(Chest. 2001;120:582-588.)
© 2001
American College of Chest Physicians
The Effect of Helium on Ventilator Performance*
Study of Five Ventilators and a Bedside Pitot Tube Spirometer
Arieh Oppenheim-Eden, MD;
Yitzhak Cohen, MD;
Charles Weissman, MD and
Reuven Pizov, MD
*
From the Departments of Anesthesia and Critical Care Medicine (Drs. Oppenheim-Eden and Pizov), Carmel Lady Davis Medical Center, Technion Medical School, Haifa; and Hadassah University Medical Center (Drs. Cohen and Weissman), the Hebrew University, Hadassah School of Medicine, Jerusalem, Israel.
Correspondence to: Arieh Oppenheim-Eden, MD, Department of Anesthesia and Critical Care Medicine, Carmel Medical Center, 7 Michal St, Haifa, 34362 Israel; e-mail: galo{at}netvision.net.il
 |
Abstract
|
|---|
Objective: To assess in vitro the
performance of five mechanical ventilators—Siemens 300 and 900C
(Siemens-Elma; Solna, Sweden), Puritan Bennett 7200 (Nellcor Puritan
Bennett; Pleasanton, CA), Evita 4 (Dragerwerk; Lubeck, Germany), and
Bear 1000 (Bear Medical Systems; Riverside CA)—and a bedside
sidestream spirometer (Datex CS3 Respiratory Module; Datex-Ohmeda;
Helsinki, Finland) during ventilation with helium-oxygen mixtures.
Design: In vitro study.
Setting: ICUs of two university-affiliated hospitals.
Methods and measurements: Each ventilator was
connected to 100% helium through compressed air inlets and
then tested at three to six different tidal volume (VT)
settings using various helium-oxygen concentrations (fraction of
inspired oxygen [FIO2] of 0.2 to 1.0).
FIO2 and VT were measured with the
Datex CS3 spirometer, and VT was validated with a
water-displacement spirometer.
Main results: The
Puritan Bennett 7200 ventilator did not function with helium. With the
other four ventilators, delivered FIO2 was
lower than the set FIO2. For the Siemens 300
and 900C ventilators, this difference could be explained by the lack of
21% oxygen when helium was connected to the air supply port, while for
the other two ventilators, a nonlinear relation was found. The
VT of the Siemens 300 ventilator was independent of helium
concentration, while for the other three ventilators, delivered
VT was greater than the set VT and was
dependent on helium concentration. During ventilation with 80% helium
and 20% oxygen, VT increased to 125% of set
VT for the Siemens 900C ventilator, and more than doubled
for the Evita 4 and Bear 1000 ventilators. Under the same conditions,
the Datex CS3 spirometer underestimated the delivered VT by
about 33%.
Conclusions: At present, no mechanical
ventilator is calibrated for use with helium. This investigation offers
correction factors for four ventilators for ventilation with
helium.
Key Words: artificial ventilation • helium • intensive care • mechanical ventilator • spirometry
|
Introduction
|
|---|
Helium
was introduced for the treatment of upper-airway obstruction and asthma
before World War II; however, its use was later discontinued
because of short supply.1
2
3
In recent years, a renewed
interest in the use of helium in asthmatic patients has arisen, and it
has been shown to be effective both in spontaneously breathing patients
and patients receiving mechanical ventilation who have increased airway
resistance.4
5
6
7
In such patients, the use of helium has
led to improved carbon dioxide removal and reduced peak and mean airway
pressures, as well as to improved oxygenation.
Helium has a density one seventh of that of air and improves
ventilation by facilitating the conversion of turbulent to laminar
flow.8
Although the effect of changing helium
concentration on flow is linear, the use of low helium
concentrations is associated with a reduced clinical
effect.9
Therefore, for helium to have a clinically
significant effect, helium concentrations of > 50% are probably
required, since at lower concentrations the difference in densities
becomes too small to exert a clinically significant effect (Fig 1
).
