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ORIGINAL ARTICLE
Adjustable breathing resistance for laryngectomized
patients: Proof of principle in a novel heat and moisture
exchanger cassette
Maartje Leemans MSc
1
| Sara H. Muller PhD
1,2
|
Maarten J. A. van Alphen PhD
1
| Wim Vallenduuk MSc
1
|
Richard Dirven MD, PhD
1
| Michiel W. M. van den Brekel MD, PhD
1,3,4
1
Department of Head and Neck
Oncology and Surgery, The Netherlands
Cancer Institute –Antoni van
Leeuwenhoek, Amsterdam, The
Netherlands
2
Department of Clinical Physics and
Instrumentation, The Netherlands Cancer
Institute –Antoni van Leeuwenhoek,
Amsterdam, The Netherlands
3
Institute of Phonetic Sciences, University
of Amsterdam, Amsterdam, The
Netherlands
4
Department of Oral and Maxillofacial
Surgery, Amsterdam University Medical
Center (AUMC), Amsterdam, The
Netherlands
Correspondence
Prof. Dr. Michiel van den Brekel,
Department of Head and Neck Oncology
and Surgery, The Netherlands Cancer
Institute –Antoni van Leeuwenhoek,
Plesmanlaan 121, Amsterdam 1066 CX,
The Netherlands.
Email: m.w.m.vandenbrekel@uva.nl
Funding information
Atos Medical AB (Malmö, Sweden)
Section Editor: Katherine Hutcheson
Abstract
Background: Due to the heat and moisture exchanger's (HME) breathing
resistance, laryngectomized patients cannot always use an (optimal) HME
during physical exercise. We propose a novel HME cassette concept with
adjustable “bypass,”to provide adjustment between different breathing
resistances within one device.
Methods: Under standardized conditions, the resistance and humidification
performance of a high resistance/high humidification HME (XM) foam in a
cassette with and without bypass were compared to a lower resistance/lesser
humidification HME (XF) foam in a closed cassette.
Results: With a bypass in the cassette, the resistance and humidification perfor-
manceofXMfoamweresimilartothoseofXF foam in the closed cassette. Com-
pared to XM foam in the closed cassette, introducing the bypass resulted in a 40%
resistance decrease, whereas humidification performance was maintained at 80% of
the original value.
Conclusions: This HME cassette prototype allows adjustment between sub-
stantially different resistances while maintaining appropriate humidification
performances.
KEYWORDS
breathing resistance, heat and moisture exchanger, HME cassette, pulmonary rehabilitation,
total laryngectomy
1|INTRODUCTION
Heat and moisture exchangers (HMEs) are used as a
standard treatment for pulmonary rehabilitation after
atotallaryngectomy.
1-5
Normally, the upper airways
condition (heat and humify) the inhaled air, but in lar-
yngectomized patients the lungs are exposed to the dry
and cold air during open stoma breathing. An HME
coveringthestomacantosomeextentimprovethepul-
monary condition. The benefits of HME use have been
Received: 15 May 2020 Revised: 27 October 2020 Accepted: 20 November 2020
DOI: 10.1002/hed.26571
This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided
the original work is properly cited.
© 2020 The Authors. Head & Neck published by Wiley Periodicals LLC.
Head & Neck. 2021;43:1073–1087. wileyonlinelibrary.com/journal/hed 1073
underlined in many studies; it does not only improve
the pulmonary functioning, such as a decrease in
mucus production, coughing, and forced expectora-
tions, but also the psychosocial functioning of laryn-
gectomized patients.
1-4,6-8
Laryngectomized patients
are recommended to continuously use an HME with
the highest possible humidification performance (the
highest water exchange).
9,10
The humidification performance of the HME, and
thus its benefits, rely mainly on the HME core material
and cassette design. The HME core material often con-
sists of a porous polymer foam impregnated with hygro-
scopic salt, which acts as a condensation and evaporation
surface.
11-13
Since the HME is a passive humidifier, its
humidification performance can primarily be improved
by increasing the width and height of the core material
or decreasing the foam's pore size. Increase of width and
height are limited by aesthetic considerations. Addition-
ally, these performance improvements have a trade off
with the HME's breathing resistance and consequently
patient acceptance. To cater to the different patient needs
and activity levels, multiple types of HMEs have been
developed, which vary in resistance and performance.
9,14
Nevertheless, complete HME compliance has not yet
been achieved in all laryngectomized patients. Laryngecto-
mized patients discontinue their (high humidification per-
formance) HME use due to the higher breathing
resistance of the HME compared to open stoma breathing,
especially periodically during physical activities.
1,10,15-19
Other reasons for laryngectomized patients to discontinue
their HME use, outside the scope of this study, include:
adhesive related skin irritation, mucus problems or the
HME's aesthetics.
1,9,14,15,17,19
Although physical exercise
can sometimes be anticipated, changing between different
HME types with varying breathing resistance is not always
an option or requires additional effort and preparation.
1,20
As a result, some patients do not use any HME at all.
Patient compliance and comfort during different
levels of physical activities could potentially be improved
by providing one HME device that enables a quick and
simple adjustment of the breathing resistance based on
the patient's activity level. During rest, a laryngectomized
patient can use the HME device with a higher resistance
and humidification performance setting. Alternatively,
during physical activities the HME device can be adjusted
to decrease its resistance, while maintaining an appropri-
ate humidification performance.
We propose a novel HME cassette concept with an
adjustable “bypass”at its base. In this study, we designed
and tested this adjustable HME cassette prototype to vali-
date that it will result in substantially different breathing
resistances with appropriate humidification perfor-
mances for each level of activity.
2|MATERIALS AND METHODS
2.1 |HME devices and prototype
In this study, we used two types of HME foams taken
from the two most commonly used HMEs at the Nether-
lands Cancer Institute –Antoni van Leeuwenhoek: the
Provox
®
XtraMoist
TM
HME (XM) and the Provox
®
XtraFlow
TM
HME (XF, both Atos Medical AB, Malmö,
Sweden). An overview of the specifications of the Pres-
sure Drop and Moisture Loss, and of the measurements
of the Water Exchange of the XM and XF are given in
Table 1. Water Exchange is a direct measure of the
humidification performance.
22
The XM is one of the
highestperformingcommonlyusedHMEs.
14
The XF is
considered to be an HME with an “acceptable”breath-
ing resistance by the majority of the laryngectomized
patients, unable to (continuously) tolerate the higher
breathing resistance of the XM.
1,10
However, the XF has
a lesser humidification performance compared to the
XM. The HME cassettes of the XM and XF are identical:
the differences in breathing resistance and performance
areduetothedifferenceincorematerial(Figure1).
In this study, we use the pressure drop as a measure for
resistance (in Appendix A, the mathematical relationship
between pressure drop, flow and resistance can be found).
Water Exchange, the amount of water an HME evaporates
during inhalation and condensates during exhalation, is
used as a measure of humidification performance.
