d realis-
ing mask
correla-
ficiency

ese data
number

oxygen

for the
e mask

It is at the higher altitude range of 18,500-
40,000 feet that the principal and critical prob-
lem area exists. Comparison in Table 3 of the
tracheal pO, for an altitude of 40,000 feet to
the values specified in CAR 4b indicate that
the mask does not provide the minimum tra-
cheal pO, as specified, and may not provide
adequate protection at this altitude.

DISCUSSION

Observation of the nitrogen recordings made
during early stages of this study indicated that
excursions of the nitrogen recorder did not
necessarily follow the respiratory cycle when a
constant flow mask was being tested, In fact,
when respiration was experimentally slowed to
an abnormally slow rate, two separate peaks
-were observed; one during inspiration and one
during expiration. Consideration of the design
characteristics of a constant flow mask provides
an explanation of this phenomenon, During
inspiration, ambient air containing nitrogen may
be drawn into the mask, upper respiratory
spaces and lungs through leakage apertures,
thus eliciting a spike on the nitrogen recorder.
During the subsequent respiratory pause, oxy-
gen flowing through the mask at 30 1/min.
(500cc/second) rapidly washes out the rela-
tively small mask dead space and the nitrogen
concentration falls. The second, or expiratory
peak, occurs as a result of nitrogen previously
inspired, since it is not probable that inboard
leakage will have occurred during the previous
respiratory pause due to the continugus flow
of oxygen delivered to the ‘mask, During ex-
piration, the same flow of oxygen is delivered
in addition to the expiratory volume thereby
eliminating any tendency for a negative pres-
sure to develop within the mask which would
permit inboard leakage. 5 /

Frequently, very large, isolated nitrog¢n con-
centration spikes were observed and were usu-
ally associated with some action on the part of
the subject, (e.g, talking, moving the head,

¢
¥

smiling or coughing). If the nitrogen concen-
tration immediately retumed to the previous
level following the respiratory pause, then the
spike was considered to be insignificant as it
was presumed that the nitrogen dilution oc-
curred only in the mask dead space. However,
if the spike was followed by a series of slowly
descending peaks before the former nitrogen
level was reached, it was obvious that a rela-
tively large quantity of nitrogen had reached
the lungs and was in the process of being
washed out.

Use of the minimum nitrogen concentration
for calculation of tracheal pO, is not justified
since the minimum concentration occurs dur-
ing the respiratory pause as a result of the con-
stant flow of oxygen washing out the mask
dead space. In contrast, utilization of the peak
nitrogen concentration is just as unrealistic
since maximum nitrogen dilution may occur
in the mask dead space. The level of pene-
tration of this nitrogen into the respiratory dead
space and lungs may vary.

An integrated mean nitrogen concentration
provides a more realistic value although it is
consistently lower than the concentration of ni-
trogen calculated from oxygen flow require-
ments. A comparison of these two values is
shown in Table 4. However, nitrogen anal-
ysis remains the more precise method, the oxy-
gen flow data providing only a crude estimate
of the nitrogen concentration,

Although the technique investigated pro-
vides a valid index of mask leakage per se, ad-
ditional data must be acquired at altitude.
This techniqie will be applied to a few trained
subjects at various simulated altitudes in an
altitude chamber. Correlation of nitrogen con-
centration, mask leakage, and blood oxygen
saturation during’ various levels of respiratory
activity will be studied. Information so gained
should assist in increasing the accuracy and
interpretation of this relatively simple and in-

expensive ground level testing technique.
, :

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TABLE 4

Comparison of nitrogen and leakage values calculated from oxygen flow data
to nitrogen analyzer data

0, flow to Min. vol, % Nitrogen % Leakage
Subject maintain (BTPS) From N; From N,
No. bag equil. Um Calculated* Analyzer Calculated** Analyzer
Fl 531/m 16.4 54 36 67.7 83
F-2 18 1/m 21.0 73 64 91.4 ° 14
F-3 5.61/m 18.7 56 57 70.0 59
F4 5.6 1/m 21.0 59 46 73.3 46
F5 10.01/m 19.9 40 20 49.7 26
F-6 5.6 1/m 21.0 59 56 73.3 72
F7 8.5 1/m 12.8 27 28 19.5 36
F-8 11.11/m 19.9 35 26 44.2 34
F9 7.0 fm 16.4 46 34 57.3 44
F-10 7.01/m 17.5 48 49 50.0 63
Mean Female
Subjects 6.7 18.46 49.7 41.6 60.6 53.7
M-1 9.8 1/m 17.5 35 22 44.0 28
M-2 5.6 1/m 14.0 48 45 60.0 58
M-3 5.6 1/m 23.4 61 51 76.0 66
M-4 3.71/m 19.9 65 52 814 67
M5 3.71/m 19.9 65 53 81.4 68
M-6 6.7 1/m 21.0 54 32 68.0 41
M-7 5.6 1/m 25.7 62 42, 78.2 54
M-8 3.7 1/m 25.7 68 48 85.6 62
M-9 3.71/m 16.4 62, 42 714 54
M-10 12.3 1/m 23.4 38 27 474A 35
Mean Male
Subjects 6.0 20.7 55.8 41.4 69.9 53.3
Mean All
Subjects 64 19.6 ; 52.7 415 65.3 53.5
ig = MV — Voge BN, = ’
Volume of N, (¥y,) = volume of ambient air x 80% ‘
Volume of ambient introduced to mask = mingte volume (MV) — volume _of onygen suplied to maintain
reservoir bag equilibrium (V,!,) /
\ ax V/ Volume of Ny (Wye) '
+ “Minute Volume (MV)
**y, = MV—V,, , ;
Volume of ambient leakage (V,) = minute volume (MV) — the volume of oxygen supplied to maintain

