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MINUTES

MINUTES

Ficune 8. Graphic comparison of altitude profile, blood-oxygen saturation, oxygen-flow rate and tracheal-oxygen
partial pressures. Shaded area indicates exercise, Subject H. H. and D. R.

= Ear check

0,000 Feet, air-breathing baseline
4,000 Feet, air-breathing baseline
Don crew mask (100% oxygen)

fold breath and don passenger mask.

G = Continue exercise at baseline. Adjust oxygen flow to mask as alti

12

xercise baseline — Adjust load to obtain desired minute and tidal volumes, Respiratory nitrogen washout,

itude is increased,




mm Hg) the sampling rate was 3 cc per minute.
The continuous sample was drawn through a
néedle valve and microcatheter tubing (PE 60)
of 0.030 inch internal diameter. The small, ex-
tremely lightweight, microcatheter tubing con-
nected to the mask did not require addition of
significant weight or extensive modification of
the mask, factors that might compromise the fit
and operational characteristics of the mask. An
integrator consisting of a small lucite reservoir
and mixing chamber was interposed in the sam-
pling tube near the mask as shown in Figures 2
and 3. In effect, this chamber integrates the
area under the curve of the rapidly changing
nitrogen concentration and provides a record of
the mean mask nitrogen concentration.

The tracheal-oxygen partial pressure is cal-
culated from the nitrogen data as follows:

= (B-47)F;
0.

2

Tracheal oxygen partial pressure

B = Ambient barometric pressure
47 = Vapor pressure of H,O at body tem-
‘Ana perature (37°C) and 100% saturation
nda:

F,=10-F,
N,

2
A= Fraction of inspired oxygen
"2
1.0 = Unity
Ry Fraction of Nitrogen inspired

2

For a more detailed account of this technique
consult references 8 and 9.

‘A Waters Conley ear oximeter Model XE-60A.
was affixed to the pinna of the subject’s ear 10
to 15 minutes prior to the flight in order to allow
warming and stabilization. The output of the
earpiece was fed into an Electronics for Medicine
oximeter amplifier and could be monitored on a
panel meter and oscilloscope.

Far-oximeter results were recorded on a 14-
channel Visicorder continuously throughout the
chamber flight.

The signal from EKG electrodes was split
and fed into an EKG monitor and cardiotach-
ometer. Both of these signals were recorded on
the Visicorder.

The output from the impedance pneumograph
electrodes was fed into a Physiograph impedance
pneumograph preamplifier and recorded on the
Visicorder.

The impedance pneumograph was included in
the experiment to attempt to determine if
changes in the respiratory activity baseline oc-
curred during subsequent ascent to altitude. At
the present time, there is no satisfactory method
of measuring respiratory volumes and activity
while wearing a passenger mask without com-
promising the performance of the mask. A typi-
cal tracing is reproduced in Figure 4. The
upper tracing shows the electrocardiograph
(EKG), ear oximeter (S), altitude (A), imped-
ance pneumograph (I), cardiotachometer (C),
and mass flowmeter (V). The EKG in the
lower tracing is erratic due to faulty electrode
conductivity, and, therefore, the cardiotachom-
eter is rendered inoperative. The calibrations
along the left margin are approximate. For
more accurate readings refer to the tables.

Motion pictures were taken of the subjects
during the maximum altitude portion of the
flights.

Closed-circuit television was also used as an
aid to observe the activity and condition of the
subjects at all times.

III. Results.

The oxygen flow of the passenger mask
NIPD (normal temperature pressure dry 70°
—760 mm — dry) and BTPS (body temperature
pressure saturated, 37° — ambient — saturated)
is shown in Table 1. The flow to the first two
subjects (J. T. and B. R.) was established at
higher rates than for subsequent: subjects since
this was the first use of the mask at altitude by
human subjects. Subsequent flows to the remain-
ing subjects were reduced to values approxi-
mately those provided by current jet transport
systems. Minute and tidal inspired volumes dur-
ing establishment of the exercise baseline at
14,000 feet are presented in Table 1. These
volume measurements were obtained using both
the mass flowmeter and dry gas meters. The dry
gas meter readings are considered to be the more
accurate of the two methods. Electronic prob-
Jems associated with the mass-flowmeter inte-
grator may have accounted for the discrepancy
between these two determinations. Unfortunately
the dry-gas meter readings of the first two sub-
jects were not recorded, but the meter was moni-
tored and the work load increased until a minute
volume of approximately 25 liter minute was
attained.