14
The high breathing resistance of an HME can be
reduced by introducing a relatively simple “bypass”in
the HME cassette, or a simple hole in the HME foam (see
Appendix A). A bypass functions as a “shortcut”for the
airflow and will therefore decrease both resistance and
humidification performance. Due to the almost quadratic
relationship between flow and resistance (Appendix A), a
bypass reduces the HME's breathing resistance consider-
ably more than its humidification performance.
Abypassshouldbedesignedwhichcaneasilybe
opened or closed and does not interfere with the HME's
speaking valve. Additionally, it is desirable that this specific
bypass can modify an XM-like HME into an HME with the
properties comparable to an XF. Therefore, the following
3D-printed (FormLabs, Form2) HME cassette designs were
used as a prototype in this study: two simplified closed
straight cylindrical cassettes without a speaking valve,
Figure 2a,b (further on called the “closed cassette”-type),
and a similar cassette with an opened bypass at its tracheal
side, Figure 2c (further on called the “cassette with bypass”-
type). The bypass consists of eight holes with a diameter of
4 mm, distributed evenly around the cassette's base, which
can quickly and easily be opened or closed by adjusting a
“twist-ring”(compare Figure 2b and 2c, similar to the
1074 LEEMANS ET AL.
“twist-ring”-concept as seen on salt shakers, Figure 2d).
This specific bypass configuration was chosen such that the
resistance of the XM foam, when the bypass is opened,
drops to the breathing resistance similar to the breathing
resistance of an XF foam in the closed cassette. The dimen-
sions of the cassettes were chosen such that the cassettes
closely fitted the HME foams.
2.2 |Equipment
The pressure drop (a measure of the HME's breathing
resistance) of the HME devices was assessed with a digi-
tal pressure indicator (DPI 705, BHGE Druck, Houston,
Texas) at different airflow rates of 30, 60, and 90 L/min
in correspondence with the ISO standards (see Table 1),
representing approximately breathing at rest and during
light and strenuous exercise.
9
Performance measurements, measuring the HME's
Water Exchange, were executed as validated by van den
Boer et al. (2013 and 2014).
14,22
The measurement proto-
col was slightly adapted to fit the objectives of this study
(see Study design). Summarizing, a healthy volunteer
breathes through a spirometer set-up with a standardized
breathing pattern, with the HME device connected to a
coupler on the other side of the spirometer (Flowhead
MLT300 AD Instruments GmbH, Oxfordshire, United
Kingdom). First, the HME is conditioned toward its
TABLE 1 Specifications of the Moisture Loss and Pressure Drop values of the Provox XtraMoist (XM) and Provox XtraFlow (XF), as
provided by the manufacturer (Atos Medical AB, Malmö, Sweden) in accordance with ISO 9360-2:2001,
21
and the humidification
performance (Water Exchange) as reported by previous studies.
Pressure Drop (Pa)
Moisture Loss
a
(mg/L)
Water Exchange (mg)
Van den Boer et al. (2014a)
14
Water Exchange (mg)
Van den Boer et al. (2014b)
21
HME
At 30
L/min
At 60
L/min
At 90
L/min
At V
T
=1L
(AH
amb-ref
= 0 mg/L)
At V
T
= 0.5 L
(AH
amb-ref
= 5 mg/L)
At V
T
= 0.5 L
(AH
amb-ref
= 5 mg/L)
Provox
XtraMoist
70 240 480 21.5 3.61 3.63
Provox
XtraFlow
40 130 290 24.0 2.89 1.95
Note: The pressure drop of the XF at a flow of 60 L/min is approximately 60% of that of the XM. The humidification performance (Water Exchange) of the XF
shows relatively less decline: approximately 80% of that of the XM.
Abbreviations: AH
amb-ref
, chosen reference value for ambient humidity; HME, heat and moisture exchanger; ISO, International Organization for
Standardization; V
T
, tidal volume.
a
The lower the moisture loss value, the better the HME's humidification performance.
FIGURE 1 The photo shows, from left to right, the original HME cassette of both the XF and XM with speaking valve (pink lid), the
3D-printed (FormLabs, Form2) closed cassette with inserted XF foam and the 3D-printed (FormLabs, Form2) cassette with bypass on the
tracheal side, with inserted XM foam (note the difference in pore size between the two different foams). A speaking valve was not included
in the 3D printed cassette designs to simplify the prototyping and to limit the scope of this proof of principle study to only the effect of the
bypass. The thicker cylinder at the base of the 3D-printed cassettes is used to connect them to the measurement set-up (spirometer). HME,
heat and moisture exchanger; XF, lower resistance/lesser humidification HME; XM, high resistance/high humidification HME [Color figure
can be viewed at wileyonlinelibrary.com]
LEEMANS ET AL.1075
equilibrium water saturation (duration of conditioning is
determined separately for each HME). After this initial
conditioning, a sequence of weight measurements is con-
ducted, alternating at the end of an inhalation and the
end of an exhalation, to determine the HME's Water
Exchange. The weight changes of the HME device are
measured using a microbalance (Sartorius MC210p,
Göttingen, Germany). The HME foam is reconditioned
for at least five breathing cycles between each weight
measurement. During the measurement sequence, the
ambient humidity and temperature of the room are
recorded by a commercial humidity sensor (Testo BV,
Almere, The Netherlands) to perform data normalization.
At the start and end of each measurement sequence, the
ambient humidity and temperature of the room is addi-
tionally monitored with a hygrometer (Philips Thermo +
Hygro, Eindhoven, The Netherlands) and digital ther-
mometer (ThermaLite Digital, E.T.I. Ltd., Worthings,
UK) and the temperature of the volunteer is measured
with an electronic ear thermometer (Braun WelchAllyn,
Kaz Inc., Marlborough, Massachusetts). In this set-up the
volunteer functions as an “artificial lung”. The tempera-
ture of the volunteer is used for normalization (see Anal-
ysis). The volunteer was asked to breath in a fixed
rectangular breathing pattern, which is guarded by the
spirometer.
2.3 |Study design
For this study, resistance (Pressure Drop) and humidifica-
tion performance (Water Exchange) measurements were
conducted for 10 XM foams (one batch, batch year: 2019)
and 15 XF foams (three batches, batch years: 2017, 2018,
and 2019) inside the two different cassette types: both the
XF and XM foams in the closed cassette and the XM foams
in the cassette with the bypass (Figure 2). All performance
measurements were performed by one healthy volunteer
(female, 27 years old, ML) for one breathing pattern under
room climate conditions. A tidal volume (V
T
)of1Landtar-
get flow of 0.33 L/s was chosen, which was a comfortable
breathing pattern for the volunteer and corresponds to the
ISO standards (see Table 1). After initial conditioning of the
HME foam, a sequence of 15 weight measurements was
conducted (starting and ending with an exhalation). This
resulted in 13 weight changes per HME since the first mea-
surement was disregarded to account for differences in con-
ditioning periods between the HME devices.
