PERFORMANCE OF A PORTABLE OXYGEN BREATHING SYSTEM
AT 25000 FEET ALTITUDE

INTRODUCTION

The amount of oxygen relative to the total amount
of atmospheric gases remains constant at approxi-
mately 21.0% to altitudes of approximately 300,000
feet. Ascent to altitude results in a drop of the
atmospheric pressure. Therefore, the partial pressure
of oxygen available to the body decreases. Both cog-
nitive and physical performance deficits are known to
occur as a result of altitude exposures above 14,000
feet (1, 2). To prevent the effects of hypoxia, the
Federal Aviation Regulations require supplemental
oxygen aboard aircraft. Portable systems have been
designed to provide the flight crew with the ability to
move throughout the cabin during conditions in
which oxygen availability has been decreased. These
systems are also used to provide supplemental oxygen
to passengers suffering cardiovascular or respiratory
distress in flight.

Numerous manufacturers build or assemble small,
lightweight, self-contained oxygen breathing systems
for use in the aviation environment. These systems
normally consist ofa small cylinder of oxygen mounted
in a carrying strap for portability. The oxygen is
delivered to the user through a continuous flow
regulator attached to appropriate delivery tubing and
mask assembly. When equipped with an open port
dilution rebreathing type of mask, the systems are
required to maintain tracheal oxygen partial pres-
sures of 100 mmHg when breathing 15 liters/minute
standardized to body temperature, ambient baromet-
ric pressure, and saturated with water vapor (BTPS)
under nonpressurized conditions (3, 4). This type of
portable system is versatile, in that it can be altered to fit
specific operational requirements by varying the volume
of oxygen available, the number of regulator outlets,
connectors, and type of constant flow mask utilized.

Performance requirements for this type of oxygen
system aboard transport category aircraft are covered
under Federal Aviation Regulation (FAR) Part
25.1443, Section a (3). Prior to use aboard an aircraft,
equipment performance must be certified, consistent

with federal regulations. Testing procedures for con-
tinuous flow oxygen systems used in aviation have
been standardized. They are described in Technical
Standard Order (TSO) C103 — Continuous Flow
Oxygen Mask Assembly (For Non-Transport Cat-
egory Aircraft) (5) that references Society of Automo-
tive Engineers Aerospace Standard (AS) 1224A —
Continuous Flow Aviation Oxygen Masks (For Non-
Transport Category Aircraft) (4). If the manufacture
orassembly of a piece of equipment or system has the
potential to alter performance, additional certifica-
tion tests may be required before the equipment is
approved for use aboard aircraft. Alterations in one
manufacturer's (O, Corporation, Wichita, KS) pro-
duction of delivery tubing and rebreathing apparatus
utilized in a portable oxygen system resulted in the
FAA Wichita Aircraft Certification Office (ACO)
requesting additional certification test support from
the hypobaric facilities within the Protection and
Survival Laboratory at the FAA Civil Aeromedical
Institute (CAMI). The purpose of the tests was to
identify performance capabilities of the modified
portable oxygen system in the context of existing
standards. The project was conducted as a compo-
nent of the FAA Aeromedical Research program.

METHODS

Test Subjects: Test subjects were drawn from
recruits that had previously participated, or expressed
interest, in altitude research studies at CAMI. Sub-
jects were recruited and paid through a contract
agreement with Aero Tech Service Associates (Okla-
homa City, OK). Prior to participation, each subject
completed a familial medical history and cardiovas-
cular risk assessment questionnaire. The subjects
were required to pass the functional equivalent of a
Class III physical exam at the CAMI clinic. A total of
22 subjects participated in the testing. The group
consisted of 11 males and 11 females between the
ages of 18 and 30. No specific ethnic background or
prior specialized training was required. Pregnancy


minute was divided into all subsequent time averages
to normalize the transcutaneous data. The results are
presented in Figures 6 and 7. The PtcO, data did
track PetO, in form during the hypoxia baseline tests.
PtcO, increased during the simulated altitude expo-
sure in the hypobaric chamber. This response was
particularly pronounced during the descent from
altitude period of the test and is consistent with the
high PetO, observed coupled with an ever-increasing
atmospheric pressure associated with descent from
altitude. Even though the pattern is consistent with

2.0

the PetO, data, the high individual to individual
variability in this measurement is clearly demon-
strated by the size of the error bars in figure 6.