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8


ance efficiency requirements in excess of similar
evaluations conducted. on the sedentary resting
subject. In an altitude experiment of this type
using jet transport flow rates, inboard mask leak-
age can only be determined at the 40,000-foot
level. At altitudes below 40,000 feet, the reduced
oxygen flow into the mask is diluted by intro-
duction of air through the ambient air valve fol-
lowing depletion of oxygen in the reservoir bag.

At 40,000 feet, 3.6 liters/minute NTPD equals
30.6 liters/minute BTPS. A subject breathing
30.6 liters/minute or less will not empty the
reservoir bag and draw in air through the am-
bient air valve, if the mask provides a good seal
to the face.

If, however, there are significant and uncon-
trolled openings around the periphery of the
mask, ambient air may be drawn into the mask
during peak inspiration rather than through the
check valve of the reservoir bag.

The percent of leakage may be calculated from
the nitrogen data by applying appropriate cor-
rections for the oxygen in the ambient air.*?

The mean nitrogen concentration in the mask
at 40,000 feet averaged 3.4% and never exceeded
5.0%. The mean tracheal-oxygen partial pres-
sures of all subjects at 40,000 feet exceeded the
air-breathing baselines established at 14,000 feet.
In addition, mean tracheal-oxygen partial pres-
sure of all subjects remained well above the 100
mm (10,000 to 18,500 feet) and 83.8 mm (18,500
to 40,000 feet) requirements of the Federal Avia-
tion Regulations.

The ear-oximeter determinations were more
variable than the mean tracheal-oxygen partial
pressures.

This wandering fluctuation of the ear oximeter
was pronounced during resting and air breathing
at 14,000 feet, become more stable with 100%
oxygen and exercise at 14,000 feet, and was ex-
hibited to a marked degree at 40,000 feet on
oxygen (Figure 4).

In general, the ear-oximeter readings appeared
to be more stable during exercise than at rest.

The ear-oximeter readings of subject E. Me.
in Table 2 were the minimum values recorded
and may reflect the effect of a transient dip in
saturation at 40,000 feet (Figure 4).

The ear-oximeter tracing of subject H. H. in-
dicated a progressive drop of saturation at 40,000
feet that was not reversed by increased oxygen
flows. The subject did not exhibit symptoms of

hypoxia commensurate with the indicated blood-
oxygen saturation. It appeared therefore that
the ear oximeter was in error.

The nitralizer method of determining tracheal
oxygen partial pressure appears to be a superior
mask evaluation technique when compared to the
ear-lobe oximeter blood-oxygenation method. It
is admitted that the maintenance of an adequate
blood-oxygen saturation is the desired end result.
Instrumentation artifacts and variations in the
physiological response of the mask wearer may
result in considerable variation in the ear-oxi-
metry indications of blood-oxygen saturation.

The function of the mask is to deliver suffi-
cient oxygen to produce an adequate tracheal
partial pressure. Since pressure breathing is not
involved in passenger systems, the mask cannot
provide partial pressures in excess of those pro-
vided by a 100% concentration of oxygen. A
hypothetical leakfree mask providing 100% oxy-
gen throughout inspiration has obtained maxi-
mum efficiency. The resulting oxygen partial
pressure therefore becomes merely a function of
the ambient barometric pressure.

It is suggested therefore that the evaluation
of the mask should be primarily based upon the
efficiency of the mask in providing an adequate
partial pressure.

This does not mean that a determination of the
blood-oxygen saturation is not important but,
until the variability of individual wearer’s physi-
ological response can be reduced and techniques
for indirect determination of blood oxygen satu-
ration can be improved, mask evaluation should
be primarily based on tracheal partial-pressure
determinations. It is desirable that the measure-
ments be supplemented by blood-oxygen satura-
tion and other physiological determinations in-
dicative of hypoxia.

