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12


sponse time of 0.024 seconds, 90% response being
obtained in 0.044 seconds. At the pressure set-
ting used (0.6 mm Hg) the sampling rate was
three cubic centimeters per minute. The con-
tinuous sample was drawn through a needle
valve and microcatheter tubing (PE 60) of 0.030
inches internal diameter. The small, extremely
light-weight microcatheter tubing connected to
the mask did not require addition of significant
weight or extensive modification of the mask,
factors which might compromise the fit and op-
erational characteristics of the mask. Mask
leakage and tracheal oxygen partial pressures
were derived from nitrogen analysis data ob-
tained using previous methods of calculation
18,22,24

A Waters Conley Ear Oximeter, Model XE-
60A, was affixed to the antihelix of the,subject’s
ear ten to fifteen minutes prior to the flight in
order to allow warming and stabilization. The
output of the earpiece was fed into an Electron-
ies for Medicine oximeter amplifier and could be
monitored on a panel meter and oscilloscope.

Ear 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 cardiotachometer.
Both of these signals were recorded on the Visi-
corder.

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

The impedance pneumograph was included in
the experiment to attempt to determine if changes
in the respiratory activity baseline occurred dur-
ing subsequent ascent to altitude. At the
present time there is, to this author’s knowledge,
no satisfactory method of directly measuring
respiratory minute and tidal volumes while wear-
ing the passenger mask without compromising
the performance of the mask.

B. Crew Mask. Evaluation of a prototype
crew mask was carried out in three phases. The
first phase consisted of exposing five crew mem-
bers to a chamber flight profile as shown in Fig-
ure 5, with a maximum altitude of 48,000 feet
while wearing the mask. The second phase con-
sisted of rapidly decompressing five crew mem-
bers from 8,000 to 40,000 feet in 45 to 50 seconds
while wearing the mask and breathing 100%

oxygen. The third phase consisted of rapidly
decompressing these same subjects from 8,000 to
40,000 feet in 45 to 50 seconds and donning the
mask during decompression. The donning signal
was delayed for fifteen seconds after passing
through 14,000 feet during the decompression.

Since subjects were well aware that they were
to be decompressed, the basic fifteen-second delay
was adopted after studying Bennett’s’ inflight
determinations of the reaction and donning time
of forty-two BOAC pilots subjected to surprise
decompressions during training flights, It was
anticipated that trained subjects would require
approximately five seconds to accomplish donning
following the donning signal. This would result
in a total donning delay of approximately twenty
seconds after passing through 14,000 feet. An
analysis of Bennett’s data by Blockley* indicates
that 99% of the pilots would have completed
donning within this time period. Instrumenta-
tion of subjects rapidly decompressed is shown
in Figure 6.

Six subjects were used in these evaluations.
Three of the subjects were transport pilots, one
a flight engineer-student pilot, the fifth a cham-
ber research technician-student pilot, and the
sixth a physiologist.

In the first phase, a standard sentence was re-
peated on command during normal and pressure
breathing, monitored and recorded for subjective
evaluation of intelligibility. Crew member per-
formance on a complex-coordinator was also
evaluated during this phase under normal and
pressure breathing conditions using commercial
and military pressure breathing schedules.

Inboard leakage was determined during de-
mand breathing by measuring the amount of
nitrogen-containing ambient air (a convenient
reference gas) entering the mask after the sub-
ject had completed respiratory nitrogen washout
while breathing 100% oxygen. Outboard leak-
age was detected and estimated during pressure
breathing by observing and measuring the slope
of the mass flowmeter recording. During exhala-
tion the recording tracing should remain hori-
zontal, indicating zero flow.

The second phase utilized a profile which in-
cluded a dwell time of 25 to 30 minutes at 8,000
feet to denitrogenate. The subjects, wearing the
mask and breathing 100% oxygen, were then
rapidly decompressed from 8,000 to 40,000 feet
in 45 to 50 seconds.