PtcCO, demonstrated the same pattern as the
PtcO, data, in that the PtcCO, response generally
followed PetCO, data during the baseline tests and
was highly variable during altitude testing. PtcCO,
did tend to be decreased during the time spent at
25,000 feet. This is consistent with the increased
PetCO, observed (Figure 5).

gost 4 4Aijppyl

00 -

Normalized Transcutaneous

2.0

a
1
|
|
|
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|
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|
|
|
|
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e
Ps
+

Partial Pressure of Oxygen (mmHg)
6
t
|
aad
I
|
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|
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A Male Baseline Data
V Male Altirude Data

0) |

‘Time (minutes)

0 4 8 12 16

‘Time (minutes)

Figure 6. PtcO, data collected during the test periods. The points and bars represent the

average, + 1 standard deviation.

1.50 1.50
i
i A Female Baseline Data 4 ‘A. Male Baseline Data
1.25 7- y Female Abinude Daa [~~ ue [Male Altitude Data ——[—— ~~~
i iE
§ 1.00 te > > & Se ae ee Tie Ebi I. LJ [4 + _Y. f+
‘7 ‘6
iT LUG I
z E075 +--+ 4-+-—-+-+- +--+ 4-+ 4---+-- 5 bors PP 4-4 |
i 3
al 8 B
050 0.50
0 4 8 12 16 0 4 8 12 16
‘Time (minutes) Time (minutes)

Figure 7. PtcCO, data collected during the test periods. The points and bars represent the

average, + 1 standard deviation.


The subject was monitored for 5 minutes at altitude.
Atthe end of 5 minutes, descent commenced at a rate
of 2,500 feet per minute until ground level was
reached.

RESULTS

None of the 25K altitude exposures had to be
terminated prematurely due to a decrease in SaO,,
which would indicate the presence of hypoxia in a
subject. All monitored variables remained within the
expected range during altitude exposures. The data
are summarized in Figures 1-7. Each figure contains
two panels. Panel A contains the female subject data
collected from the tests; panel B represents the male
subject data. Data from both altitude exposure and
ground level hypoxia are presented on each panel.

Summary graphs of SaO, levels, grouped by female
and male responses, are presented in Figure 1. The
data points in this, and other figures, represent minute
averages + 1 standard deviation. Due to adaptive
responses resulting from physiological control sys-
tems and delays associated with equipment response
times, the most representative comparisons can be
made between minutes 2 and 5 of the 25K altitude
exposure using the portable oxygen system and the
last 2 minutes of the hypoxic conditions created
using mixed gases. As expected, exposure to the
mixed gases resulted in a decrease in SaO,. The

14.4% oxygen mixture resulted in an average SaO, of
93.5% in both females and males between minutes 2
and 4 of exposure. The 13.1% oxygen gas mixture
resulted in an average SaO, of 90.0% for females and
89.7% for males between minutes 2 and 4. Exposure
to a simulated altitude of 25,000 feet while using the
portable oxygen system resulted in SaO, levels of
98.5% and 97.7% for females and males, respec-
tively, between minutes 2 and 5 at altitude. These
SaO, levels are consistent with values expected in
healthy humans at or near sea level. The male subjects
SaO, responses appeared more variable than the fe-
males for all conditions tested.

Heart rate responses were consistent with what one
would expect, given the experimental protocol used
in the tests (Figure 2). At rest, the average female
heart rate was slightly higher than the males. Both
groups’ heart rate increased with the hypoxic stimuli.
The increased heart rate observed during altitude
exposure is indicative of the low-level exercise task
the subjects were required to perform. This response
may have been somewhat attenuated by the high
oxygen levels while breathing from the portable system.