reservoir bag e§eilibrium (V,q)

Per cent leakage (%L) =

-15-

minute volume (MV)

volume of ambient leakage (V, )



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the lucite mixing chamber (integrator).

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Ficure 3. Two small nitrogen sampling devices designed for insertion in thé nasal cavity and



Nitralyzers

Ficure 1. Schematic sketch of oxygen mask leakage testing system showing crew mask being

Vacuum Pumps

utilized for pulmonary nitrogen washout. Demand System.

a dilution of the oxygen concentration in the
mask and is immediately evident by continuous
nitrogen analysis.

The use of nitrogen analysis to obtain mask
leakage information appears to be the method
of choice since the nitrogen analyzer provides
accurate, rapid response unavailable in other
gas analysis equipment.’ In addition, unlike
many other gas sampling devices, the nitrogen
analyzer utilized in this study is unaffected by
barometric pressure changes and operates as
efficiently at altitude as at sea level.

A secondary, but integral, phase of this
study is the evaluation of a proposed passenger
oxygen mask design. All data presented were
obtained utilizing a proposed mask design.

The use of specific minimum performance
characteristics based upon tracheal partial pres-
sures, as detailed in Part 4b of the Civil Air
Regulations (CAR 4b), was in part adopted
due to the difficulty involved in accurately
measuring and determining alveolar partial
pressures and blood oxygen saturation. Since
alveolar partial pressure and blood oxygen sat-
uration are directly related and determined by
the inspired or tracheal partial pressure, the
latter value was chosen as a basis for the regu-
lation.

It is recognized that the alveolar partial pres-
sure and blood oxygen saturation may exhibit
a lower or higher value than theoretically cal-
culated from tracheal oxygen partial pressure
based upon nitrogen concentration in,the mask.
However, tracheal oxygen: partial pressure was
used in order to evaluate the proposed mask
design in terms of perfornjance requirements
as specified by CAR 4b,‘

y 3

METHOD _ , }
The leakage testing technique is based upon
a preliminary respiratory washout of pulmonary
nitrogen utilizing aviators’ oxygen (99.5%) as
illustrated in Fig. 1, Following pulmonary ni-
» trogen washout to an asymptotic level, nitrogen’
entering the system is primarily introduced by
inboard eakade of ambient air. Tissue nitrogen
elimination is an exception which normally oc-
_ curs at such a low rate and constitues such a
small percent of the continual gas composition

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= 46 9
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= = j

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Ficure 2, Nitrogen washout. Two separate matched and
calibrated nitrogen analyzers connected symetrically to the

left and right side of ‘crew type oxygen mask.
System. Seale €-1008. Ny.

as to be negligible. Crew type oxygen masks
and_ demand yegulators were utilized for pul-
monary nitrogen washout and the time-course
of the recorded nitrogen observed.

EQUIPMENT

Two Custom Engineering and Development
Company, Model 300AR, nitralyzers were used
in the study. These instruments exhibit an
initial response time of 0.024 seconds, 90% re-
sponse being obtained in 0.044 seconds. At the



| |
REFERENCES
| 1. White, C. S., Lovelace, W. R., Clark, J., 2. Final Report on The Investigation of Mask i
| and Hirsch, F. G., “Recent Advances of In- Leakage in Passenger Oxygen Masks,
strumentation of Interest in Aviation Medi- Pioneer-Central Div., The Bendix Corpora-
cine” in Collected Papers on Aviation Medi- tion, FAA Contract Number FA-885, 8 Feb.
cine, Butterworth’s Scientific Publications, > nr umiber id e.
p. 96, 1955. 1962.
1
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Doc
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ig system. Continuous Flow

System.