The use of helium is associated with erroneous and
unpredictable delivery of tidal volume (VT) by various
mechanical ventilators,10
11
and also may lead to
inaccurate delivery of fraction of inspired oxygen
(FIO2), because of the difference in
physical properties between helium and air, and different ventilator
design. In previous studies, helium was delivered either in a
fixed concentration, or when 20% oxygen in helium was connected to the
air inlet of the ventilator.10
11
To date, no mechanical
ventilator has been produced that addresses the problem of delivering
gases or gas mixtures with variable densities.
In the present study, we examined the performance of five
currently used mechanical ventilators while delivering various
VTs at different helium-oxygen concentrations, with special
emphasis on high helium concentrations: Siemens 300 and 900C
(Siemens-Elma, Solna, Sweden), Puritan Bennett 7200 (Nellcor Puritan
Bennett, Pleasanton, CA), Evita 4 (Dragerwerk, Lubeck, Germany), and
Bear 1000 (Bear Medical Systems, Inc., Riverside CA). We also studied
the function of a commercially available Pitot tube sidestream
spirometer (Datex CS3 Respiratory Module; Datex-Ohmeda, Helsinki,
Finland). Unlike previous reports, we used pure helium because of
unavailability, at the time, of helium-oxygen mixture.
|
Materials and Methods
|
|---|
Five mechanical ventilators and Pitot tube sidestream
spirometer were studied, as described above. The Datex CS3
spirometer was calibrated prior to each experiment using
manufacturer-recommended procedures. The ventilatory circuit was
checked for the absence of leaks prior to each test. All tests were
performed at 700 mm Hg barometric pressure, at room temperature.
FIO2 Measurements
FIO2 was measured by
the Datex CS3 spirometer (a rapid-response paramagnetic oxygen
analyzer), and the measurement used in the analysis is termed
delivered FIO2.
VT Measurements
VT measurements were performed using a
density-independent volume monitor that relies on measurement of water
displacement (sidestream spirometry tester, Datex-Ohmeda). At each of
the conditions tested, measurements were made visually by two
independent investigators simultaneously, who were blinded to
FIO2 and gas mixture. Measurements
were made during three breaths cycles at each condition, and the mean
of six measurements was calculated and is termed actual
delivered VT for that specific condition.
Following each change in ventilatory parameters or
FIO2, 15 min were allowed
for equilibration of the system. Inspiratory VT
was recorded from the Datex CS-3 module.
Study Protocol
The performance of each ventilator was examined
separately (Fig 2 ). Helium was connected to the air inlet of each ventilator, and oxygen
was connected to the oxygen inlet. The Siemens 900C ventilator was also
studied while helium was administered through the nitrous oxide inlet.
The Evita 4 ventilator was studied with the flow sensor and leak
compensation turned off, as recommended by the manufacturer. The
ventilator was connected using standard tubing to the spirometry
transducer of the Datex CS3 monitor (including heat and moisture
exchange device), to a standard 90°-angle elbow and through an 8.5-mm
internal-diameter endotracheal tube to the water-displacement
spirometer. The ventilators were tested in volume-control mode.
Preliminary evaluation of the ventilators using pressure-control
ventilation and positive end-expiratory pressure (PEEP) showed that
unlike during volume-control ventilation, no difference between helium
and air was found (data not shown). Each ventilator was studied at
three to six preset VTs that were achieved using at least
two inspiratory flow rates. The helium and oxygen were mixed so that
FIO2 values between 0.21 and 1.0 were
achieved.
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Figure 2.. Schematic drawing of the study apparatus. Helium
at 140 barometric pressure was connected to the air-supply port of each
ventilator through a pressure-reducing valve. The Datex CS3 sidestream
spirometer (SSS) was connected distal to the Y-piece, and through an
elbow connector to an 8.5-mm inner-diameter endotracheal tube that was
attached to the water-displacement spirometer.
|
|
Mathematical Correction of VT Measurement by
the Datex CS3 Spirometer
During rapid, turbulent, inspiratory flow such as
occurs during artificial ventilation, the relation between the driving
pressure and flow velocity and pressure is calculated according to the
Bernoulli theorem8
:
 | (1) |
where 
= density, P = driving pressure, and
U = flow velocity. Rearranging this equation directly calculates flow
velocity:
 | (2) |
Integration of the flow velocity over time, with
knowledge of the cross-section of the resistor, yields VT.