Ventilatory frequencies during the tests are pre-
sented in Figure 3. During the baseline hypoxia
exposures, the trend was for the females to have a
slightly higher ventilatory rate than the males. This
pattern was not as clear during altitude exposure.
Both females and males demonstrated a slight increase

“Tittiy, ,yry Trrvyy*
im see: e tfttrry
i 95 +--———— diy To : 95 Tt pene ty nl -
Joop pte Llp pet life
: A Female Baseline Data i & Male Baseline Data
i 85 - w Female Altinde Data “PTT 3 85 Male Alinde Data ~~~]

Time (minutes)

‘Time (minutes)

Figure 1. Subject SaO, responses during the different test scenarios. The data clearly
indicate that Sa O2 levels were maintained during altitude exposure using the portable
oxygen system. Panel A — Females. Panel B — Males. The points and bars represent the

average, + 1 standard deviation.


was a disqualifying characteristic for the female par-
ticipants. Recruits were fully informed of the details
of the study, including risks inherent to altitude
exposure, before giving informed consent for partici-
pation in the testing. Subjects were free to withdraw
from participation in the testing at any time without
penalty. All test protocols and consent forms were
reviewed and approved by the CAMI Institutional
Review Board (IRB) for human subject use.

Oxygen Equipment: The portable oxygen equip-
ment tested consisted of the delivery tubing,
rebreathing apparatus, and open port dilution mask
assembly distributed by O, Corporation attached to
oxygen cylinders sold by Scott Aviation (Buffalo,
NY) and Puritan Bennett Aero Systems (Lenexa, KS).
Each cylinder contained approximately 22 ft? of
oxygen when fully charged.

Monitoring Equipment: The primary monitoring
equipment used in this study was a pulse oximeter
(Nellcor, model N-200). This provided continuous
measurement of blood oxygen saturation (SaO,) and
heart rate (HR) during all testing. During tests utiliz-
ing hypoxic gas mixtures, end tidal oxygen (PetO,)
and carbon dioxide (PetCO,) values were collected
using fast responding gas analyzers (Applied Electro-
chemistry, S-3A/I and CD-3A, respectively). Trans-
cutaneous pressures of oxygen (PetO,) and carbon
dioxide (PetCO,) were monitored using a Sen-
sormedics Transend II. PetO, and PetCO, values
were recorded from a mass spectrometer (Marquette,
MGA-1100) during altitude tests. A Novametrix
System 800 was used to record transcutaneous gas
pressures at altitude. Each subject’s electrocardio-
gram (Marquette, MAC 6) was also monitored dur-
ing altitude exposures. Electronic signals from the
measurement instruments were collected using a digi-
tal computer (Intel architecture) and data acquisition
boards (National Instruments, AT-MIO-16F and
AT-MIO-64F). The data collection programs were
written using the LabVIEW graphical programming
environment (National Instruments).

Test Protocol: At least 24 hours prior to altitude
testing, subjects had their SaO, response to a hypoxic
stimulus measured. The SaO, values obtained were
used as a baseline reference of each individual’s re-
sponse to hypoxia. All baseline data were collected
with the subject in a seated position at rest. For the

baseline tests, the subject breathed compressed gas
mixtures through diluter demand regulators (Type
CRU 68/A) set to the 100% oxygen position. Regu-
lator flows were directed through a multidirectional
flow valve to allow immediate manual switching
among gas mixtures. The subject breathed different
gas mixtures through the regulator-based breathing
system using a unidirectional breathing valve (Hans
Rudolf, Kansas City). Ground level exposure (Okla-
homa City, approximately 1300 feet above sea level)
initiated the baseline tests. At 4 minutes, the inspired
gas was switched to a mixture consisting of 14.4%
oxygen. Physiological variables were monitored for 4
minutes and until steady state levels were observed.
The breathing gas was then switched to a 13.1%
oxygen/nitrogen mixture for 4 minutes. The subject
was then switched back to breathing ambient air. The
values obtained were taken to represent the altitude
exposures of 10,000 and 12,500 feet, respectively.
The reference guidelines for this testing require
that SaO, and tracheal PO, estimates be maintained
at required levels. Failure of the breathing system to
maintain these physiological parameters at any point
during the altitude tests required immediate termina-
tion and an analysis of failure given for that particular
test trial of the system being evaluated. The perfor-
mance tests for the portable oxygen breathing equip-
ment required the subject be taken to a simulated
altitude of 25,000 feet in the hypobaric chamber.
The first step in this process was a check to verify that
the subject could clear ear and sinus passages. This
consisted of decompressing the hypobaric chamber
to an altitude of 5,000 feet. Upon successful comple-
tion of the ear and sinus check, the subject was
instrumented with transducers required to make the
necessary test measurements. Testing consisted of the
hypobaric chamber initially being decompressed to
an altitude of 7,000 feet above sea level. This pressur-
ization level was maintained for five minutes. The
chamber was then decompressed at a rate of 12,500
feet per minute to an altitude of 25,000 feet above sea
level. At a simulated altitude of 10,000 feet, the
subject was instructed to don the portable oxygen
system’s mask and start the flow of oxygen from the
cylinder. An oxygen flow rate of 4 liters per minute
was used for all tests. Upon reaching the 25,000 feet
altitude, the subject was instructed to begin self-
paced pedaling of a cycle ergometer at a work rate of
15 watts to mimic movement in an aircraft cabin.