ion @-INITIAL WASHOUT TWENTYFIVE MINUTES

PRIOR TO DECOMPRESSION FROM 8,000 -
\ 40,000 FT. QUICK DON CREW MASK, N=4
707 |
i O-WASHOUT DURING DECOMPRESSION FROM
\i 8,000- 40,000 FT. FOLLOWING 2 MIN,
60- \\\ BREATHING AIR AT 8,000 FT. AND DONNING
\y MASK AT 29,000- 31,000 FT. QUICK DON
b i CREW MASK, N=4
a hl
© 50-4 Vt A-WASHOUT DURING DECOMPRESSION FROM
ow \\ 6,000- 41,000 FT. NO PREVIOUS
uu \\) DENITROGENATION. QUICK DON CREW MASK,
“ it Ne6
| 404 \4
2
Ww \
5 \
« 304 |
a \
2 | e
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20- \
fe) A
~\
107 q e
~s
OL, g
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T T T T T T 7
I5 30 45 60 75 90 105 120
TIME-SECONDS

Fieure 8. Respiratory nitrogen washout of subjects at ground level (1,273 feet) compared to subsequent washout
during decompression following 2 minutes of air breathing. The third curve of respiratory nitrogen washout
during decompression with no prior denitrogenation was obtained from a prior, unpublished evaluation of a quick
don crew mask.

16


There was significant delayed transient de-
crease in blood oxygen saturation as indicated
by the oximeter without loss of consciousness fol-
lowed by rapid recovery.

The rapid decompression and mask donning
experiments, in addition to providing informa-
tion relative to unproven crew equipment, also
point out practical limits of protection provided
within the parameters of the decompression alti-
tude and times evaluated.

Jet transport passenger and crew masks are
used operationally only in an emergengy and for
brief durations. This does not allow operational
experience at altitude to be gained with these
masks prior to a pressure loss emergency. This
emphasizes the importance of human evaluation
and testing of new, unproven, continuous flow,
oxygen mask designs at altitude prior to approval
and certification.

REFERENCES

1, Aro Equipment or Caxir.: Qualification Test Report,
Passenger Oxygen Mask. QTR 59-6, Los Angeles,
Calif., June 24, 1959.

2, Awnospace Inpustems Asgn. or Awer.: National
Aerospace Standard, NAS-1179.

3. Barron, ©. I., Coox, T. Effects of Variable De-
compressions to 45,000 Feet. Aerospace Med. 36 :425,
1961.

4, Brocxtey, W. V., Haniran, D, T.: An Analysis of
the Oxygen Protection Problem at Flight Altitudes
Between 40,000 and 50,000 Feet. Final report on
Contract FA-955, Federal Aviation Agency, Feb. 20,
1961.

. Bennert, G.: Reactions and Performance of Pilots
Following Decompression. Aerospace Med. 32:184,
1961,

6, Bryan, C. A. and Leacu, W. G.: Physiologic Effects

of Cabin Pressure Failure in High Altitude Pas-

senger Aircraft. Aerospace Med. 31:267, 1960.

Bryan, ©. A.: Aircrew Oxygen Requirements in

High Altitude Transport Aircraft. Report No. IAM

59/4, RCAF Institute of Aviation Medicine, Toronto,

Canada, Sept. 15, 1959.

8. Donatpson, R. T., Canter, E. T., Britrnes, C. E., and

Hironcock, F, A.: Acute Hypoxia During Rapid

Decompression and Emergency Descent in a Com-

mercial Jet Aircraft. Aerospace Med. 31:842, 1960.

Drayton, F. A. and Carson, L. D.: Evaluation of

Prototype Passenger Oxygen Mask Assemblies, Re-

port No. D6-1954. Boeing Airplane Company, Trans-

port Division, Feb. 9, 1959,

10. Feprrat Aviation RecuLations, Part 25, Airworthi-
ness Standards, Transport Category Airplanes,

1. Feperat Aviation Acency, Technical Standard Order
C-64, Aug. 23, 1961.

12. Geppes, L. A., Horr, H. E., Hickman, E. M., and
Moore, A. G.: The Impedance Pneumograph, Aero-
space Med. 33:28, 1962.

13, Korster, N. B., and Exrmann: Emergency Oxygen
Mask Development and Tests. Scott Engineering Re-
port No. 554, Nov. 30, 1956,

14, McFappen, E. B.: Time Factors in Passenger Utiliza-
tion of Emergency Oxygen. Office of Aviation Safety,

a

a

2

Civil Aeronautics Administration, Preliminary Re-
port, May 1955.

15. McFapven, BE. B., RaeKe, J, W., and Youne, J. W.:
An Improved Method for Determining the Efficiency
of Crew and Passenger Oxygen Masks. Report 62-21.
Federal Aviation Agency, Civil Aeromedical Research
Institute, Oklahoma City, Oklahoma, Nov, 1962.