2.4 |Analysis
All performance measurements were normalized to the
reference ambient humidity of 5 mg/L and a reference
humidity at the tracheal side of 32 mg/L (see Appendix
B).
22
An independent sample ttest was conducted using
IBM SPSS Statistics 25 (SPSS, Chicago, IL) to compare
the average performances of the different HME devices.
3|RESULTS
An overview of the average resistance (Pressure Drop)
and the humidification performance (Water Exchange) of
all XF and XM foams in the two different HME cassette
types are shown in Table 2 and Figure 3.
FIGURE 2 The two HME cassette types. A, Design of the closed cassette for the XF foam measurements. B, Design of the closed
cassette for the XM foam measurements. The bypass on the tracheal side of the cassette is closed off with a “twist-ring.”C, 3D-design of the
cassette with opened bypass for the XM foam measurements. The specific bypass consists of eight d= 4 mm holes at the base of the cassette
and can be opened or closed by adjusting the “twist-ring.”D, “Twist-ring”concept as seen on salt shakers. The bar at the base and the two
small holes at the top of the cassettes, intended for inserting a pin, keep the HME foam in place during the measurements. The thicker
cylinder at the base of the 3D-printed cassettes is used to connect them to the measurement set-up (spirometer). HME, heat and moisture
exchanger; XF, lower resistance/lesser humidification HME; XM, high resistance/high humidification HME
1076 LEEMANS ET AL.
In the closed cassette, the average pressure drops and
Water Exchange values of the XM foam are higher than
that of the XF foam. When the bypass was introduced in
the XM foam's cassette, the pressure drop of the XM
foam decreased to a pressure drop similar to the XF
foam in the closed cassette. The average Water
Exchange of the XM foam in the cassette with bypass
was slightly lower than the average Water Exchange of
the XF foam in the closed cassette (not significant,
P> .05). Compared to the XM foam in the closed
cassette, the bypass resulted in pressure drop of approxi-
mately 60% the original pressure drop value, thus a 40%
decrease in resistance, whereas the humidification per-
formance was maintained at approximately 80% of the
original Water Exchange value of the XM foam.
4|DISCUSSION
This proof of principle study shows that introducing a
bypass in the base of an HME cassette can substantially
decrease the resistance of a high resistance/high humidi-
fication HME (XM) foam to the lower breathing resis-
tance of a lower resistance/lesser humidification HME
(XF) foam in the closed cassette, while humidification
performance stays at an acceptable level.
Intuitively, one would expect that creating holes in an
HME cassette (which lets the air bypass the HME's foam)
will decrease the HME's resistance and consequently its
humidification performance to a level where the HME
will become “useless”for the pulmonary rehabilitation of
laryngectomized patients. However, both the theory stat-
ing the (almost) quadratic relationship between pressure
and flow (Appendix A), as the results of this study indi-
cate that a bypass will decrease the resistance much more
than the humidification performance. Additionally, care-
ful examination of existing HMEs shows that the cas-
settes already (coincidentally) have “bypasses”in their
designs and still these HMEs have good Water Exchange
values.
14
For example, the Provox
®
Luna
®
HME (Atos
Medical AB, Malmö, Sweden) clearly has two side open-
ings acting as “bypasses.”
In this proof of principle study, we used a cassette
without speaking valve. However, cassettes without a
speaking valve are nowadays often not acceptable to
patients with a voice prothesis.
15
In Appendix B.4,
Table B2, a comparison is made between the
TABLE 2 Overview of the average resistance (pressure drop) and normalized humidification performance (water exchange) of the XM
and XF foams in the two different cassette types.
HME device Pressure Drop in Pa (SD) Water Exchange in mg (SD)
HME foam type HME cassette type At 30 L/min At 60 L/min At 90 L/min
At V
T
=1L,F= 0.33 L/s,
AH
amb-ref
= 5 mg/L, and AH
ts
= 32 mg/L
XM foam Closed cassette 50 (2) 158 (7) 325 (13) 5.70 (0.42)
Cassette with bypass 29 (1) 95 (5) 201 (11) 4.77 (0.40)
XF foam Closed cassette 26 (1) 93 (3) 196 (4) 4.91 (0.35)
Note: The tidal volume (V
T
) and airflow rates of the pressure drop measurements correspond to the ISO standards (see Table 1). The different airflow rates of
30, 60, and 90 L/min represent approximately breathing at rest and during light and strenuous exercise.
9
The SDs of the Water Exchange measurements of the
HME devices are comparable to those previously reported by van den Boer et al. (2013).
22
For the XF foam, a weighted mean and SD were calculated to
represent the three different batches in equal proportion.
Abbreviations: AH
amb-ref
, reference ambient humidity; AH
ts
, reference humidity at the tracheal side of the HME; F, flow; HME, heat and moisture exchanger;
V
T
, tidal volume; XF, lower resistance/lesser humidification HMESD, standard deviation; XM, high resistance/high humidification HME.
FIGURE 3 Resistance (Pressure Drop at 60 L/min) against
normalized humidificationperformance (Water Exchange at
V
T
= 1 L) of the different HME devices. The horizontal and vertical
error bars indicate the standard deviations from the average
Resistance and Water Exchange, respectively. Abbreviations: HME,
heat and moisture exchanger; XF, lower resistance/lesser
humidification HME; XM, high resistance/high humidification
HME; V
T
, tidal volume [Color figure can be viewed at
wileyonlinelibrary.com]
LEEMANS ET AL.1077
performance measurements found in this study (Table 2),
with the humidification performance values of with the
HMEs with speaking valve found by van den Boer et al.
(2014a, 2014b) and the manufacturer's specifications
(Table 1).
14,23
Additionally, unpublished experiments'
results were included in Table B2, performed in the Neth-
erlands Cancer Institute –Antoni van Leeuwenhoek dur-
ing the past 3 years. The humidification performance
results with and without speaking valve are very similar.
Therefore, we predict that a final prototype with speaking
valve will have a similar clinically acceptable humidifica-
tion performance. The assessment of the user functional-
ity and compliance, important device considerations for a
final prototype with speaking valve, requires the support
of a manufacturer and was outside the scope of this
study. Such a study with laryngectomized patients, in
which the effectiveness of the final prototype is evalu-
ated, is recommended as the next step.
This proof of principle shows that an adjustable HME is
feasible.SuchanHMEwouldhaveseveralimportant
advantages. In the first place, it can be used by the laryngec-
tomized patients to modify the breathing resistance, which
eliminates the need to remove or switch HME types based
on activity level. Even if the novel HME cassette is used
solely on the lowest resistance setting, it still has a clinically
acceptable humidification performance similar to an XF. If
laryngectomized patients are not able or willing to switch
HMEs, an adjustable HME enables a lower breathing resis-
tance during physical activity and an optimal HME with a
higher breathing resistance during nonstrenuous activities.