60 TT ] 60
y

50 py t nae ann oo 3 50 p+ —--S — YF 7-4-4 —--——
21] Y| 2°yoryyyrr titty
B40 + 4t—-——-p -- 4 - Be ppp _——-F-——
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S39 A Female BaselineDam _} 9 _>__ je 30 A Male Baseline Data __|___.__.__|_-_
& ‘¥ Female Altitude Data £ Male Altitude Data
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0 - + ‘ 0) a

0 4 8 12 16 0) 4 8 12 16

Time (minutes)

Time (minutes)

Figure 4. End-tidal oxygen data collected during the tests. The points and bars represent

the average, + 1 standard deviation.

12

Percent Carbon Dioxide in End-Tidal Air

‘Carbon Dioxide in End-Tidal Air

Percent

12

10 }-—-y——-----—-+-—--~-4---—--|

12
Time (minutes)

12
‘Time (minutes)

Figure 5. PetCO2 data collected during the test periods. The points and bars represent
the average, + 1 standard deviation. Where the bars are not present, the standard
deviation was below a level demonstrable on the scale of the graph.

mmHg, respectively. The oxyhemoglobin dissocia-
tion curve predicts that PO, values of 65 and 58
would result in SaO, values of 93 and 90. These
values are consistent with the test data. As antici-
pated, there were large fluctuations in PetO, at alti-
tude using a continuous flow oxygen system and
open port dilution mask, The average value was
between 50 and 60% O, for females and consistently
near 50% O, for males. Atan altitude of 25,000 feet,
this represents an alveolar PO, of approximately 140
mmHg and is consistent with SaO, values being
maintained at ground level values.

PetCO, data were consistent with physiological
responses anticipated under the test conditions. It
increased during the mild exercise at altitude and
demonstrated a slight decrease during the mixed gas
exposures. This probably reflects a mild hyperventi-
lation resulting from the hypoxic stimulus. In addi-
tion to end-tidal values, transcutaneous partial
pressures of O, and CO, were also tracked.

PtcO, and PtcCO, measures are qualitative at best.
The values are best used to track changes or trends.
Therefore, to allow meaningful grouping of indi-
vidual responses, the average value from the first


DOT/FAA/AM-98/27

Office of Aviation Medicine
Washington, D.C. 20591

Performance of a Portable

Oxygen Breathing System
at 25,000 Feet Altitude

Robert P. Garner
Richard E. Murphy
Chad B. Hudgins
Joseph G. Mandella, Jr.

Civil Aeromedical Institute
Oklahoma City, OK 73125

November 1998
Final Report

This document is available to the public
through the National Technical Information

Service, Springfield, Virginia 22161.

@

U.S. Department
of Transportation

Federal Aviation
Administration


Fulco, C.S. and Cymerman, A. Human perfor-
mance and acute hypoxia. Defense Technical In-
formation Center Report. November 1987. Fort
Belvoir, Virginia.

Ernsting, J. and Sharp, G.R. Hypoxia and Hyper-
ventilation. In Ernsting, J. and King, P. (eds.)
Aviation Medicine, 2°4 ed. Butterworth-
Heinemann, Oxford (England), 1988.

Garner, R.P. Performance of a continuous flow
passenger oxygen mask at an altitude of 40,000
feet. Office of Aviation Medicine Report, FAA
Report no. DOT/FAA/AM-96/4. Washington,
DC, February 1996. Available from: National
Technical Information Service, Springfield, VA
22161. Order # N96-22217.