16. McFapven, BE. B., Tana, P. C., and RarxKe, J. W.:
Continuous Functional Testing of Passenger Masks
at Ground Level and During Rapid Decompression.
Presented at the Space and Flight Equipment Associ-
ation, National Flight Safety, Survival and Per-
sonal Equipment Symposium, Oct. 28, 1964 (Unpub-
lished).

17, Mappocx, R. W.: A Design Analysis of a Jet Trans-
port Passenger Oxygen System, Report No. SM-42564,
Missile and Space System Division. Douglas Aircraft
Company, Santa Monica, Calif, Mar. 1968.

18, Nevitte, J. R.: Continuous Functional Testing of
Oxygen Breathing Equipment. Aerospace Med.
341020, 1963.

19. Newnerry, P. D., Aziz, J. P., and Srusss, C. D.: A
Method for Continuous Monitoring of the End-Tidal
Partial Pressure of Oxygen. Med. Serv. J. Canada
20 :745, Oct. 1964,

20. Preston, F. S.: Results of Decompression Trials in
Chamber Appraisal of 3 Types of Aircraft Oxygen
Mask Suspensions. British Eurpean Airways, Apr.
1959.

21, Revtersxiorp, C.: Project Bag, A Passenger Oxygen
Mask For High Altitude Aircraft. Scandinavian Air-
lines System. Stockholm, Nov. 1956.

22, Sorzamr, B. J.: Final Report on the Investigation of
Mask Leakage in Passenger Oxygen Masks. Pioneer-
Central Division, The Bendix Corporation. Federal
Aviation Agency Contract FA-885, Feb, 1962,

23. SweaRINncEN, J. J.: An Adhesive Type Oxygen Mask.
J. Aviation Med, 28 :19-22, 1957.

24. Turrie, A. D., Marparcer, J. P., and Lurt, U. C.:
KS Disposable Oxygen Mask. J. Aviation Med, 22:
265, 1951.

25, USAF ScHoon or AVIATION MEDICINE: Respiratory
Physiology in Aviation. Sept. 1954,


tude of 14,000 feet while exercising at the pre-
determined baseline level. These tests indicated
that exercise reduced the 14,000 feet air-breathing
ear oximeter baseline readings by an average of
5%.

Following a flow curve slightly lower than
used in jet transports'’, and increasing flow as
required, oximeter readings of the subjects were
maintained between an average of 85% at 21,500
to 80% at 40,000 feet during exercise, using an
average oxygen flow of from 1.8 to 3.65 liters/
minute NTPD.

Inboard mask leakage at 40,000 feet was low,
with the nitrogen concentration averaging 3.4%
and never exceeding 6%. The calculated mean
tracheal oxygen partial pressures averaged 90
mm Hg.

B. Grew Masks. During the stepwise altitude
profile in the first phase of the evaluation of a
prototype crew mask, three subjects used an oxy-
gen breathing regulator incorporating a reduced
pressure breathing schedule. At 43,000 feet mask
differential pressures using this regulator aver-
aged 4.6 mm Hg upon inspiration and 11.5 mm
Hg upon expiration.

Performance on the complex coordinator, re-
quiring higher cerebral function dropped sig-
nificantly. Although oximeter readings were not
available in this phase, subjects exhibited subjec-
tive hypoxic symptoms to the extent that after
three minutes at 43,000 feet, a descent was made
and the remainder of the profile below 43,000
feet cancelled. The two remaining subjects, us-
ing an MD-1 regulator incorporating a military
pressure breathing schedule, completed the cham-
ber flight without incident.

In the second phase consisting of wearing the
mask and breathing 100% oxygen during rapid
decompression from 8,000 to 40,000 feet, oximetry
indicated a slight (2-3%) drop in blood oxygen
saturation. “ Tracheal oxygen partial pressures
were also adequate. Immediately after decom-
pression the impedance pneumograph amplitude
increased by a factor of 3.5 times the baseline
value.