Furthermore, since clinically acceptable breathing resistance
does not only vary between physical activity levels but also
between laryngectomized patients
1,10
,thisnovelHMEcas-
sette concept could also be employed to gradually train lar-
yngectomized patient to a (higher) HME resistance over
time (eg, by using the “twist-ring”in an intermediate set-
ting). Altogether, this might increase overall HME compli-
ance and pulmonary rehabilitation in laryngectomized
patients.
5|CONCLUSION
By introducing a bypass, this novel HME cassette prototype
allows adjustment between substantially different HME
resistances while maintaining appropriate humidification
performances. The advantage of the specific bypass in the
prototype is that it can easily be opened, closed or adjusted
by the laryngectomized patient. This potentially facilitates
physical exercise without changing or removing the HME
and might therefore increase overall patient compliance.
Currently, this adjustable “bypass”-principle is not yet
available in any commercial HME cassette. We hope that
this prototype will be developed further into an effective
medical device.
ACKNOWLEDGMENTS
We would like to thank Atos Medical AB (Malmö, Sweden)
for providing the HMEs to The Netherlands Cancer Institute
–Antoni van Leeuwenhoek. Also, we would like to thank
the Verwelius 3D lab of The Netherlands Cancer Institute for
making the 3D-printing of the HME cassettes possible. The
Netherlands Cancer Institute's Department of Head and
Neck Oncology and Surgery receives a research grant from
Atos Medical AB (Malmö, Sweden), which contributes to the
existing infrastructure for quality of life research of the
Department of Head and Neck Oncology and Surgery.
CONFLICT OF INTEREST
Atos Medical AB had no role in the concept, study
design, and drafting of this manuscript. The authors have
no other funding, financial relationships, or conflicts of
interest to disclose.
DATA AVAILABILITY STATEMENT
The data that support the findings of this study are avail-
able from the corresponding author upon reasonable
request.
ORCID
Maartje Leemans https://orcid.org/0000-0003-2976-
5368
Michiel W. M. van den Brekel https://orcid.org/0000-
0002-6338-6743
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Hilgers FJ. A new heat and moisture exchanger for laryngecto-
mized patients: endotracheal temperature and humidity. Respir
Care. 2011;56(5):604-611.
21. International Standards Organization. Anaesthetic and respira-
tory equipment—heat and moisture exchangers (HMEs) for
humidifying respired gases in humans. Part 2: HMEs for use
with tracheostomized patients having minimal tidal volume of
250 mL. ISO, Geneva; 2001. pp. 9360-9362.
22. van den Boer C, Muller SH, Vincent AD, Züchner K, van den
Brekel MW, Hilgers FJ. A novel, simplified ex vivo method for mea-
suring water exchange performance of heat and moisture exchangers
for tracheostomy application. Respir Care. 2013;58(9):1449-1458.
23. van den Boer C, Muller SH, Vincent AD, Züchner K, van den
Brekel MWM, Hilgers FJM. Ex vivo water exchange perfor-
mance and short-term clinical feasibility assessment of newly
developed heat and moisture exchangers for pulmonary reha-
bilitation after total laryngectomy. Eur Arch Otorhinolaryngol.
2014;271(2):359-366.
How to cite this article: Leemans M, Muller SH,
van Alphen MJA, Vallenduuk W, Dirven R, van
den Brekel MWM. Adjustable breathing resistance
for laryngectomized patients: Proof of principle in
a novel heat and moisture exchanger cassette.
Head & Neck. 2021;43:1073–1087. https://doi.org/
10.1002/hed.26571
LEEMANS ET AL.1079
APPENDIX A: THEORETICAL RESISTANCE
AND PERFORMANCE OF AN HME WITH
ALEAK
Introduction
In this appendix, we derive the theoretical impact of a
bypass (eg, through the center of an HME, Figure A1) on
resistance and humidification performance (Water
Exchange) of the HME.
For the calculation of resistance, we consider the
HME as the combination of the remaining foam and the
hole. For the calculation of humidification performance
(Water Exchange), we use the remaining foam only.
Derivation of parallel resistance for a power law
relationship between air flow and pressure
difference
Analogous to the derivation of parallel electrical resis-
tance using Ohm's law, we can derive the “parallel”
resistance (R
//
) of the resistance remaining foam of the
HME (R
HME
) and the resistance of the hole (R
Hole
)
(Figure A1).
1
Using a power law relationship with an exponent a,
the following equations apply:
dP =RHME FHMEað1Þ
dP =RHole FHoleað2Þ
F=FHME +FHole ð3Þ
dP =R== Fa:ð4Þ
Nomenclatures
dP: pressure difference over the HME (Pa)
F: combined flow (L/s)
R: resistance (Pa/[m
3
/s]
a
)
1
R
//
: combined resistance of the parallel resistances R
HME
and R
Hole
(Pa/[m
3
/s]
a
)
Verkerke et al. (2001) found a mixture of a linear and
a quadratic relationship and discussed the theoretical
background of the linear and quadratic terms.
2
For the
HMEs used in this study, the linear term is small so that
the pressure data can be fitted with a power law with an
exponent aof about 1.8.
1
From Equation (1) it follows
that for a flow (F) of 1 L/s (60 L/min), the dP (in Pa) is
numerically equal to the resistance.
Combining Equations (1) and (2) yields:
FHMEa=RHole
RHME
FHoleaor FHME =RHole
RHME
1=a
FHole ð5Þ
Combining Equations (3) and (4) yields: R
//
=dP/
F
a
=dP/(F
HME
+F
Hole
)
a
. Using Equation (2) followed by
Equation (5) yields the resistance of the remaining foam
and hole together:
R== =RHole Fa
Hole
FHME +FHole
ðÞ
a=RHole Fa
Hole
FHole RHole
RHME
1=a+FHole
a
=RHole
RHole
RHME
1=a+1
a=RHole RHME
RHole
ðÞ
1=a+RHME
ðÞ
1=a
a
ð6Þ
FIGURE A1 Combining two resistances into one parallel resistance for an HME foam with a hole, a “bypass,”through the center. dP
is the pressure difference over the HME, Fthe total flow (volume/time), F
HME
the flow through the foam, F
Hole
the flow through the hole
[Color figure can be viewed at wileyonlinelibrary.com]
1080 LEEMANS ET AL.
For a= 2, this simplifies to:
R== =RHME RHole
ffiffiffiffiffiffiffiffiffiffiffi
RHME
p+ffiffiffiffiffiffiffiffiffiffi
RHole
p
2
Relation between resistance and cross
sectional area
For a homogeneous air stream (assuming a homogeneous
flow profile) inside a homogeneous HME, the flow (F)is
proportional to the cross sectional area (A). Combining
this with Equations (1) and (2) gives:
RA
1
ðÞ
A1a=RA
2
ðÞ
A2að7Þ
For a= 2, this means that resistance is proportional
to the inverse square of the area, so with the inverse of
the fourth power of the radius of a cylindrical HME. Con-
sequently, resistances decrease very quickly if the HME's
radius is enlarged.