DISCUSSION

The aviation industry is expected to see continued
growth in the coming decades. Many current avia-
tion safety equipment designs will remain on aircraft.
Undoubtedly, new and/or improved components
will continue to be incorporated into these systems,
since the ability of innovators to develop and modify
systems to be more efficient or cost-effective in ac-
complishing a given task has limitless potential. The
difficulty for the FAA, or other aviation regulatory
organizations, is making the determination as to
whether or not a given change has the potential to
result in a substantive alteration in the performance
of the equipment. Any modification to an aviation
safety system must be evaluated on its own merits. If
there appears to be a question as to the ability of a
safety device to function properly, the equipment
should be thoroughly tested in a manner consistent
with its intended use and existing regulatory stan-
dards. Alterations in the production and assembly of
an open port dilution system led to this series of tests
to determine if performance capabilities were main-
tained at an altitude of 25,000 feet.

Open port dilution masks are described in AS
1224A (4). This type of mask is fed by a rebreather
bag that works to catch oxygen rich air previously
contained in dead space volumes associated with the
previous breath. The air in the rebreather bag can
then be inhaled into the gas exchanging regions of the
lung during the next breath. The inhaled air is diluted
with ambient air that flows in through holes designed
into the body of the mask. These orifices do not have
any type of valve associated with them, thereby allow-
ing the free flow of gas into and out of the mask cavity
as a function of the pressure differential between the
mask cavity and the ambient environment. This
arrangement is the reason that open port dilution
masks are not a highly effective means of supplying
supplemental oxygen at altitudes beyond 25,000 to
30,000 feet.

The physiological data collected during the baseline
tests were consistent with the responses that would
have been anticipated, given the mild hypoxic stimu-
lus presented the subjects (6, 7). The altitude data
were consistent with there being a supply of supple-
mental oxygen sufficient to maintain blood oxygen
saturation levels equivalent to ground level condi-
tions. In fact, the data suggest that hypoxia protection
would be provided at even higher altitudes while

using this equipment at a flow of 4 liters per minute.
Previous work, recently performed at CAMI, that
evaluated a continuous-flow passenger oxygen mask
system supports this contention (8). Additional, more
detailed experiments would have to be performed to
truly determine the rational safety limits of this type
of portable oxygen system.

Equipment testing at altitude must represent a
balance between subject safety and determining ac-
ceptable performance in an inherently dangerous
environment. The dangers of altitude exposure to
35,000 to 45,000 feet often prevent the use of a test
protocol consistent with the emergency altitude ex-
posure conditions that the safety equipment is de-
signed to protect individuals against. In contrast, this
set of altitude tests was conducted in the context of
how the system might be used aboard an aircraft. In
theory, a user would don a portable system during the
decompression and continue using it until safe cabin/
flight altitudes are again achieved. For flight altitudes
of 25,000 feet and below, the tested equipment
worked very well. The maintenance of oxygenation
levels at altitude while using the portable oxygen
system indicates that the manufacturer’s production
changes did not negatively influence the system’s
ability to provide sufficient supplemental oxygen at
an altitude of 25,000 feet.

REFERENCES

1. Ernsting, J. Prevention of hypoxia — acceptable
compromises. Aviat. Space Environ. Med. 49(3):
495-502, 1978.

2. Mohler, S.R. The pilot: An air breathing mam-
mal. Flight Crew. 2:56-9, 1984.

3. Federal Aviation Regulation Part 25.1443 Mini-
mum mass flow of supplemental oxygen. Federal
Aviation Administration, Washington, DC.

4, Aerospace Standard 1224A — Continuous flow
aviation oxygen masks (for non-transport cat-
egory aircraft). 1978. Society of Automotive En-
gineers, Warrendale, PA.

5. Federal Aviation Administration: Technical Stan-
dard Order C103 — Continuous flow oxygen mask
assembly (for non-transport category aircraft), April
1984. Department of Transportation, Federal Avia-
tion Administration, Washington, DC.


55 + ---—-4+--—-—+ —----4------] 55 f+ 4-b--+4------
& Female Baseline Data & Male Baseline Data
‘v Female Altinude Data A ‘Vv Male Altitude Data B
40 - 40
0 4 8 12 16 0 4 8 12 16
Time (minutes) ‘Time (minutes)

Figure 2. Heart responses during the experimental treatments. Panel A— Females. Panel
B - Males. The points and bars represent the average, + 1 standard deviation.