The spirometer is also equipped with a gas analyzer that analyzes
oxygen (using a paramagnetic analyzer) and carbon dioxide (infra-red
spectroscopy). The algorithm assumes that the balance gas is nitrogen,
and the mean density of the gas mixture is calculated and inserted into
equation 2 . The use of helium instead of nitrogen as the balance gas
leads to erroneous flow and VT calculations because helium
has a lower density than nitrogen. Similar errors were described with
the use of xenon, which has a greater density than
nitrogen12
:
 | (3) |
where U' =erroneous flow calculated during helium
ventilation. To calculate the true flow velocity (Utrue), the density
of the gas mixture must be known:
 | (4) |
Dividing and rearranging equation 3
and equation 4 yields:
 | (5) |
As VT is calculated by integration of flow
velocity over time:
 | (6) |
Because both gas mixtures (helium-oxygen
[He:O2] and nitrogen-oxygen [air])
behave as ideal gases under physiologic conditions, their
density is proportional to their mean molecular weight (Fig 1)
.
Therefore, molecular weight (MW) can be substituted for density in
equation 6 , and a correction factor for VT measurement by
the Datex CS3 monitor can be reached:
 | (7) |
Data Analysis
Data analysis was performed as a function of
FIO2 rather than fraction of inspired
helium. The density of the inspired gas was represented by the
FIO2, which is the only measurable
independent variable (fraction of inspired
helium = 1 - FIO2). Best-fit
curve analysis and regression analysis were performed with software
(Excel; Microsoft Corporation; Redmond, WA).
|
Results
|
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During helium ventilation, a total of 84 different
conditions were studied using the Siemens 900C ventilator, 18 with the
Siemens 300 ventilator, 45 with the Evita 4 ventilator, and 25 with the
Bear 1000 ventilator. Five measurements were made using the Puritan
Bennett 7200 ventilator; however, VT output of this
ventilator was very low, and thus the machine was excluded from further
study since it is unsuitable for ventilation with helium. Both
delivered FIO2 and actual
VT were also measured during air ventilation, and all five
ventilators plus the Datex CS3 spirometer were in agreement with
FIO2 and VT settings as
well as water-displacement volume measurements during ventilation
with air (data not shown).
Effect of Helium on Delivered FIO2
FIO2 measurements from
the Datex CS3 spirometer and from the oxygen analyzers of each of the
ventilators equipped with oxygen analyzers were in agreement (data not
shown). Measured FIO2 during helium
ventilation with the Siemens 300 and 900C ventilators (helium
administered through the air inlet) deviated from the
FIO2 set on the ventilator in a
linear fashion (Fig 3
, top left, a, and top right,
b; Table 1
). During the study of the Evita 4 and Bear 1000 ventilators, the
relationship between the set and delivered
FIO2 was not linear, but
was described by a power function (Fig 3
, bottom left,
c, and bottom right, d; Table 1 ). The
relation between set and delivered
FIO2 was also
nonlinear during helium administration through the
N2O port of the Siemens 900C (Fig 3
, top
left, a; Table 1
); however, in this instance, the
relationship was best described by an exponential equation.
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Figure 3.. Delivered FIO2 as a
function of set FIO2 during ventilation with
helium and oxygen for four mechanical ventilators at all study
conditions. Top left, a: Siemens 900C
ventilator (triangles = helium connected to the N2O
inlet; squares = helium to the air inlet). Top right,
b: Siemens 300 ventilator. Bottom left,
c: Evita 4 ventilator. Bottom right,
d: Bear 1000 ventilator. Mathematical description and
r2 values appear in Table 1
.
|
|
Effect of Helium Ventilation on VT Delivery
VT was unaffected by helium during
ventilation with the Siemens 300 ventilator (Fig 4
, top right, b; Table 1
) and actual
VT did not deviate by > 5% from set
VT. However, with the Siemens 900C, Evita 4, and
Bear 1000 ventilators, actual VT was affected by
the helium concentration in a nonlinear, dose-dependent fashion (Fig 4
,
top left, a; top right, b; bottom right, d; Table 1
). With
the Siemens 900C ventilator, alteration in actual
VT was independent of the inlet through which
helium was administered and exceeded the set VT
by about 25% when 80% helium was used. However, with the Evita 4 and
Bear 1000 ventilators, actual VT exceeded the set
VT by almost 100% when 80% helium was
delivered. VT was unaffected by the inspiratory
flow rate (data not shown).