The mask during various phases of the respira-
tory cycle contains oxygen introduced by con-
tinuous flow and. the reservoir bag reserve. In
addition, nitrogen from the ambient air due to
leakage or dilution, or both, may be present as
well as carbon dioxide in the expired air. It
would appear that the carbon dioxide would have
a significant effect on the calculated tracheal
partial pressure; however, since the dead space
of the mask is very small (100 cc) and the oxy-
gen flow at critical altitudes through the mask
very high (30,000 ce/minute) during expiration



The first subject experienced Grade 1 bends
in his right knee at 35,000 feet. This condition
was not relieved by discontinuing the exercise.
Reduction in altitude, however, relieved the con-
dition. The remaining portion of this flight to
40,000 feet was cancelled.

The tidal volume of subject D. D. appeared
abnormally high during exercise. This subject
has previously demonstrated a very large vital
capacity and was breathing very deeply at ap-
proximately one-half the normal resting respira-
tory rate.

The impedance pneumograph factor in Table
1 is the ratio by which the mean amplitude
varies from unity, which was for the purpose of
these tests established at 14,000 feet wearing
the passenger mask, When attempting to as-
sess this factor, one must keep in mind that the
respiratory frequency, which increased with alti-
tude, also affects minute or ventilation volume
per unit of time, providing the tidal volume re-
mains constant.

The electrocardiograph and cardiotachometer
indicated an increast in heart rate at the maxi-
mum altitude attained. There was also a pre-
dictable increase of heart rate with exercise
(Table 2).

The National Aerospace Standards (NAS)
recognized gas analysis and blood-oxygen-satu-
ration determination as the two principal alter-
nate methods to be used in altitude-chamber
evaluations of passenger masks.

In this study, the experiments were so designed
that both of these parameters were measured
simultaneously.

The tracheal oxygen partial pressures and ear
oximetry data are summarized in Table 2.

A more detailed summary of the ear oximetry
data is presented in Table 3.

Exercise time prior to ascending to altitude
was held to a minimum in order to reduce the
potential development of bends and reduce fa-
tigue. Therefore the air-breathing baselines at
10,000 and 14,000 feet were carried out under
resting conditions.

The NAS standard states that the baselines
established at 10,000 and 14,000 feet should be
conducted with the subject engaged at the same
level of activity as during the altitude tests.

In order to investigate this factor, five air-
breathing subjects were exposed to an altitude

of 14,000 feet while resting and also exercising
at the predetermined baseline level (Table 4).

These tests indicated that exercise reduced the
air-breathing baseline ear oximeter reading by an
average of 4.9%.

It would appear, therefore, that the resting,
air-breathing baselines determined in conjunction
with the altitude-exercise experiments may be ap-
proximately 5% too high and should be reduced
by this factor for valid comparison.

Tracheal-oxygen partial pressure, blood-oxy-
gen saturation and oxygen flow as related to the
flight-altitude profile are plotted for each subject
in Figures 5, 6, and 7.

IV. Discussion.

Previous passenger-mask, high-altitude evalua-
tions have been carried out with the subjects in a
resting or sedentary condition. In some previous
evaluations, a brief episode of voluntary hyper-
ventilation was carried out in order to elevate
minute volume to 30 liters/minute. This pro-
cedure is recommended in NAS 1179, but it is
practically impossible for a sedentary subject
to maintain this level of respiration for more
than 2 to 3 minutes without experiencing severe
symptoms of hypocapnia (dizziness, paraesthesia,
muscular cramps, etc.). In addition, the reduc-
tion of alveolar pCO, unrealistically provides for
an increased alveolar pO. Drastic changes in
blood chemistry and cerebral blood flow due to
hyperventilation also detract from its usefulness
in mask evaluations.

A controlled and measured work load was used
in these experiments in order to stimulate respi-
ration to the 30 liters/minute standard without
imposing severe changes in respiratory and
blood-gas composition and chemistry.

It is admitted that the increased work load
produces an increase in oxygen consumption.
The level of work load used in these experiments
should produce an increase in oxygen consump-
tion of approximately 350 to 500 cc above the
resting value.

One disadvantage of using exercise in mask
evaluations at altitude is the increased suscepti-
bility to the development of bends. The degree
of denitrogenation, altitude profile, and expos-
ure time must be carefully considered in rela-
tion to the use of exercise.