The third phase consisted of donning the mask
during rapid decompression from 8,000 to 40,000
feet in 49-51 seconds. Mask donning was accom-
plished in 29-84 seconds from the start of the
decompression at altitudes of 28,000 to 31,000
feet. Data and time-motion analysis of motion
pictures indicated donning was achieved in an

average time of 4.80 seconds from the donning
signal, with a range of 2.50 to 6.29 seconds. This
data is in general agreement with Barron and
Cook® who stated that their trained subjects re-
quired a minimum of three to four seconds to
accomplish donning.

A sharp upward sweep in several of the mass
flowmeter tracings shortly before initiation of
the first breath after donning, indicated that the
necessary altitude (28,000-31,000 feet) had been
attained to activate the automatic pressure breath-
ing characteristics of the MD-1 regulator. The
system became free flowing until the mask was
donned.

The oximeter demonstrated a marked transient
drop in blood oxygen saturation 6 to 12 seconds
after mask donning, being slightly more pro-
nounced in the subject who had not denitroge-
nated (Figure 7). This delayed transient reduc-
tion in blood oxygen saturation was anticipated,
having been demonstrated and investigated by
Luft, Bryan, Donaldson, Barron and others®**4,

A transient oximeter reading of 84% was the
lowest recorded. After one minute at 40,000 feet
the average saturation had returned to 96% with
a mean mask pressure of 7.6 mm Hg.

The impedance pneumograph exhibited an av-
erage increase in amplitude of nine times the
baseline value.

III. Discussion.

A. Passenger Mask. Previous high altitude
evaluations of passenger masks have been car-
ried out with the subjects in a resting or sedentary
condition. In some of these evaluations a brief
episode of voluntary hyperventilation was car-
ried out in order to elevate minute volume to 30
liters/minute. This procedure is suggested in
NAS 1179, but it is practically impossible for a
sedentary subject to maintain this level of venti-
lation for more than two or three minutes with-
out experiencing severe symptoms of hypocapnia
(dizziness, paraesthesia, muscular cramps, etc.).
In addition, the reduction of alveolar pCO, may
unrealistically provide for an increase in alveolar
p02 Changes in blood chemistry and cerebral
blood flow due to hyperventilation also detract
from its usefulness in mask evaluations‘.

A controlled and measured workload was used
in this passenger mask evaluation in order to at-
tempt to stimulate respiration to 80 liters/minute
with an 1,100 cc tidal volume, without imposing



The output of the Waters Ear Oximeter was
fed into an Electronics for Medicine amplifier,
displayed on a panel meter and oscilloscope and
also fed into one of the two direct readout visi-
corders. The impedance pneumograph signal
was amplified by a Physiograph and also re-
corded on one of the visicorders.

In the third phase four of the subjects were
exposed to a profile similar to that of the second
phase with the exception that the mask was re-
moved after 25 to 28 minutes of denitrogenation
at 8,000 feet and the subjects breathed air at 8,000
feet for two minutes prior to decompression. A
fifth subject was rapidly decompressed from 8,000
to 40,000 feet without previous denitrogenation.

Prior to the decompression and during the time
the subject. was breathing air, the chamber safety
observer would hang the mask back up in its re-
tention device. During the decompression as the
chamber passed through 14,000 feet an audible
alarm bell alerted the subject. Fifteen seconds
later and at approximately 25,000 feet a red don-
ning signal light was activated and the subject
donned the mask, Readout of the mass flow-
meter and mask pressure transducer data indi-
cated subject’s completed mask donning at an
altitude of 28,000 to 31,000 feet during decom-
pression.

II. Results.

The intent of this paper is to describe several
methods and techniques of evaluating unproven
prototype passenger and crew masks at altitude
in terms of applicable standards and regulations.
Space does not allow presentation of detailed
data obtained in each of a number of mask eval-
uations conducted. General results as applicable
to critical analysis of two techniques are de-
scribed.

Although the Federal Aviation Regulations
specify a minute volume of 30 liters/minute and
a tidal volume of 1,100 cc, this relationship was
rarely obtained in our experiments. During pre-
vious evaluations carried out at ground level on
ten female and ten male subjects*’, the tidal vol-
ume specified was attained by female subjects at
an average minute volume of 18.5 liters/minute.
At an average minute volume of 20.7 liters/min-
ute male subjects exhibited a tidal volume of
1,430 ce.

During the altitude profile in Figure 2, the
workload was increased during establishment of

a baseline lével in order to stimulate respiration
to the 30 liters/minute level. An average minute
volume of 27 liters/minute was attained with a
concurrent tidal volume of 2,600 cc, over twice
the 1,100 cc value specified in the Federal Avia-
tion Regulations.

