Minimizing Stomach Inflation Versus Optimizing
Holger Herff, MD
Peter Paal, MD
Achim von Goedecke, MD, MSc
Thomas Mitterlechner, MD
Thomas Danninger, BSc
Volker Wenzel, MD, MSc
During cardiopulmonary resuscitation (CPR), ven-
tilation of patients with an unprotected airway is often
performed too aggressively, with excessive airway
pressures resulting in subsequent stomach inflation,
regurgitation, and aspiration of stomach contents.1–3
To reduce the risk of stomach inflation in an unpro-
tected airway, an inspiratory flow-limiting, bag-valve
device was developed that decreases peak airway
pressure, in turn decreasing the likelihood of stomach
During CPR with 100 chest compressions over 60 s
with a 50% duty cycle,5the secure time window for
ventilation during the chest decompression phase is
only 0.3 s. Although the inspiratory flow-limiting,
bag-valve device is valuable to minimize stomach
inflation in an unprotected airway, in a protected
airway high flow ventilation may be needed if chest
compressions are not interrupted for ventilation.
The purpose of this study was to assess effects of
the inspiratory flow-limiting, bag-valve device and an
adult self-inflating, bag-valve device during simulated
CPR with uninterrupted chest compressions after in-
tubation. Inspiratory times were 0.25, 0.3, and 0.5 s.
Our null hypothesis was that the different bag-valve
In a bench model, we evaluated a bag-valve device (Smart Bag®MO) with limited
maximum inspiratory gas flow developed to reduce the risk of stomach inflation in
an unprotected airway. During simulated cardiopulmonary resuscitation with
uninterrupted chest compressions, ventilation with the “disabled” Smart Bag®MO
or an adult self-inflating bag-valve device provided only adequate tidal volumes if
inspiratory time was 0.5 s. Ventilation with the “enabled” Smart Bag®MO, even in
ventilation windows of 0.5 s, provided inadequate tidal volumes during simulated
cardiopulmonary resuscitation and would result in hypoventilation in a patient.
(Anesth Analg 2008;106:535–7)
device settings would have comparable effects on the
study end points tidal volume and respiratory mechanics.
The inspiratory flow-limiting, bag-valve device
(Smart Bag®MO, O-Two Medical Technologies, Mis-
sissauga, Ontario; Fig. 1) is an adult bag-valve device
that reduces inspiratory gas flow and in consequence
peak airway pressures during ventilation of an unpro-
tected airway. When a lever is rotated on the side of
the gas flow reducer, the piston of this element is fixed
in its original position, which turns off the flow-
limiting feature, converting the Smart Bag®MO to a
standard adult bag-valve device (manual override mode).
To simulate ventilation of an intubated adult patient
in a bench model, the Smart Bag®MO in its enabled
flow-limiting state, the Smart Bag®MO in its disabled
state, and an adult bag-valve device were directly at-
tached to the air inlet of an adult test lung (MI Instru-
ments, Grand Rapids, MI; Fig. 2). The test lung was
connected to a personal computer using Pneuview stan-
dard software (MI Instruments, Grand Rapids, MI).
To simulate respiratory system conditions during
prolonged continuing CPR, respiratory system com-
pliance was set to 20 mL/cm H2O.6In a second
approach simulating ventilation of a supine anesthe-
tized adult, respiratory system compliance was set to
60 mL/cm H2O, which simulates respiratory system
mechanics shortly after onset of cardiac arrest.7With
both respiratory compliance settings, upper and lower
airway resistance were adjusted to values resulting in
a total airway resistance of 5 cm H2O ? L?1? s?1, which
is a physiological value for an intubated patient with
no airway obstruction.7In each respiratory compli-
ance setting, the test lung was randomly ventilated
with an enabled versus disabled Smart Bag®MO
versus adult bag-valve device. The enabled Smart
This article has supplementary material on the Web site:
From the Department of Anesthesiology and Critical Care
Medicine, Innsbruck Medical University, Innsbruck, Austria.
Accepted for publication October 8, 2007.
Supported, in part, by the Science Foundation of the Austrian
National Bank grant 11448, Vienna, Austria.
Address correspondence and reprint requests to Dr. Holger
Herff, Department of Anesthesiology and Critical Care Medicine,
Innsbruck Medical University, Anichstrasse 35, 6020 Innsbruck,
Austria. Address e-mail to email@example.com.
Copyright © 2008 International Anesthesia Research Society
Vol. 106, No. 2, February 2008
Bag®MO, the disabled Smart Bag®MO, and the adult
bag-valve device were squeezed to produce inspiratory
times of 0.25, 0.3, or 0.5 s. Only results within a margin of
0.01 s above or below the target times were accepted. In
both respiratory system compliance settings for each
inspiratory time, 20 successful ventilation attempts were
recorded for each bag valve device. The rescuer was
blinded to all monitor tracings except inspiratory time.