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Figure 4.. True VT as monitored by
water-displacement spirometry (VTactual), as a function of
delivered FIO2 during ventilation with helium.
Each line represents a single VT setting. The set
VT equaled the actual VT measured at
FIO2 of 1.0. Top left, a:
Siemens 900C ventilator. Top right, b: Siemens 300
ventilator. Bottom left, c: Evita 4 ventilator.
Bottom right, d: Bear 1000 ventilator. Mathematical
description and r2 values appear in Table 1
.
|
|
The Performance of a Pitot Tube Spirometer During Helium
Ventilation
The function of the Datex CS3 respiratory monitoring
module was assessed during helium ventilation. At each condition,
FIO2 as well as the inspired and
expired VT measurements were taken directly from the Datex
CS3 monitor. The inspired VT measured by the Datex SC3
monitor was compared to actual VT. The actual
VT/inspired VT ratio correlated inversely in a
nonlinear way to FIO2 during
ventilation with helium (Fig 5
), and reached a value of 1.5 during ventilation with 80% helium.
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Figure 5.. The ratio of VT monitored by
water-displacement spirometry (Vt-actual) and inspired VT
(Vti) monitored by the Datex CS3 spirometer as a function of delivered
FIO2 during helium ventilation
(y = 0.96x- 0.3;
r2 = 0.96).
|
|
True VT, which was calculated from inspired VT
according to equation 7
, using relative density from Figure 1
,
correlated to actual VT (true
VT = 1.06 x actual VT + 7.6;
r2 = 0.986; Fig 6
).
|
Discussion
|
|---|
The present study demonstrates that the function of
different respiratory equipment (both ventilators and monitors) is
affected by the use of helium. Mathematical correction factors were
developed for the equipment studied, and these as well as the graphic
results can be used to guide ventilatory therapy with helium when a
pure helium source is used.
Changes in Gas-Flow Dynamics and Their Effect on the
Ventilator
Because of its much-reduced density (about one seventh of
that of air), helium may alter gas flow dynamics in ventilator tubing
and blenders. Gas flow is generally either laminar or turbulent, and
gas density is one of the variables that determines flow pattern.
Laminar flow predominates when gas with low density flows through a
straight, wide, smooth tube, at slow rates. During laminar flow, flow
velocity is directly proportional to the pressure gradient, and is
independent of gas density as shown by the Hagen-Poiseuille low:
 | (8) |
where P = driving pressure, U = flow
velocity, 
= gas viscosity, and l and r = length and radius of
the tube, respectively. The transition from laminar to turbulent flow
occurs when a critical flow velocity for the specific system is
reached, as defined by Reynolds number:
 | (9) |
where Re = Reynolds number, and
= density.
For example, gas flow through a smooth, straight tube
will be laminar if the Reynolds number is < 2,000 and will be
turbulent if it is > 4,000.8
11
Once the Reynolds number
exceeds the critical limit for the system and turbulent flow
ensues, the physical laws governing flow velocity can be derived from
the Bernoulli principle (equation 1)
. The inspiratory-flow regulators
and valves, which control the delivery of a preset
FIO2 and VT, function
under turbulent flow conditions because of their physical
design.11
As can be seen from the Bernoulli equation
(equation 3)
, under these conditions flow becomes density dependent.
Therefore, the use of helium instead of either air or
N2O can be expected to erroneously elevate helium
flow, thereby reducing FIO2 and
increasing VT. However, monitoring equipment, which relies
on the flow properties of the gas to measure VT, will read
falsely reduced VT during helium ventilation.