The concept of a 30 liters/minute volume and
1,100 ce tidal volume implies a respiratory rate of
27. The Federal Aviation Regulations were in
part based on the assumption that an excited
post-decompression passenger would exhibit a
marked increase in respiratory rate with a mod-
erate increase in tidal volume.

An empirical impedance pneumograph factor
was calculated consisting of the ratio by which
the mean amplitude varies from unity, which, for
the purpose of these tests, was established as a
baseline at 14,000 feet wearing the passenger
mask.

The average impedance pneumograph factor
decreased to 0.83 at 40,000 feet, indicating a
slight reduction in tidal volume; however, the
average respiratory rate increased from 16 to 18
off-setting the apparent decrease in tidal volume.

The impedance pneumograph amplitude is de-
pendent upon changes in transthorasic impedance
and has been shown to be directly related to the
tidal volume”.

As the oximeter indicated an average blood
oxygen saturation of approximately 80% at
40,000 feet, it was expected hypoxic hyperventi-
lation would occur. Continuous recording of the
impedance pneumograph and respiratory rate
failed to indicate a significant increase in venti-
lation during exercise at 40,000 feet over that of
the corrected baseline obtained during exercise
at 14,000 feet (average 89%) wearing the pas-
senger mask.

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 initial air-breathing base-
lines at 10,000 and 14,000 feet were carried out
under resting conditions.

According to NAS 1179, the air-breathing
baselines are to be conducted with the subject
engaged in the same level of activity as subse-
quently attained during evaluation of the mask
at higher altitudes, but permits this to be carried
out separately.

In order to investigate this factor, subjects
breathing air were separately exposed to an alti-




rapid response characteristics to allow end ex-
piratory determination of the p0,%57°.

Early versions of polarographic sensors have
been used in several of our evaluations of pas-
senger oxygen masks with discouraging results,
due to the very rapid and extreme changes of
gas composition in the mask.

The Mass Spectrometer would be applicable to
this situation were it not for the great expense in-
volved.

In our experience the nitrogen analyzer with
very rapid response and low sensing head pres-
sures of 0.6 mm Hg (being at all times lower
than the altitude chamber pressure) has been an
easily calibrated and reliable instrument for
measuring inboard mask leakage and indirectly
estimating the inspired tracheal oxygen partial
pressure.

B. Crew Mask. The prototype crew mask
evaluated provided good protection at 43,000 feet
when it was used with a regulator incorporating
sufficient pressure to meet physiological require-
ments,

The suspension system and mask facepiece de-
sign limited inboard leakage during demand
breathing and outboard leakage during pressure
breathing to a minimal value. In one instance
outboard leakage developed spontaneously during
pressure breathing at 40,000 feet. Leakage was
calculated at 10.4-10.9 liters/minute BTPS and
ceased after two minutes. The pressure in the
mask was unaffected by leakage of this magni-
tude.

Nitrogen washout following mask donning
during decompression is shown in Figure 8.
The initial washout at the beginning of denitro-
genation at 8,000 feet is compared to the washout.
during rapid decompression from 8,000 to 40,000
feet in 49-51 seconds following two minutes of
air breathing. The two-minute period of air
breathing followed twenty-five minutes of 100%
oxygen.

The third curve was derived from unpublished
data obtained during evaluation of a similar type
mask involving rapid decompression from 6,000
to 41,000 feet in 41-49 seconds. Subjects were
breathing air at 6,000 feet with no previous de-
nitrogenation prior to decompression.

These curves point out the delay imposed by
equipment, anatomical and physiological dead
space in reaching a high alveolar pO. after mask
donning.

The breathing tube and mask at these decom-
pression rates and donning altitudes would not
be expected to contribute appreciably to the curve
since the breathing tube had been flushed by
oxygen two minutes previously; also the pressure
breathing characteristics of the automatic regu-
lator were observed to have been activated and
the equipment flushed by free-flowing oxygen
prior to donning.

The decompression rates of 40-50 seconds used.
in evaluation of the crew masks do not represent.
the maximum possible, but were decided upon
after review of the calculated pressure loss rates
for the B-707-131 (40 seconds) following loss of
a cabin window, as presented to the FAA relative
to mask approval.