Kolmogorov-Smirnov analysis. For statistical analysis
between self-inflating, bag-valve devices in each set-
ting, Student’s t-test for unpaired data was used.
Further, for each self-inflating, bag-valve device in a
given setting, results of 0.25 s were compared with
those of 0.3 s, and those of 0.3 s were compared with
those of 0.5 s using the paired Student’s t-test. Data are
presented as mean ? sd; statistical significance was
considered as P ? 0.05.
of datawas determinedusing
With a respiratory system compliance of 20 mL/cm
H2O, tidal volume provided with an enabled Smart
Bag®MO increased from 83 ? 3 mL (mean ? sd) in
0.25 s to 96 ? 2 mL in 0.3 s (P ? 0.05) and to 161 ? 7
mL in 0.5 s (P ? 0.05). With the disabled Smart Bag®
MO, tidal volume increased from 212 ? 13 mL to
271 ? 16 mL (P ? 0.05) and then to 473 ? 28 mL (P ?
0.05), and with an adult bag-valve device from 277 ?
28 mL to 283 ? 22 mL (P ? 0.05) and then to 503 ? 26
mL (P ? 0.05) in 0.25, 0.3, and 0.5 s, respectively.
With a respiratory system compliance of 60 mL/cm
H2O, tidal volume provided with the enabled Smart
Bag®MO increased from 120 ? 3 mL in 0.25 s to 140 ?
4 mL in 0.3 s (P ? 0.05) and to 190 ? 7 mL in 0.5 s (P ?
0.05). With the disabled Smart Bag®MO tidal volume
increased from 221 ? 16 mL to 285 ? 16 mL (P ? 0.05)
and then to 452 ? 30 mL (P ? 0.05). With an adult
bag-valve device, tidal volume increased from 244 ?
32 mL to 337 ? 21 mL (P ? 0.05) and to 515 ? 29 mL
(P ? 0.05) in 0.25, 0.3, and 0.5 s, respectively (Fig. 3, A
and B). (Further data on respiratory mechanics are
available in the Table in the online supplement avail-
able at www.anesthesia-analgesia.org.)
In this bench model, tidal volumes applied with the
enabled Smart Bag®MO were ?200 mL even with an
inspiratory time of 0.5 s, and would mainly result in
dead-space ventilation, leading to hypoventilation,
hypoxia and, in consequence, probably to death.8,9
Thus, while the Smart Bag®MO has the advantage of
preventing stomach inflation with an unprotected
airway, a sufficient inspiratory time of ?0.5 s is
required, which may simply not be available when
unsynchronized chest compressions are performed at
the recommended rate of 100 per minute.
Therefore, we suggest that the enabled Smart Bag®
MO not be used in asynchronous ventilation settings
during CPR after intubation. The solution may be to
switch a Smart Bag®MO to the manual override mode
or even to use a standard adult bag-valve device. This
ensures adequate tidal volumes of about 500 mL if an
inspiratory time of 0.5 s can be secured.
Study limitations include the use of a test lung
model and the fact that only one experienced res-
cuer provided ventilation. However, while test
Figure 1. Smart Bag®MO limits inspiratory gas flow. (A)
Medium manual pressure on the Smart Bag®MO1leaves the
flow limiting element open2and results in a medium
inspiratory gas flow.3(B) Forceful compression of the Smart
Bag®MO1closes the flow-limiting element2and reduces
inspiratory gas flow.3
Figure 2. Experimental bench model. The tested bag-valve
device (BVD) was attached to the air inlet of the test lungs.
The proximal airway pressure, lung tidal volume, and
respiratory mechanics were measured and recorded on a
personal computer (PC). Airway resistance was adjusted
with two modules simulating upper and lower airway
resistance to a total of 5 cm H2O ? mL?1? s?1.
Stomach Inflation Versus Adequate Ventilation
ANESTHESIA & ANALGESIA
lungs cannot exactly replicate respiratory anatomy Download full-text
and physiology of a cardiac or respiratory arrest
patient, they are established to study simulated
In conclusion, ventilation with the disabled Smart
Bag®MO or an adult self-inflating bag-valve device
provided only adequate tidal volumes in short venti-
lation windows during simulated CPR if inspiratory
time was 0.5 s. Ventilation with the enabled Smart
Bag®MO, even in ventilation windows of 0.5 s,
provided inadequate tidal volumes during simu-
lated CPR and would result in hypoventilation in a
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Figure 3. Tidal volumes achieved with the different ventilation
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