Helium differs from air not only in density, but also in
viscosity and thermal qualities. The viscosity of helium is about 10%
higher than that of air, but the viscosity of gas mixtures is difficult
to predict and does not always follow a linear pattern. The specific
heat and thermal conductivity of helium are much higher than those of
nitrogen and oxygen. Therefore, instruments that rely on the thermal
properties of the gas to measure flow are liable to produce grossly
inaccurate measurements. The flow regulator of the Puritan-Bennett 7200
ventilator is controlled by heated-wire flowmeters and therefore
perceives the helium flow to be much higher than actual. This causes
the gas flow delivered to the ventilator to be reduced, leading to
small VTs. Additionally,
FIO2 is increased. This has been
demonstrated previously.10
Alternatively, during
ventilation with helium-oxygen mixtures, volume monitors that rely on
heated-wire technology will greatly overestimate VT.
Similar results were shown during xenon ventilation.12
The Effect of Helium on FIO2
Delivered FIO2 deviated
from the set FIO2 for all the
ventilators studied. This is in contradiction to the findings in an
article by Tassaux et al.11
However, this difference can
be explained by the fact that Tassaux et al11
used a
mixture of 78% helium and 22% oxygen delivered to the ventilator air
inlet, while in the present study 100% helium was delivered through
the air inlet. This difference in methodology explains our findings for
the change in FIO2 during the use of
Siemens 300 and 900C ventilators (helium administered through the air
supply inlet), where FIO2 roughly
equaled 1.2 x (set FIO2) - 0.2
(Fig 3
, top left, a, and top right,
b; Table 1
). The use of the N2O port
for the delivery of helium is attractive, because such use would reduce
the risk for inadvertent delivery of hypoxic gas mixture. However, when
helium was thus administered using the Siemens 900C ventilator,
FIO2 behaved in a nonlinear
relation. This was probably due to the high density of
N2O, which requires special compensation for the
varying density of the inspired gas (Fig 3
, top left,
a; Table 1
). During helium ventilation with both the Bear
1000 and Evita 4 ventilators,
FIO2 was nonlinear (Fig 3
,
bottom left, c, and bottom right,
d; Table 1
). These results deviate from previous findings,
which were based on the use of a gas mixture of 22% oxygen in 78%
helium introduced into the air inlet. Such a gas mixture has a density
of more than twice that of helium (Fig 1)
. The difference in gas
density flowing through the air-flow regulator may explain why our
results show a more extreme nonlinear relationship between set
FIO2 and delivered
FIO2.11
Effect of Helium Administration on VT
The use of helium, as has been shown previously,
did not influence the VT output of the Siemens 300
ventilator.11
This is probably because the flow-regulating
valve has a built-in compensatory mechanism for changing gas density
due to change in temperature when gas is supplied from a high-pressure
source.11
The VT output of the Siemens 900C
ventilator was dependent in a nonlinear way on the
FIO2; when actual
FIO2 was 20%, VT was
increased by roughly 25%. Tassaux et al11
showed an
increase of 40% in VT under similar conditions. However,
the output of the machine was independent of the gas supply port used
for the delivery of helium (air or N2O).
The Bear 1000 and Evita 4 ventilators also had increased
VT with increasing helium concentration, which
behaved nonlinearly. Unlike the Siemens900 C ventilator,
however, both ventilators had almost doubled their output when 80%
helium was delivered, which agrees with the findings by Tassaux et
al11
concerning the Evita 4 ventilator. For the Evita 4
ventilator, Tassaux et al11
showed a linear relationship
between set and delivered VT at
FIO2 > 50%; however, such
gas mixtures would not be clinically useful. During higher helium
concentrations, they state that a nonlinear relationship was found, but
as no detailed description is available, those results cannot be
compared to the present findings. The difference between delivered and
set VT at various FIO2
levels can be attributed to the different types of the inspiratory
valves, flow regulators, and compensatory mechanisms.11
The present study addressed volume-control ventilation.