Performance
Performance, defined as the Water Exchange, is in first
order proportional to the volume of air passing through
the HME (F
HME
) and to the mass of remaining foam.
3,4
Combining Equations (1) and (4), we can calculate
the flow through the remaining foam F
HME
(and simi-
larly from Equations (1) and (3) the flow through the
hole, F
Hole
):
FHME =FR==
RHME
1=a
ð8Þ
Example: Calculated impact of a bypass on the
resistance and performance of an HME
FortheexampleweusetheresultsfromTable2.The
high resistance—high performance HME (the XM foam
in the closed cassette, Figure 2) has a Water Exchange
of 5.70 mg and a resistance of 157. The resistance was
determined by fitting the three pressure drop measure-
ments (Table 2) to Equation (1) using a=1.8,whichis
approximately numerically equal to the pressure drop
at 60 L/min. The measured resistances (pressure drops)
of our HME devices (Table 2) are substantially lower
than those of the clinical XM and XF HME device
(Table 1) due to the absence of a speaking valve in our
HME cassette designs. Nonetheless, the relative pres-
sure drop between the XM and XF foam is similar
(XF/XM = 0.54 with speaking valve and 0.59 without
speaking valve).
The XM foam is cylindrical and has a diameter of
21 mm and a height of 11 mm. In this HME foam, we
introduce a bypass, for instance a simple hole with a
diameter of about 4 mm in the middle of the HME foam
(which has approximately the same effect on the HME's
resistance as the type of bypass used in the Main paper).
The area of this hole is 3% of the area of the XM foam.
The remaining 97% of the foam will thus have a resis-
tance of 166 (Equation 7). If we assume a resistance of
about 1000 for this hole (based on pressure drop values
measured over small pipes), we can calculate with
Equation (6) that the HME with bypass will have the
intended resistance of 94 (similar to that of the “low resis-
tance”reference HME, the XF foam in the closed cas-
sette). The resistance has thus decrease to 60% of the
original resistance value. Using Equation (8) and the
knowledge of the first order proportionality between the
HME's performance and the volume of air passing
through the HME and the mass of the remaining foam,
we find that the performance is reduced to approximately
73% of the original performance value, corresponding to
Water Exchange of 4.2 mg Water Exchange.
*
*
We expect the performance of this HME with bypass
(hole through its foam) to be slightly higher than calcu-
lated in this example; during experiments we have made
the observation that Water Exchange increases when
local air speed (not to be confused with total air flow)
decreases, but the order of this secondary effect is to be
further investigated. This flow dependence was not taken
into account in previous studies (van den Boer et al.
[2014a, 2014b]).
3,4
1.Walker IS, Wilson DJ, Sherman MH. A comparison
of the power law to quadratic formulations for air infil-
tration calculations. Energy and Buildings. 1997;27
(3):293-299.
2.Verkerke G, Geertsema A, Schutte H. Airflow Resis-
tance of Airflow-Regulating Devices Described by Inde-
pendent Coefficients. The Annals of otology,rhinology,
and laryngology. 2001;110:639-645.
3.van den Boer C, Muller SH, Vincent AD, van den
Brekel MW, Hilgers FJ. Ex vivo assessment and valida-
tion of water exchange performance of 23 heat and mois-
ture exchangers for laryngectomized patients. Respiratory
Care. 2014; 59(8):1161–1171.
4.van den Boer C, Muller SH, Vincent AD, Züchner K,
van den Brekel MW, Hilgers FJ. A novel, simplified
ex vivo method for measuring water exchange perfor-
mance of heat and moisture exchangers for tracheostomy
application. Respiratory care. 2013;58(9):1449-1458.
LEEMANS ET AL.1081
APPENDIX B: COMPARING WATER
EXCHANGE (WE)ANDMOISTURE
LOSS (ML)
Nomenclatures
AH: Absolute Humidity (mg/L)
HME: Heat and Moisture Exchanger
ML: Moisture Loss (mg/L)
RH: Relative Humidity (%)
V
T
: Tidal volume (L)
WE: Water Exchange (mg)
WEV: Water Exchange per V
T
(mg/L)
Subscripts:
1
: standardized conditions for normalization
2
: conditions during the performance measurement
alv
:“alveolar”
amb
: ambient conditions
HME
: test HME, measured
HMEcalc
: test HME, calculated
ISO
: output tube of the ISO rig
midex
: the position between the ISO rig and test HME
during exhalation
midin
: the position between the ISO rig and test HME
during inhalation
ts
: on the tracheal side of the HME
Even though Water Exchange (WE) and Moisture
Loss (ML) are both a measure of the HME's humidifica-
tion performance (the amount of water recovered by an
HME), comparison between the two performance mea-
sures is complicated. Water Exchange is measured in
vivo, whereas Moisture Loss is measured ex vivo. More-
over, the performance of an HME depends on the AH
amb
and AH
ts
during the measurement, and on V
T
. This
appendix described the steps required for a reliable com-
parison between WE and ML:
•Normalization of WE to standardized AHs: AH
amb
and
AH
ts
(Appendix B.1).
•Conversion between different values of V
T
(Appendix B.2)
•Theory: conversion between WE and ML (Appen-
dix B.3)
•Comparison of WE and ML values for this study
(Appendix B.4)
B.1. Normalization of Water Exchange to
standardized AHs (AH
amb
and AH
ts
)
The performance (defined as Moisture Loss ML or Water
Exchange WE) of an HME depends on the absolute humid-
ities (AHs) on both sides of the HME; on the AH at the tra-
cheal side of the HME (AH
ts
) and on the ambient AH
(AH
amb
). Therefore, the measured WE must be normalized
and converted to standardized conditions to be able to com-
pare measurements under different conditions.
Normalization of the WE data between different
values of AH
ts
and AH
amb
is done using a generalized
equation of van den Boer et al. (2013):
WE @AHts1 and AH amb1 =WE@AHts2 and AHamb2
AHts1 −AH amb1
AHts2 −AH amb2
ð9Þ
B.1.1. Standardized ambient Absolute Humidity
(AH
amb
)
Water Exchange is specified by van den Boer et al. (2013,
2014a, and 2014b, Table 1) at a low but realistic AH
amb
of
5 mg/L, which we also used in this study as the standard-
ized AH
amb
to normalized the HMEs' performance to
(except in Appendix B.3, Table B1).
B.1.2. Standardized Absolute Humidity at the
tracheal side of the HME (AH
ts
)
Using the data from Scheenstra et al. (2010 and 2011)
4,5
measured in laryngectomized patients, we estimate that
for laryngectomized patients the humidity on the tracheal
side of the HME at tracheostoma level (1 cm behind the
HME) is 34 mg/L at 34.5C(RH = 90%, see Figure B1).