IV. Conclusions.

The 1,100 cc tidal and 30 liters/minute
volumes specified for passengers in the Federal
Aviation Regulations were not duplicated by ex-
ercise. ‘The 1,100 cc tidal volume was attained
well in advance of the 30 liters/minute volume.

This does not imply that the regulation is in-
adequate, since all phase dilution masks, exhibit-
ing an acceptable level of leakage and providing
no less than 83.8 mm Hg inspired tracheal oxy-
gen partial pressure, maintained subjects in a sat-
isfactory physiological condition.

The characteristics of the phase dilution mask
and the continuous high flows at the maximum
altitude were sufficient to provide adequate pas-
senger protection at tidal volumes imposed by
exercise well in excess of those required by the
Federal Aviation Regulations.

A mask design which allowed existence of
significant and uncontrolled openings around the
periphery of the mask was found to be unaccept-
able. Following redesign of the facepiece and
valving, the mask provided adequate protection.

Compared to tracheal oxygen partial pressure,
oximetry appeared to be a more variable but no
less reliable index of the physiological condition
of the mask wearer. Tracheal oxygen partial
pressure appears to be a more reliable and practi-
cal index of passenger mask performance.

During rapid decompression from 8,000 to
40,000 feet in 49 to 51 seconds, crew mask don-
ning was accomplished in 29 to 34 seconds from
the start of decompression at an altitude of from
28,000 to 31,000 feet. Donning was accomplished
in 2.5 to 6.3 seconds following the donning signal.



AM 67-3

DEVELOPMENT OF TECHNIQUES FOR EVALUATING
THE PHYSIOLOGICAL PROTECTIVE EFFICIENCY
OF CIVIL AVIATION OXYGEN EQUIPMENT

Ernest B. McFadden, M.S.

Approved by Released by
é
J. Rozert Dittz, M.D. P. V. Srzezn, M.D.
Curzr, Civ. AzROMEDICAL Fevers Arr Surcron
InstiruTe
April 1967

Office of Aviation Medicine

FEDERAL AVIATION ADMINISTRATION



PASSENGER MASK EVALUATION
40+

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

357

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ALTITUDE (thousand feet)

hoon PASSENGER MASK
DON CREW MASK

0 q q q q T
10) 10 20 30 40 50 60
TIME (minutes)

Ficure 2. Basic altitude profile as designed to minimum altitude increments and duration of NAS 1179. Oximeter,
minute and tidal volume baselines were continued until stabilized and were of variable duration.

10


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severe changes in respiratory and blood gas com-
position and chemistry.

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 exposure
time must be carefully considered in relation to
the use of exercise.

In an altitude experiment, using jet transport
flow rates and masks, true inboard leakage can
normally be determined only at the 40,000 foot
level or with an oxygen flow sufficient to prevent
depletion of oxygen in the reservoir bag during
peak inspiration. At altitudes below 40,000 feet
the reduced flow into the mask may be deliber-
ately diluted by introduction of air through the
ambient air valve following depletion of oxygen
in the reservoir bag. When determining true
leakage the reservoir bag must be visually ob-
served to confirm that it is not being emptied by
the subject’s tidal volume. If, however, there
are significant and uncontrolled openings around
the periphery of the mask, ambient air may be
drawn into the mask during peak inspiration
rather than through the non-return valve of the
reservoir bag.

Continuous recording of mask nitrogen may
indicate a sharp spike of up to 60-70% nitrogen.
Continuous or transient dilution levels of alveolar
oxygen are indicated if the spike is preliminary to
establishment of a new and higher continuous
level of dilution in the mask, or there is a sig-
nificant nitrogen washout of the portion of am-
bient air introduced by a single leak during one
inspiration.

Conversely, a simple spike without washout
indicates the leakage has only penetrated the
mask and anatomical dead space, having no ef-
fect on the alveolar p0..

At lower altitudes or increased tidal volumes,
oxygen flow must be sufficient, so that the propor-
tion of ambient air introduced does not lower the
tracheal oxygen partial pressure below the speci-
fied level.

Determination of end expiratory nitrogen also
provides a convenient estimation of dilution and
calculation of tracheal oxygen partial pres-
sure®?:24,

Considering the 1,100 ce volume of the reservoir
one would deduct that a tidal volume in excess
of 1,100 ce would result in introduction of am-
bient air during inspiration. However, at the

maximum flow of 30 liters/minute BTPS at
40,000 feet, an additional 1000 cc BTPS may be
introduced into the reservoir during the two sec-
onds of inspiration.