The increased minute and tidal volumes de-
veloped during exercise impose mask-perform-


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PASSENGER MASK EVALUATION

CHAMBER {Per FAA TSO-C64
PROFILE: Per. NAS II79

357

307

20; MV,TV
BASELINE

O25 BASELINE

ALTITUDE (thousand feet)

hon PASSENGER MASK

DON CREW MASK

OT T T T T T
0 10 20 30 40 50 60

TIME (minutes)

Ficure 1. Altitude-chamber profile used in evaluation of the Puritan prototype disposable passenger mask.
Subjects were resting until the crew mask was donned and exercising the remainder of the flight
until descent from 40,000 feet was initiated.

7



AM 66-7

EVALUATION OF THE PHYSIOLOGICAL PROTECTIVE
EFFICIENCY OF A NEW PROTOTYPE DISPOSABLE
PASSENGER OXYGEN MASK

Ernest B. McFadden, M.S.

Approved by Released by
hte (Ui ege€ md
J. Rosert Ditxe, M.D. P. V. Srecen, M.D.
Omer, Crvin Avromeprcan InsriroTe Feperay Air Surceon
April 1966

FEDERAL AVIATION AGENCY
Office of Aviation Medicine



Mass Op Flow I Liter NTPD

40-
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OXYGEN-EXERCISE- 40,000 FT. PASSENGER MASK

Ficure 4. Reproduction of portions of @ typical recording. Subject E. Mc. EKG — Electrocardiogram, § —
Saturation — Ear oximeter. X — Minute volume — Mass flowmeter. C — Heart rate — Cardio-
tachometer. A — Altitude. I — Impedance pneumograph. Scales to left of record are approximate.

10


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and the subsequent pause, the carbon dioxide of
the expired gas is rapidly washed from the mask.

If one assumes, for example, that the mask
wearer is receiving an oxygen flow rate of 30
liters/minute, the oxygen flow through the mask
during exhalation would approximate 500 cc/
secend, rapidly washing the carbon dioxide from
the mask dead space.

A control experiment at 14,000 feet breathing
air and exercising indicated that the baseline
resting blood-oxygen saturations at an altitude
of 14,000 feet in Tables 2 and 3 and Figures
5, 6 and 7 were approximately 5% too high for
valid comparison and should be corrected ac-
cordingly.

The discrepancy between constant-altitude ex-
periments as presented in this report and the
dynamic physiological changes that occur during
a rapid decompression as related to protective
efficiency of passenger masks have never been
completely resolved. Experiments have been con-
ducted in this area by Bryan and Donaldson"?
in an effort to bridge this gap in knowledge.

V. Conclusions,

1. The prototype passenger mask demon-
strated an adequate capability to maintain
human subjects in a satisfactory physiological
condition at 40,000 feet for the duration of dwell
at this altitude. The increased minute and tidal

volumes developed during exercise impose. mask-
performance efficiency requirements in excess of
previous evaluations carried out with sedentary
subjects.

2, The mask demonstrated the low leakage
characteristics desirable at the maximum altitude
of 40,000 feet.

8. The mean tracheal partial pressure ex-
ceeded the requirements of FAR-25, TSO-C64,
and NAS 1179 in all tests.

4. Control subjects breathing air at 14,000
feet and exercising indicated an average of 5%
reduction in blood-oxygen saturation when com-
pared to similar tests conducted on resting sub-
jects.

5. Blood-oxygen saturation as determined by
ear oximetry was subject to considerable varia-
tion. Three of the five subjects ascending to
40,000 feet maintained a blood saturation in ex-
cess of the 14,000-foot air-breathing baseline cor-
rected for the effects of exercise.

6. In order to stimulate respiration to the
level required by applicable regulations and
standards, measured and controlled exercise
should be the method of choice during ground
level testing. The use of exercise at altitude is
subject to certain limitations and its use should
be considered with respect to the experimental
design.

REFERENCES

1, Federal Regulations, Part 25. Airworthiness Stand-
ards, Transport Category Airplanes.

2. Evaluation of Prototype Passenger Oxygen Mask As-
semblies. Boeing Airplane Company Report D6-1954,
Transport Division, Renton, Washington, D.C., Feb.
9, 1959.

3. Qualification Test Report, Passenger Oxygen Mask,

QTR-59-6. Aro Equipment Company of California,

Los Angeles, Calif., Jun, 24, 1959.

National Aerospace Standard, 1179. Revised. Aero-

space Industries Association of America, Washing-

ton, D.C., Mar. 31, 1961.