†
The HMEs' Water Exchange data in our study and that of
van den Boer et al. (2014a and 2014b) were measured
with volunteers, with the HME connected to the volun-
teer's mouth using a spirometer.
2,3
This means that the
temperature and humidity at the tracheal side of HME at
spirometer level are slightly lower than at tracheostoma
level. Comparing temperature observations made during
volunteer experiments with those made in laryngecto-
mized patients, we estimate that the humidity at the tra-
cheal side of the HME at spirometer level is 32 mg/L at
33.4C(RH = 88%, see Figure B2).
‡
In this volunteer
study, we therefore normalized the HMEs' performance
data to a humidity of 32 mg/L at the tracheal side of the
HME at spirometer level (AH
ts
).
B.1.3. Converting Water Exchange values between
volunteers and patients
Water Exchange values measured in volunteers (specified
at AH
ts
= 32 mg/L and AH
amb
= 5 mg/L) can be
converted to the WE values which would be found in lar-
yngectomized patients (specified at AH
ts
= 34 mg/L and
AH
amb
= 5 mg/L) using Equation (9). These values will
be (34-5)/(32-5) = 7% higher.
1082 LEEMANS ET AL.
TABLE B1 Normalized input and verification data of different HMEs for the determination of the conversion from the normalized
Water Exchange (WE) to Moisture Loss (ML).
All values in mg/L WEV32;0
HME ML
HME
ML
HMEcalc
Ref. for WE
HME
Year HME type @32; 0 mg/L (V
T
= 1 L) @44; 0 mg/L (V
T
= 1 L) @44; 0 mg/L (V
T
=1L)
2014 Hiflow 4.49 24.4 23.9 Van den Boer et al. (2014b)
2
2014 Normal 4.39 23.7 23.9 Van den Boer et al. (2014b)
2
2014 XtraFlow 4.49 24 23.9 Van den Boer et al. (2014b)
2
2014 XtraMoist 7.58 21.5 22.1 Van den Boer et al. (2014b)
2
2014 Hiflow 4.44 24.4 23.9 Van den Boer et al. (2014a)
1
2014 Normal 5.37 23.7 23.4 Van den Boer et al. (2014a)
1
2014 XtraFlow 5.92 24 23.1 Van den Boer et al. (2014a)
1
2014 XtraMoist 7.09 21.5 22.4 Van den Boer et al. (2014a)
1
2016 XtraFlow 5.01 24 23.6
a
2016 XtraMoist 6.79 21.5 22.6
a
2017 XtraFlow 4.41 24 23.9
a
2017 XtraMoist 5.58 21.5 23.3
a
Note: Normalized WEV
HME
(WEV32;0
HME) values at V
T
= 1L were calculated (see Appendix B.1 and B.2) from the values as measured by van den Boer et al.
(2014a, 2014b).
1,2
ML
HME
values were provided by the manufacturer (Atos Medical, Malmö, Sweden) in accordance with ISO 9360-2:2001.
3
ML
HMEcalc
was
calculated from WE
HME
using Equation (21) and WEV44; 0
ISO = 17.8 mg. For, abbreviations, see nomenclature in Append ix B.3.
Note: 1. van den Boer C, Muller SH, Vincent AD, van den Brekel MW, Hilgers FJ. Ex vivo assessment and validation of water exchange performance of 23 heat
and moisture exchangers for laryngectomized patients. Respiratory Care. 2014; 59(8): 1161-1171.
Note: 2. van den Boer C, Muller SH, Vincent AD, Züchner K, van den Brekel MWM, Hilgers FJM. Ex vivo water exchange performance and short-term clinical
feasibility assessment of newly developed heat and moisture exchangers for pulmonary rehabilitation after total laryngectomy. European Archives of Oto-Rhino-
Laryngology. 2014;271(2):359-366.
Note: 3. International Standards Organization. Anesthetic and respiratory equipment—heat and moisture exchangers (HMEs) for humidifying respired gases in
humans. HMEs for use with tracheostomized patients having minimal tidal volume of 250 mL. Geneva: ISO; 9360-2:2001.
a
The observations in 2016 and 2017 were made following the protocol of van den Boer (internal communication).
FIGURE B1 Humidity and
temperature values of the upper and
lower respiratory tract. For
abbreviations, see nomenclature in
Appendix B [Color figure can be viewed
at wileyonlinelibrary.com]
LEEMANS ET AL.1083
B.2. Conversion for tidal volume (V
T
)
The HME's Water Exchange WE strongly depends on the
V
T
, because in the first order the amount of water vapor
that can be condensed or evaporated will be proportional
to the volume of air that goes through the HME. Usually,
the HME's WE is reported at V
T
= 0.5 L, which is compa-
rable to the V
T
of a laryngectomized patient at rest
(Table 1, Main Paper). Manufacturers often specify the
HME's ML only at a V
T
of 1.0 L.
§
When comparing the
HME's WE (mg) and ML (mg/L), we first have to convert
WE to the WE per Tidal Volume:
WEV = WE
VTð10Þ
However, neither WEV nor ML is independent of V
T.
When V
T
increases, WE increases less than linear with
V
T
(see, for example, fig. 2 in Van den Boer et al. 2014a)
3
,
thus WEV decreases with increasing V
T
. The ISO norm
**
also assumes that ML may depend on V
T
. Therefore,
WEV and ML must be compared at the same V
T.
WEV at
V
T
= 0.5 L has been converted to WEV at V
T
=1.0Lusing
the WE data fits as a function of volume (see Van den Boer
et al. [2014a], fig. 2 and Appendix 2). To avoid additional
conversions, we have chosen to perform our Water
Exchange measurements in this study directly at V
T
=1.0L.
B.3. Comparison of in vivo Water Exchange and ex
vivo Moisture Loss
WEV and ML have the same unit (mg/L), but the com-
parison of the HMEs' WEV values and ML values as
specified by the manufacturer (in accordance with ISO
9360-2:2001)
6
is still complicated. WE and WEV values
are measured in vivo in volunteers and laryngectomized
patients. In a human, the trachea is an active (heated and
moisture providing) HME, and thus provides a constant
humidity on the tracheal side of the HME. In contrast,
ML values are measured ex vivo with an artificial lung,
the ISO rig. The “trachea”output tubing in the artificial
lung is a less efficient passive HME. Therefore, the Abso-
lute Humidity (AH) at the tracheal side of the HME is
not constant; it will increase when a higher performance
test-HME is placed in this ISO rig, and vice versa, and
will thus influence the tested HME's performance results.
Using a compartment model of the ISO rig (see
Figure B2, B.3.1 and B.3.2) we can derive the relationship
between ML and WEV (see B.3.4).