In general, the ear oximeter, heart rate and
impedance pneumograph readings were stabilized
by exercise.

The wandering fluctuation of the ear oximeter
was pronounced at 14,000 feet breathing air at
rest, making establishment of baselines difficult,
and generally was more variable than the calcu-
lated tracheal oxygen partial pressure.

Ground level evaluation of a passenger mask
design*® is also valuable in predicting the per-
formance of a passenger mask at altitude. Deter-
minations of mask leakage and tracheal partial
pressyres in one previous study at ground level®
indicated that a mask design was inadequate.
Subsequent rapid decompression of six subjects
from 12,000 to 40,000 feet in 36-47 seconds wear-
ing this mask design was carried out during
resting and exercise, Subjects wearing the
mask and lightly exercising lost useful conscious-
ness in 36-98 seconds as determined by inability
to maintain sequential counting and the onset,
frequency and duration of encephalographic
slow waves. Three of the resting subjects dem-
onstrated loss of useful consciousness in 67-250
seconds. One showed 6-7 cycle encephalographic
slow waves only and two had no indication of
loss of useful consciousness or encephalographic
changes. One additional subject decompressed
from 12,000 to 35,000 feet in 36 seconds exhibited
intermittent bursts of 6-7 cycle slow waves, pro-
gressing to 3-4 cycle bursts, and returning to
6-7 cycle bursts, with no loss of useful conscious-
ness detected.

This mask design exhibited leakage of sufficient
magnitude that in only one instance, even when
resting quietly, did the calculated tracheal p0.
meet the minimum requirements of the Federal
Aviation Regulations.

Standards for jet transport passenger mask
performance are specified in terms of inspired
tracheal oxygen partial pressures. In terms of
physiological adequacy determination of alveolar
p0. is a more desirable measure. Measurement
of this parameter is generally not possible in the
evaluation of passenger masks without compro-
mising the performance characteristics of the
mask. Polarographic oxygen sensors are being
developed which hopefully may have sufficient


and characteristics of the regulator. In addition,
compatibility with eyeglasses and/or smoke
goggles, peripheral visibility, comfort and com-
munication must be considered.

Light, non-pressurized, supercharged aircraft

* are also being designed and manufactured with
increased altitude capability. Manufacturers of
this type of aircraft are re-assessing current
available oxygen equipment searching for new
equipment acceptable for use by relatively un-
trained private pilots and passengers. On Janu-
ary 11, 1966, at Upland, California, a stock model,
single engine, Cessna Turbo-System Centurion set
a new world altitude record for light aircraft of
39,334 feet. The flight was witnessed by repre-
sentatives of NASA and recognized by the Fed-
eration Aeronautique Internationale, which cer-
tifies records in aviation.

This increased altitude capability of unpres-
surized light aircraft places a greater moral re-
sponsibility upon the oxygen equipment manu-
facturers, airframe manufacturers, aircraft
dealers, medical organizations, and the Federal
Aviation Administration, to provide the private
pilot with simple, reliable and adequate oxygen
equipment, plus a widespread and higher level
of training and education in its utilization.

I. Methods.

A. Passenger Masks. Utilizing a variety of
techniques, a number of evaluations of newly de-
veloped prototype passenger masks have been
conducted. One of these was designed around
minimum altitude increments and dwell time at
altitude permissable under the National Aero-
space Passenger Mask Standard (NAS 1179).
The two specified alternative methods of deter-
mining mask performance (inspired tracheal oxy-
gen partial pressure and oximetry) were simul-
taneously and continuously monitored and re-
corded. In addition, an attempt was made to
maintain the 1,100 cc tidal and 30 liter/minute
volumes specified in the Federal Aviation Regu-
lations, using light exercise instead of voluntary
hyperventilation.

The altitude chamber flight profile is shown in
Figure 2.

After a preliminary test of the subject’s capa-
bility to equalize ear pressures, the subject rested
quietly at 10,000 feet until air-breathing 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. Six subjects were in-
strumented as shown in Figure 8. A chamber
safety observer accompanied each subject. Im-
mediately following crew mask donning, exercise
on a bicycle ergometer was initiated. The exer-
cise level in RPM (Speed) and Watts (Load)
was increased or decreased to stimulate and ob-
tain the desired respiratory activity (approxi-
mately 25-80 liters/minute BTPS). This is re-
garded as a light to moderate workload approxi-
mately equivalent to walking at 3.0 to 3.5 miles
per hour.