5. Federal Aviation Agency, Technical Standard Order
C-64, Aug. 23, 1961.

6. Oxygen Equipment for Aircraft. Aerospace Infor-
mation Report 825. Society of Automotive Engineers,
New York, N.Y., Feb. 25, 1965.

7. A Design Analysis of a Jet Transport Passenger-
Oxygen System. Douglas Report SM—42564, Missile

»

and Space Systems Division, Douglas Aircraft Com-
pany, Santa Monica, California, Mar. 1963.

8. Final Report on the Investigation of Mask Leakage
in Passenger Oxygen Masks. Pioneer-Central Divi-
sion, The Bendix Corporation. Federal Aviation
Agency Contract FA-885, Feb. 1962.

. McFanpen, E. Bt, Rarxe, J. W., Youna, J. W.; An
Improved Method for Determining the Efficiency of
Crew and Passenger Oxygen Masks, Report 62-21,

Federal Aviation Agency, Civil Aeromedical Re-
search Institute, Oklahoma City, Oklahome, Nov.

1962,

10, NASA Life Sciences Data Book, First Edition. Jun.
1962,

11. Bryan and LeacH: Physiological Effects of Cabin
Failure in High Altitude Passenger Aircraft. Aero-
space Med. 31 (4) : 267-275. Apr. 1960.

12. Donatpson, R. T.: A Study of Areterial Oxygen Sat-
uration During Rapidly Changed Barometric Pres-
sure. Ohio State University. M. S. Thesis.

2


Table 4. Chamber fight to 14,000 fect to establish air-breathing baselines during resting,
work and recovery.

Subject % Saturation — Ear Oximeter
and Time — Minutes
Condition Altitude Preflight 1 2 3 4 5 6 7 8 9 Mean
H.
Resting 1,273 100.0
Resting 14,000 93.0 92.0 91.0 93.0, 92.0 92.0 92.2
Exercise 35 rpm, 40 W 14,000 87.0 87.0 87.0 87.0 91.0 87.0 87.6
Resting — Recovery 14,000 01.0 02.0 92.0 920 92.0 92.0 910 917
D. R.
Resting 1,273 97.0
Resting 14,000 88.0 90.0 88.0 880 87.0 860 87.0 850 830 869
Exercise 35 rpm, 40 W 14,000 81.0 80.0 81.0 80.0 80.0 80.0 80.0 80.0 80.3
Resting — Recovery 14,000 80.0 79.0 78.0 76.0 79.0 76.0 75.0 77.0 18
E. Me.
Resting 1273 99.0
Resting. 14,000 95.0 95.5 955 95.0 940 94.0 94.0 95.0 94.8
Exercise 35 rpm, 40 W 14,000 92.0 93.0 925 93.0 91.0 92.5 90.0 880 91s
Resting — Recovery 14,000 90.0 910 920 920 93.0 940 940 940 92.5
D. D.
Resting 1,273. 985
Resting 14,000 95.0 940 945 94.0 94.0 94.0 940 94.0 94.2
Exercise 40 rpm, 40 W 14,000 930 915 910 915 905 905 91.0 90.0 oll
Resting — Recovery 14,000 940 95.0 940 94.0 945 940 940 940 930 941
J. s.
Resting 1273 98.5
Resting 14,000 91.5 90.0 87.0 85.0 85.0 86.0 85.0 84.0 86.7
Exercise 35 rpm, 40 W 14,000 810 79.0 76.0 78.0 820 80.0 79.3
Resting — Recovery 14,000 84.0 83.0 84.0 83.0 82.0 83.0 840 820 83.1
MEANS
N=5
Resting 14,000 — Resting — Breathing Air 90.9
Exercise 14,000 Exercise — Breathing Air 86.0
Resting — Recovery 14,000 Difference = 4.9 87.8

20 67-3290


8. Oxygen concentrations approaching 100%,
which are required at 35,000 to 40,000 feet, may
be obtained with moderate and reasonable flow
rates.

One basic disadvantage of all continuous-flow
oxygen systems is their inability to adjust auto-
matically to the respiratory changes associated
with changes in emotional and physical activity
of the wearer.

A healthy young male breathing air at rest
normally exhibits an approximate (volume/
breath) tidal volume of 550 ce and a minute
volume (volume/minute) of 7,700 ce, or 7.7 liters.
Emotional or physical activity, or both, may
cause values to increase greatly.