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Respiratory Rate (breathes/min.)

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10 +-—---———+-----4 —---+4------ 10 +-—-————}+—-~~-~-——+-——--——4--—~—-—_—
‘A Female Baseline Data
S7- ow Female Altitude Data [——— SF nnn |
A B
0 0 +
0 4 8 12 16 0 4 8 12 16
Time (minutes) Time (minutes)

Figure 3. Breathing frequency during the different test periods. The points and bars
represent the average, + 1 standard deviation.

in ventilatory rate while breathing the hypoxic gas Ventilatory rates were tallied from end-tidal read-
mixtures. Subjects’ ventilatory frequency tended to ings of oxygen (PetO,) and carbon dioxide (PetCO,).
be increased in the first minute of altitude exposure. These data are expressed as percentages of O, and
The contrast with rest while breathing air with atmo- CO, in Figures 4 and 5, respectively. The oxygen data
spheric oxygen concentration was particularly pro- are very consistent for both groups during the baseline
nounced in the male subjects. The first minute of tests and predict the SaO, data exceptionally well.
data collection represents the time after the subjects PetO, values were at 8.9% during the last minute
have donned the mask and the hypobaric chamber exposure to the 14.4% oxygen/nitrogen mixture and
was still being decompressed towards the final alti- 7.8% during the last minute of exposure to the 13.1%
tude of 25,000 feet. oxygen/nitrogen mixture. This is indicative of lung

oxygen partial pressures of approximately 65 and 58




Technical Report Documentation Page

T. Report No. 2. Government Accession NO. 3. Recipient's Catalog No.
DOT/FAA/AM-98/27

4. Title and Subtitle 5. Report Date
Performance of a Portable Oxygen Breathing System at 25,000 Feet November 1998
Altitude

6. Performing Organization Code

7. Author(s) 8. Performing Organization Report No.
Garner, R.P., Murphy, R.E., Hudgins, C.B., and Mandella, J.G., Jr.

9. Performing Organization Name and Address 10. Work Unit No. (TRAIS)

FAA Civil Aeromedical Institute
P.O. Box 25082

Oklahoma City, OK 73125 11. Contract or Grant No.

12. Sponsoring Agency name and Address 13. Type of Report and Period Covered
Office of Aviation Medicine
Federal Aviation Administration
800 Independence Ave., 5.W.
Washington, D.C. 20591

14. Sponsoring Agency Code

15. Supplemental Notes

16. Abstract

A portable oxygen system utilizing open port dilution rebreathing mask technology was tested for its ability to deliver an
adequate supply of oxygen at an altitude of 25,000 feet above sea level. Twenty-two subjects, 11 females and 11 males,
participated in the study. Blood oxygen saturation (SaO,) baseline levels for hypoxic exposure were established for each
subject. Altitude testing consisted of the subject being placed in a hypobaric chamber and it being decompressed to an
altitude of 25,000 feet. Immediately after the start of the decompression, the subject was instructed to don the oxygen
mask and start the flow of oxygen from the portable cylinder. Oxygen flow to the mask was continuous at 4 liters per
minute. Once at altitude, the subjects pedaled a cycle ergometer at a resistance of 15 watts for five minutes. SaO, and
other physiological variables were monitored throughout the altitude exposure. SaO, levels were maintained at ground
level values for all subjects throughout the altitude exposures. At no point during the testing did oxygenation levels
approach baseline levels for hypoxic exposure. The portable oxygen system tested provided protection from hypobaric
hypoxia at an altitude of 25,000 feet.

17. Key Words 18. Distribution Statement
Altitude, Portable Oxygen System, Hypoxia, Supplemental | Document is available to the public through the National
Oxygen Technical Information Service
Springfield, Virginia 22161
19. Security Classif. (of this report) 20. Security Classi. (of this page) 21, No. of Pages 22. Price
Unclassified Unclassified 11

Form DOT F 1700.7 (8-72) Reproduction of completed page authorized



NOTICE

This document is disseminated under the sponsorship of
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