Exercise was continued until the desired min-
ute volume as indicated by a calibrated dry gas
meter was obtained and stabilized. A mass flow-
meter (Technology, Inc. Model MFM-150-1)
located 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 denitroge-
nated 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 held his breath, removed the crew mask,
and rapidly donned the prototype passenger
mask as shown in Figure 4. The flow of oxygen
to the mask was regulated by an altitude sensi-
tive regulator of the type used in multi-passenger
oxygen systems of jet transport aircraft. The
flow from this regulator instead of being trans-
mitted directly to the subject was first routed
outside the chamber through a flowmeter and
needle valve arrangement in order to obtain pre-
cise measurement and control of flow.

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 end expiratory mask nitro-
gen. These instruments exhibit an initial re-


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DEVELOPMENT OF TECHNIQUES FOR EVALUATING THE
PHYSIOLOGICAL PROTECTIVE EFFICIENCY OF
CIVIL AVIATION OXYGEN EQUIPMENT

Ernest B. McFadden, M.S.

With the increased altitude capability of civil
aircraft there has been a continuing and cor-
responding evolvement of new oxygen systems
and masks.

The introduction of post World War IT pres-
surized commercial transport aircraft flying at
higher altitudes resulted in development of the
KS passenger mask by United Air Lines. De-
signed to meet physiological requirements for
supplementary oxygen and decompression to
25,000 feet, the mask was evaluated and intro-
duced by Tuttle, Marbarger and Luft in 1951.%

The impending introduction of commercial jet
transports flying at altitudes of up to 41,000 feet
stimulated a hurried development of passenger
oxygen systems and masks in the years between
1955 and 1959. 3% 1223 Blockley* describes
some thirteen mask evaluations conducted at alti-
tude during this period of time.

The possibility of decompression to the maxi-
mum flight altitude of this generation of aircraft
dictated that passenger systems be capable of pro-
viding oxygen concentrations approaching 100%.
This fostered the design of new types of con-
tinuous flow reservoir oxygen masks and auto-
matically actuated presentation systems.

Studies of the physiological effects of rapid
decompressions on passengers using this passen-
ger mask design have been conducted by Donald-
son® and Bryan.*

Disregarding inboard leakage, the continuous
flow reservoir or phase dilution mask provides
100% oxygen at the most advantageous point in
the respiratory cycle, that is, at the beginning of
inspiration. If the oxygen flow is too low and
the reservoir bag volume inadequate to accom-
modate the wearer's tidal volume, dilution, which
occurs, does so toward the end of inspiration.
Dilution so occurring may involve only the mask
and anatomical dead space (Figure 1).

Assuming an adequate oxygen flow, the pri-
mary determinant of mask efficiency of this mask
design becomes the existence and extent of in-
board mask leakage and its effect on the inspired
tracheal oxygen partial pressure. One basic dis-
advantage of all continuous flow oxygen systems
is their inability to adjust automatically to the
respiratory changes associated with variations in
the emotional and physical activity of the wearer.

Concern with this problem is reflected in the
Federal Aviation Regulations, 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
with a 80 liter (BTPS) minute volume for alti-
tudes of 18,500 to 40,000 feet.

Standards for materials, testing and perform-
ance of this type of mask were set forth in a
National Aerospace Standard (NAS 1179) in
1959.2. This subsequently became a part of FAA
Technical Standard Order C-64.

Crew oxygen masks used in commercial jet
transports have also undergone evolutionary
change. Early versions of quick-donning masks
were designed to be worn in the ready position
on the person over the sternum. Crews com-
plained that the masks, when worn in the ready
position, compromised comfort and activity.
Complaints have also been received that masks
so worn soon become unsanitary depositories for
food crums, spilled coffee, cigarette ashes and
constitute a fire hazard.

In recent years the trend has been toward the
quick-donning, hanging crew mask which is sus-
pended in the cockpit within reach of the crew-
member.

The possibility of rapid decompression dictates
the importance of design as related to the ease
and speed of donning, composition and volume
of gases in the mask and breathing tube,® * %7 2°



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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