a short
“refractory” period required to reduce the risk of triggering a breath before exhalation is

16
complete. If a signal from the patient (indicating an inspiratory effort) occurs within this
trigger window, inspiration starts and is defined as a patient triggered event. See
intermittent mandatory ventilation, machine triggering, patient triggering,
synchronization window
ventilatory pattern A sequence of breaths (CMV, IMV, or CSV) with a designated
control variable (volume or pressure) for the mandatory breaths (or the spontaneous
breaths for CSV).
ventilatory period The time from the start of inspiratory flow of one breath to the start
of inspiratory flow of the next breath; inspiratory time plus expiratory time; the reciprocal
of ventilatory frequency. Also called total cycle time or total breath cycle.
volume control A general category of ventilator modes for which both inspiratory flow
and tidal volume are predetermined by a targeting scheme to have particular waveforms
independent of respiratory system mechanics. Usually, flow and tidal volume may be set
directly by the operator. Alternatively, the ventilator may determine tidal volume based
on operator preset values for frequency and minute ventilation or the ventilator may
determine inspiratory flow based on operator set tidal volume and inspiratory time. When
inspiratory volume and flow are preset, we further specify that inspiration must start out
with the preset flow to avoid confusion with dual targeting that may switch from a preset
pressure to a preset flow and volume (eg. Volume Assured Pressure Support). See dual
targeting scheme. Note that setting tidal volume is a necessary but not sufficient
criterion for volume control. The reason is that some ventilators use pressure control with
adaptive targeting and allow the operator to set a tidal volume but not an inspiratory flow.
In this case, the tidal volume setting refers to the between-breath tidal volume target, not
a within-breath target. See adaptive targeting scheme. Likewise, setting inspiratory
flow is also a necessary but not sufficient criterion for volume control. For example, the
Bird Mark 7 ventilator requires an inspiratory flow setting but has no tidal volume
setting. Instead the operator sets the inspiratory pressure, which is also the cycle variable.
Hence, breaths are pressure controlled, and changing lung mechanics change the rate of
pressure rise, the inspiratory time, and hence the delivered tidal volume as in other
examples of pressure control. According to the equation of motion, volume control means
that both volume and flow are predetermined as the independent variables and pressure is
thus the dependent variable. See pressure control and equation of motion.
volume cycling Inspiratory time ends when inspiratory volume reaches a preset
threshold (ie, tidal volume).
VC-CMV Volume controlled continuous mandatory ventilation.
VC-IMV Volume controlled intermittent mandatory ventilation.
volume target A preset value for tidal volume that the ventilator is set to attain either
within a breath or as an average over multiple breaths.

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volume triggering The starting of inspiratory flow due to a patient inspiratory effort
that generates an inspiratory volume signal larger than a preset threshold (ie, the trigger
sensitivity setting).
work of breathing The general definition of work is the integral of pressure with
respect to volume during an assisted inspiration. There are two general components of
work related to mechanical ventilation. One kind is the work performed by the ventilator
on the patient, which is reflected by a positive change in airway pressure above baseline
during inspiration. The second component is the work the patient does on the ventilator to
(eg, to trigger inspiration), which is reflected by a negative change in airway pressure
below baseline during inspiration.