Unfortunately, the WEV value of the output tube of
the ISO rig is not specified, so we need an additional step
to determine this value. Using the WE values of different
HMEs for which also the ML values are known, the WEV
of the output tube can be determined (see B.3.3).
B.3.1. Properties of the ISO rig
The ISO rig maintains water at 37C, so the “alveolar”
absolute humidity (AH
alv
) is 44 mg/L. The ambient abso-
lute humidity (AH
amb
) is 0 mg/L. We consider the output
tube of the ISO rig as a (passive) HME
ISO
with Water
Exchange WEV44; 0
ISO if no test HME is present on the ISO
rig (the superscripts denote the humidities on either side
of the output tube). The test HME has a measured
FIGURE B2 Left: Schematic of the ISO rig (“artificial lung”), in accordance with ISO 9360-2:2001,
6
with the test HME placed on the
right hand side. The output tube is considered as a (passive) HME
ISO
. Right: Simplified model of the ISO rig and relations between the
equilibrium WEV
ISO
and WEV
HME
values during inhalation and exhalation of the artificial lung. For abbreviations, see nomenclature in
Appendix B [Color figure can be viewed at wileyonlinelibrary.com]
1084 LEEMANS ET AL.
WEV
HME
and a normalized Water Exchange WEV 32; 0
HME
(see also Appendix B.1). We normalize the WEV
HME
values to an AH
amb
of 0 mg/L in accordance with the ISO
standards (instead of 5 mg/L as used in the clinical
articles
1-3
), to enable an easy conversion between Water
Exchange and Moisture Loss values (see Appendix B.3.4).
When the test HME is mounted on the ISO rig, the
WEV values of the output tube and test HME adapt and a
dynamic equilibrium situation is established. Figure B2,
right image, shows a model of the relations between the
equilibrium WEV
ISO
and WEV
HME
values during inhala-
tion and exhalation of the artificial lung.
B.3.2. Basic equations of the ISO rig model
During inhalation, the test HME increases the Abso-
lute Humidity of the incoming airstream by WEV
HME
.
In ISO conditions, the AH
amb
is equal to 0 mg/L (see
Figure B2), thus:
AHmidin =AH
amb + WEVHME = WEVHME ð11Þ
The Moisture Loss of the test HME is:
ML
HME
=AH
midex
−AH
midin
=AH
midex
−WEV
HME
(see
Figure B2), which can be rewritten to:
WEVHME =AH
midex –MLHME ð12Þ
In the equilibrium situation during exhalation, the output
tube HME
ISO
reduces the Absolute Humidity of the outgoing
airstream of the ISO rig by WEV
ISO
(see Figure B2). Thus
AH
midex
=AH
alv
−WEV
ISO
,andwithAH
alv
=44mg/L:
WEVISO =44–AHmidex ð13Þ
Using the normalization equation for the Water
Exchange from Appendix B.1 (Equation 9), we find:
WEVISO = WEV44;0
ISO 44−WEVHME
ðÞ
44 ð14Þ
WEVHME = WEV32;0
HME AHmidex
32 ð15Þ
B.3.3. Determination of WEV44;0
ISO
Unfortunately, WEV44; 0
ISO is not specified for the ISO test
rig. However, if both the normalized ML
HME
and
WEV32; 0
HME values of a reference HME are known, we can
determine WEV 44; 0
ISO as a function of ML
HME
and WEV32; 0
HME,
by eliminating the unknown variables WEV
ISO
,WEV
HME
,
and AH
midex
from Equations (12) to (15).
Eliminate WEV
ISO
by combining (14) and (15):
44−AHmidex = WEV44; 0
ISO 44−WEVHME
ðÞ
44 ð16Þ
Eliminate WEV
HME
by substituting Equation (15) into
Equation (16):
44 –AHmidex = WEV44;0
ISO
44 –WEV32;0
HME AHmidex=32
=44 ð17Þ
Combining Equations (12) and (15):
AHmidex –MLHME = WEV32;0
HME AHmidex
32
Rewrite :AHmidex =MLHME
1−
WEV32;0
HME
32 ð18Þ
Eliminate AH
midex
by substituting Equation (18) into
Equation (17) and multiplying both sides by (1- WEV 32; 0
HME
/32)*44*32:
44 44 32 –44 WEV32;0
HME –32 MLHME
= WEV44;0
ISO
ð44 32 −WEV32;0
HME –WEV32;0
HME MLHME
Rewriting yields WEV44; 0
ISO as a function of WEV 32; 0
HME
and ML
HME
:
WEV44;0
ISO =44 44 32−44 WEV32; 0
HME −32 MLHME
44 32−44 WEV32; 0
HME −MLHME WEV32;0
HME
ð19Þ
Using the WEV32;0
HME values of different HMEs for
which also the ML
HME
values are known, the WEV44; 0
ISO of
the output tube can be determined.
In order to minimize the effect of measurement errors
in the Water Exchange values,
2,3
we used all available
reference HMEs data for which both the WE32;0
HME and
ML
HME
values are known: the Water Exchange values as
measured by van den Boer et al. (2014a, 2014b,
2,3
normal-
ized to 32 mg/L; 0 mg/L as described in Appendix B.1),
and the Moisture Loss values as specified by the manu-
facturer, respectively. Table B1 (columns “WEV32; 0
HME
”and
“ML
HME
”) gives an overview of the available data we
used. ML values are only specified for V
T
= 1 L so that
WEV44; 0
ISO will only be determined for V
T
=1L.
The WEV32; 0
HME values were converted into ML
HMEcalc
values using Equation (21) (see below). By using a sum of
LEEMANS ET AL.1085
least squares solver over the difference between the cal-
culated ML
HMEcalc
values and the ML
HME
values as speci-
fied by the manufacturer, the optimal WEV44; 0
ISO was
determined.
We find that WEV44; 0
ISO = 17.8 mg/L at V
T
= 1 L for the
ISO rig as used by the manufacturer Atos Medical
(Malmö, Sweden).
B.3.4. Conversion between Water Exchange and
Moisture Loss values
Knowing WEV44; 0
ISO , Equation (19) can be rewritten to cal-
culate the test HME's ML
HMEcalc
from WEV32; 0
HME:
Similarly, Equation (19) can also be rewritten to cal-
culate WEV32; 0
HMEcalc if ML
HME
is known:
WEV32;0
HMEcalc =44 32 WEV44;0
ISO −44−MLHME
44 WEV44;0
ISO + WEV44;0
ISO MLHME −44 44
:
ð21Þ
All values in mg/L and at the same V
T
(at which
WEV44; 0
ISO also must be known).
In this article we use V
T
= 1 L and WEV44; 0
ISO
= 17.8 mg/L for the ISO rig of Atos Medical (Malmö, Swe-
den, see Appendix B.3.3).