Pressure-control ventilation and PEEP were also assessed, but only in a
preliminary way (data not shown). During pressure-control ventilation
using the current lung model, VT is dependent on the weight
of the displaced water column; therefore, the VT would not
differ between helium and air ventilation. For the same reason, there
would be no difference in the effect of PEEP between the two gases.
This is in agreement with previous results.11
Effect of Helium Ventilation on the Performance of a Pitot
Tube Spirometer
During the study, VT was monitored through
the Datex CS3 sidestream spirometer unit. This spirometer uses the
Pitot tube principle. Pressure is monitored continuously before and
after a constriction, which creates resistance to flow and induces
turbulence. Once turbulence ensues, the Bernoulli principle is used to
calculate the VT.3
12
However, during slow
flow (such as during most of the expiration), flow remains laminar and
is therefore calculated according to Poiseuille law, using viscosity
rather than density. The Datex CS3 spirometer resorts to tabulated
results during laminar flow3
; therefore, we decided to
analyze the inspired rather than the expired VT of the
Datex CS3 spirometer as has been done by others.12
Our
results show that with simple mathematical correction, the Datex CS3
spirometer can be used to monitor controlled ventilation with
helium-oxygen mixtures (Fig 6)
.
Potential Hazards During the Use of Helium for Mechanical
Ventilation
Although helium ventilation may prove beneficial to
patients with small-airway obstruction, its uncontrolled use may be
hazardous as ours and previous studies suggest. Helium
ventilation leads to severe hypoventilation by some ventilators, while
other ventilators may deliver excessively large VTs. The
use of helium may lead to erroneous
FIO2 delivery with the risk of
delivering hypoxic gas mixtures. Using heliox rather than pure helium
instead of air would be safer, as it reduces this risk. However, the
most important factor in the use of helium, or otherwise tampering with
ventilatory equipment, is vigilant monitoring. It is essential to
monitor not only the saturation and arterial blood gas levels of the
patient, but also to monitor continuously the
FIO2. While no bedside volume monitor
has been shown to function properly with helium, the Datex CS3
sidestream spirometer can be used with appropriate mathematical
conversion according to the true density of the delivered gas mixture.
|
Summary
|
|---|
At present, no mechanical ventilator is calibrated for
the use of helium. Although some mechanical ventilators perform in a
predictable fashion during helium ventilation, the uncontrolled use of
helium could be hazardous for the patient. The Puritan-Bennett 7200
ventilator does not function during helium ventilation, while the
Siemens 300 ventilator is the most accurate machine presently available
for ventilation with helium. The Datex CS3 spirometer cannot be used to
monitor VT during helium ventilation, unless mathematical
corrections for gas density are applied. However, when the clinical
situation calls for ventilation with helium, the ventilators studied
can be used according to either the mathematical or graphic
corrections. The true VT delivered to the patient can also
be calculated from that measured by the Datex CS3 spirometer. Further
studies are required to validate these findings under clinical
conditions.
|
Acknowledgements
|
|---|
We thank Rina Ankori and Alex Tuchband for
technical assistance.
|
Footnotes
|
|---|
Abbreviations:
FIO2 = fraction of inspired oxygen;
PEEP = positive end-expiratory pressure; VT = tidal
volume
This work was presented in part in the European Society of Anaesthesia
7th annual meeting, May 29–June 1, 1999, Amsterdam, The
Netherlands.
Received for publication May 25, 2000.
Accepted for publication February 23, 2001.
|
References
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Gluck, E, Onorato, D, Castriotta, R (1990) Helium-oxygen mixtures in intubated patients with status asthmaticus and respiratory acidosis. Chest 98,693-698[Abstract/Free Full Text]
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Houck, JR, Keamy, MF, III, McDonough, JM (1990) Effect of helium concentration on experimental upper airway obstruction. Ann Otol Rhinol Laryngol 99,556-561[Medline]
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McArthur, C, Adams, A, Suzuki, S (1996) Effects of helium/oxygen mixtures on delivered and died tidal volume during mechanical ventilation [abstract]. Am J Respir Crit Care Med 153,A370
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TOP
Abstract
Introduction