Concern with the problem is reflected in the
Federal Aviation Regulation Part 25 (formerly
Part 4b) 25.1443' which requires maintenance of
a mean tracheal oxygen partial pressure of 83.8
mm Hg at a tidal volume of 1,100 cc, and a
30-liter body temperature pressure saturated,
(BTPS) minute volume for altitudes of 18,500
to 40,000 feet.

With the introduction of jet-transport passen-
ger aircraft certified to operate at high altitudes,
new oxygen systems and masks were formulated
and evaluated.

Subsequently, standards for passenger oxygen
masks were compiled and published. The Na-
tional Aerospace Standard 1179* and Federal
Aviation Agency Technical Standard Order
C-64° set forth manufacturing, material, and
testing standards for passenger oxygen masks.

An excellent description of the basic physi-
ology of oxygen in aircraft as related to oxygen
equipment design has been prepared by the SAE
A-10, Aircraft Oxygen Equipment Committee.®

An additional report describes the basic crite-
ria and design philosophy of jet transport pas-
senger systems,”

II. Methods.

The altitude chamber flight profile is shown
in Figure 1. (All figures and tables are in the
appendix.) Six subjects were instrumented as
shown in Figure 2, with the exception that the
mask was not donned until air-breathing base-
lines were established at 10,000 and 14,000 feet.
A chamber safety observer accompanied each
subject.

After a preliminary test of the subject’s capa-
bility to equalize ear pressures, the subject rested
quietly at 10,000 feet until ear oximeter readings

indicated a stabilization of blood saturation. The
chamber then ascended to 14,000 feet to establish
a similar baseline at this altitude.

When it appeared the blood saturation had
stabilized at 14,000 feet, the subject donned a
crew-type demand oxygen mask and commenced
breathing 100% oxygen. Immediately following
crew-mask donning, exercise on a bicycle ergo-
meter was initiated. The exercise level in rpm
(speed) and watts (load) was increased or de-
creased to stimulate and obtain the desired res-
piratory activity (approximately 25 to 30 liters/
minute). This is regarded as a light to moderate
work load approximately equivalent to walking
at 3.0 to 3.5 mph.

Exercise was continued until the desired min-
ute volume as indicated by a dry gas meter was
obtained and stabilized. A mass flowmeter lo-
cated in the mask hose also sensed and recorded
the inspired tidal and minute volumes of the
subject. The output of the mass flowmeter was
fed into an integrator so that, when a predeter-
mined volume was sensed, the unit would dis-
charge and repeat. The subjects were denitro-
genated during this period in an attempt to
attenuate the increased bends potential due to
exercise at the subsequent higher altitudes to be
attained.

Continuing the exercise at the baseline level,
the subject removed the crew mask and rapidly
donned the Puritan prototype passenger mask
(Part Number 11401902) as shown in Figure 3.
The flow of oxygen to the mask was regulated
by an altitude-sensitive regulator of the type
used in multipassenger oxygen systems of trans-
port aircraft. The flow from this regulator, in-
stead of being transmitted directly to the subject,
was first routed outside the chamber through a
flowmeter and needle-valve arrangement in order
to obtain precise measurement and control of the
flow (Figure 3).

The subject continued to exercise at the pre-
determined level as the altitude was increased to
40,000 feet. The chamber was leveled off and
readings were taken at 14,000, 21,500, 29,000,
35,000, and 40,000 feet.

Two Custom Engineering and Development
Company Model 300AR nitralizers were used to
continuously measure the mask nitrogen. These
instruments exhibit an initial response time of
0.024 second, 90% response being obtained in
0,044 second. At the pressure setting used (0.6


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Ficune 7. Graphic comparison of altitude profile, blood-oxygen saturation, oxygen-flow rate and tracheal-oxygen
partial pressure, Shaded area indicates exercise. Subjects E. Me, and D. D.

A= Ear check

Cc
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0,000 Feet, air-breathing baseline
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F = Hold breath and don passenger mask

G = Continue exercise at baseline.

‘Adjust oxygen flow to mask as altitude is increased.

13



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15



EVALUATION OF THE PHYSIOLOGICAL PROTECTIVE EFFICIENCY
OF A NEW PROTOTYPE DISPOSABLE PASSENGER OXYGEN MASK

I. Introduction.

This report describes altitude chamber experi-
ments conducted with human subjects using new
Puritan protype disposable passenger oxygen
masks applicable for emergency use to 40,000-
foot altitudes.