B.4. Comparison between Water Exchange and
Moisture Loss values for this study
After normalization to standardized AHs and conversion
to the appropriate V
T
, the HME's WEV values can be
converted into corresponding the ML values, and vice
versa using Equations (20) and (21).
TABLE B2 Comparison of the data measured in this study with the Water Exchange and Moisture Loss values (in accordance with ISO
9360-2:2001)
6
of the HMEs.
All values in
mg/L, at
V
T
=1L
Water Exchange/V
T
,
normalized to 32/5 mg/L
Moisture Loss,
normalized to 44/0 mg/L
This study
Van den
Boer et al.
(2014a)
3
Van den
Boer et al.
(2014b)
2
NKI-AVL
unpublished
2016
NKI-AVL
unpublished
(averaged
over 2016,
2017 and 2018)
This study,
calculated Atos Medical
Cassette foam Narrow fitting,
no speaking
valve
with speaking
valve
with speaking
valve
with speaking
valve
Narrow fitting,
no speaking valve
Narrow fitting,
no speaking
valve
with speaking
valve
XtraMoist 5.70 5.98 6.40 5.73 5.47 22.6 21.5
XtraFlow 4.91 4.99 3.79 4.23 n.a. 23.2 24.0
Note: All Water Exchange data from Van den Boer et al.
2,3
were normalized to AH
amb
= 5 mg/L and AH
ts
=32 mg/L (Appendix B.1) and converted to V
T
=1L
(Appendix B.2). Van den Boer et al. measured in volunteers, so the actual AH
ts
during the measurements was about 32 mg/L. However, they performed their
data normalization with an AH
ts
of 44 mg/L (AH in the alveoli of the lungs). Using the appropriate AH
ts
value, only has a minor impact on the results of van
den Boer et al. (2014a and 2014b); the WE increases with approximately 4% and the rating of HMEs stays the same.]) Moisture Loss data for our HME devices
were determined using Equation (21) (Appendix B.3.4). For comparison with the ML values, the table shows WE/V
T
values (at V
T
= 1 L, numerically equal
to WE).
Abbreviations: NKI-AVL, Netherlands Cancer Institute –Antoni van Leeuwenhoek (Amsterdam, The Netherlands); also see nomenclature in Appendix B.
Note: 1. International Standards Organization. Anesthetic and respiratory equipment—heat and moisture exchangers (HMEs) for humidifying respired gases in
humans. HMEs for use with tracheostomized patients having minimal tidal volume of 250 mL. Geneva: ISO; 9360-2:2001.
Note: 2. van den Boer C, Muller SH, Vincent AD, Züchner K, van den Brekel MWM, Hilgers FJM. Ex vivo water exchange performance and short-term clinical
feasibility assessment of newly developed heat and moisture exchangers for pulmonary rehabilitation after total laryngectomy. European Archives of Oto-Rhino-
Laryngology. 2014;271(2):359-366.
Note: 3. van den Boer C, Muller SH, Vincent AD, van den Brekel MW, Hilgers FJ. Ex vivo assessment and validation of water exchange performance of 23 heat
and moisture exchangers for laryngectomized patients. Respiratory Care. 2014; 59(8): 1161-1171.
MLHMEcalc =44 32 WEV44;0
ISO −32 44−WEV44; 0
ISO WEV32;0
HME +44WEV32;0
HME
WEV44;0
ISO WEV32;0
HME −44 32
:ð20Þ
1086 LEEMANS ET AL.
InTableB2,acomparisonismadebetweentheperfor-
mance measurements found in this study (Main paper,
Table 2), with the performance values of the HMEs found
by van den Boer et al. (2014a, 2014b) and the manufac-
turer's specifications (Main paper, Table 1) for a V
T
=1L.
2,3
Furthermore, unpublished experiments' results were
included in Table B2, performed in the Netherlands Cancer
Institute - Antoni van Leeuwenhoek (NKI-AVL, Amster-
dam, the Netherlands) during the past 3 years. Table B2
shows that overall the performance results, even if variable,
areverycomparableandthedifferenceinperformancewith
and without the speaking valve, if any, is small.
†
The values at the tracheostoma level are valid for
tidal volumes of about 0.5 to 1.0 L and for HMEs with the
typical performance of current HMEs. For HMEs with a
much better performance, the temperature and the abso-
lute humidity at the tracheostoma level will be higher.
The impact of dead space on AH at the end of inspiration
has been neglected.
‡
The actual body temperature of the volunteer (or lar-
yngectomized patient) will also influence the AH
ts
.We
used the measured body temperature of the volunteer
(which was stable within 1C) to normalize AH
ts
to the
value corresponding with a body temperature of 37C.
The measured body temperature of the volunteer is
corrected with a constant (−3.6C = 37-33.4) to estimate
the temperature T
ts2
at spirometer level (see Figure 5).
Based on this T
ts2
, the AH
ts2
is calculated (with a refer-
ence RH
ts
of 88%) and used in Equation (9).
§
Manufactures specifications are performed in accor-
dance with ISO 9360-2:2001
6
. ISO 9360-2:2001 offers the
choice to perform the measurements at three different
tidal volumes (VT = 0.5, 1.0, and 1.5 L).
1. van den Boer C, Muller SH, Vincent AD, Züchner K,
van den Brekel MW, Hilgers FJ. A novel, simplified ex vivo
method for measuring water exchange performance of heat
and moisture exchangers for tracheostomy application.
Respiratory care. 2013;58(9):1449-1458.
2. van den Boer C, Muller SH, Vincent AD,
Züchner K, van den Brekel MWM, Hilgers FJM. ex vivo
water exchange performance and short-term clinical fea-
sibility assessment of newly developed heat and moisture
exchangers for pulmonary rehabilitation after total laryn-
gectomy. European Archives of Oto-Rhino-Laryngology.
2014;271(2):359-366.
3. van den Boer C, Muller SH, Vincent AD, van den
Brekel MW, Hilgers FJ. ex vivo assessment and validation
of water exchange performance of 23 heat and moisture
exchangers for laryngectomized patients. Respiratory
Care. 2014;59(8):1161-1171.
4. Scheenstra RJ, Muller SH, Vincent A,
Sinaasappel M, Hilgers FJ. Influence of breathing resis-
tance of heat and moisture exchangers on tracheal cli-
mate and breathing pattern in laryngectomized
individuals. Head & Neck. 2010;32(8):1069-1078.
5. Scheenstra RJ, Muller SH, Vincent A, Ackerstaff
AH, Jacobi I, Hilgers FJ. A new heat and moisture
exchanger for laryngectomized patients: endotracheal
temperature and humidity. Respiratory care. 2011;56
(5):604-611.
6. International Standards Organization. Anesthetic
and respiratory equipment—heat and moisture
exchangers (HMEs) for humidifying respired gases in
humans. HMEs for use with tracheostomized patients
having minimal tidal volume of 250 mL. Geneva: ISO;
9360-2:2001.
LEEMANS ET AL.1087