The specific functional characteristics of con-
tinuous-flow oxygen masks in terms of human
respiration are frequently not well understood.
Although less costly to manufacture than a crew
mask and deceptively simple in appearance, the
continuous-flow passenger mask involves physio-
logical performance characteristics that are rela-
tively complex. Continuous-flow masks may be
generally divided into the following categories.

A. Rebreathing Mask. A manual or automatic
continuous flow of oxygen is delivered to the
mask. A fraction of the expired gas from the
dead spaces of the mouth and trachea that con-
tain unused oxygen and air is collected and in-
spired as a part of the next inhalation. The
remainder of gas required is obtained either from
the oxygen supply or from dilution ports or
valves. If the oxygen flow is excessively low or
the respiratory pattern modified, the rebreathed
gas may contain significant quantities of carbon
dioxide from the lungs.

B. Continuous flow dilutor mask. As in the
above mask, a continuous flow of oxygen is de-
livered manually or automatically. Dilution of
oxygen by air occurs in the mask by use of ori-
fices of a predetermined diameter or through a
porous material. This, mask, if not equipped
with a reservoir bag, wastes oxygen since the
flow must be sufficiently high to provide the
volume required during the peak flow of in-
spiration. The flow that continues during the
respiratory pause and exhalation phase is vented
to the ambient atmosphere and wasted.

C. Continuous-flow reservoir mask. As in the
previous masks, oxygen is delivered in a con-

tinuous flow; however, a reservoir is interposed
between the delivery tube and the mask. The
reservoir is separated from the mask by a sensi-
tive check valve. The continuous flow of oxygen
fills the reservoir bag during the respiratory
pause and exhalation. The flow also continues
at the same rate during inspiration. The mask
wearer inspires and receives the 100% oxygen
content of the reservoir until inspiration is com-
plete or the bag emptied or both. If the reser-
voir is emptied, a spring-loaded valve in the
mask opens, and ambient air is introduced in
order to provide sufficient volume to meet the
remainder of the inspiration. The flow of 100%
oxygen is provided at the most advantageous
point in the respiratory cycle; that is, at the be-
ginning of inspiration. For example, if a human
subject’s tidal volume is 500 cc and the reservoir
contains only 350 ce at the beginning of inspira-
tion, the 350 ec of 100% oxygen will be inspired
first and delivered to the active areas of the
lungs. The ambient air valve will then open and
deliver 150 cc of air, which will enter the mouth,
trachea, and other “dead” or inactive spaces of
the respiratory system. Upon expiration, this
dead-space air is the first to exit through the
exhalation valve. This is repeated with each
respiratory cycle. In practice, the reservoir bags
normally are capable of containing a maximum
of 1,100 ce which, along with the volume intro-
duced by the continuing flow, provide for in-
creased tidal and minute volumes. It may be
readily seen that this type of mask offers the fol-
lowing advantages:

1. Oxygen economy is afforded by use of a
reservoir bag that fills and retains the oxygen
flow during the respiratory pause and exhalation,
allowing for the use of lower flow rates.

2, Reduced oxygen flow rates at lower altitudes
may be utilized, providing air-dilution of oxygen
in a more predictable and controlled manner.



ACKNOWLEDGMENTS

The author wishes to acknowledge the assistance provided by C. M.
Brake, M.D.; H. F. Harrison; J. M. Simpson; J. W. Fitch, and F. F. Winters.
In addition, acknowledgement is due B. Hansford, D. H. Grimm, and G. Gish
of the Aerospace Division of the Puritan Equipment Company for their co-
operation and assistance,

Qualified requestors may obtain Aviation Medical Reports from Defense Documentation
Center. The general public may purchase from Clearinghouse for Federal Scientific
and Technical Information, U.S. Dept. of Commerce, Springfield, Va, 22151


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As Ear check

10,000 Feet, air-breathing baseline

14,000 Feet, air-breathing baseline

Don crew mask (100% oxygen)

Exercise baseline — Adjust load to obtain desired minute and tidal volumes. Respiratory nitrogen washout,
Hold breath and don passenger mask.

G = Continue exercise at baseline. Adjust oxygen flow to mask as altitude is increased.

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