Ficurr 7. Schematic sketch of passenger oxygen mask testin,

»

leakage to the extent that the reservoir may act
merely as an extension of the supply line. As
a consequence, the system may then be unable
to furnish the required volume of oxygen de-
manded at peak inspiratory flow, except at
prohibitively large oxygen-supply flow rates.

The test mask used in this study utilized a
reservoir bag of light weight plastic film and a
mask facepiece constructed of semi-rigid
polyethylene.

TECHNIQUE

The preliminary nitrogen washout and sub-
sequent testing is carried out with the subject
walking on a treadmill at a slow rate (1.8
22 mph). Preliminary nitrogen washout is
continued until the curve becomes essentially
asymptotic. The curve in a normal, healthy
individual becomes flat (slope less than .6)
in approximately 5-6 minutes or less, Tidal
and minute volumes are recorded and the
treadmill speed adjusted until the desired res-
piratory response is obtained as specified for
passenger oxygen. equipment utilized at alti-
tudes of 18,500 to 40,000 feet (tidal volume of
1100ce, per CAR 4b),

After pulmonary nitrogen washout is accom-
plished, the subject is instructed to hold his
breath following inspiration. During breath-
holding, the crew mask is removed, the test
mask is donned with oxygen flowing at
30 1/min. BTPS and the test continued as illus-
trated in Fig. 7 at the activity level previously
established. Since in all cases the nitrogen
washout equilibrium ‘is at a lpwer level with
the crew masks than with th¢ test masks, the
nitrogen concentration recording is observed
to climb to a new and greater level, Short
sections of nitrogen analysis, recordings, dyring
which the subject was wearing the proposed
passenger mask, are shown in Fig.'8 and 9,

The oxygen supply flow rate of 301/min.
BTPS is continued for 5-6 minutes and the,
leakage level determined at equilibration. The
Seygen flow is then reduced to 15 1/min, BTPS
(per CAR 4b foraltitudes of 10,000-18,500 ft.)
and the nitrogen recording is again observed
to climb and equilibrate at a new and higher
level.

- -9-

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Ficune 8. Nitrdgen analysis recording of subject F-2
wearing the passenger mask. This subject demonstrated.
the Highest mean nitrogen level. Integrator in the lower
channel. Oxygen continuous flow 30 1/min. Scale 0-100% N,.

The flow of dxygen is then decreased to the
point gt which the reservoir bag can be main-
tained at a volume equilibrium, Oxygen flow
is adjusted and equilibrium obtained when the
reservoir bag is neither allowed to distend or
‘collapse. If the leakage about the face is low,
then the required volume flow of oxygen ap-
proaches the minute volume. If, in contrast,
the leakage is high about the face with am-
bient air being drawn into the mask in prefer-
ence to oxygen from the reservoir bag, then the



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pressure setting used (0.6 mm, Hg) the sampl-
ing rate was 3 cubic centimeters per minute.
The continuous 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 through the mask wall
did not require the addition of signficant weight
(thereby influencing the mask fit) or extensive
modification of the mask (compromising the
operational characteristics of the mask) as may
be the case with other types of gas analysis
equipment. A typical dual nitralyzer record-
ing of pulmonary nitrogen washout in a subject
wearing a crew mask is illustrated in Fig. 2

A small lucite reservoir and mixing chamber,
shown in Fig. 3, was designed and added to one
analyzer channel. In effect, the chamber in-
tegrates the area under the curve of the rapidly
changing nitrogen concentration and provides
a record of the mean mask nitrogen concentra-
tion. The effect on the nitrogen washout may
be seen in Fig. 4. Two small sampling devices
designed for insertion in the nasal cavities are
also shown in Fig. 3 but were not utilized in
this study. The microcatheter sampling tube
openings were orientated directly over the ex-
halation valve.

TEST MASK

Known leakage characteristics are frequently
incorporated in the design of oxygen masks.
Control of tracheal oxygen partial pressure
(pO,) must then take into account’ oxygen
flow rate as related to dilution port size and
area, uncontrolled leakage as'a result of mask
fit, and respiratory activity of the mask wearer.

When nitrogen dilution is not desired and an
inspired oxygen ¢oncentratibn of 99.5%+ Js re-
quired, particular attention must be fdcused
upon providing a sufficient quantity of/oxygen
to meet instantaneous requirements of the peak
inspiratory flow rate. Volume flow during
peak inspiration is normally in excess of three

dimes that of the minute volume. To accom-
modate peak inspiratory flow, several current,
constant-flow, Sygen masks utilize a reservoir
bag, The function of the reservoir bag is to
collect oxygen during the exhalation phase and
‘thereby permit a greater non-diluted inspiratory

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Ficure 4, Nitrogen washout. Integrator added to system
(upper channel), Demand System. Scale 0-108 N,.

Crew oxygen mask.

flow of oxygen than would be possible with a
purely constant flow system. A valve normally
separates the mask from the reservoir so that
the latter doesnot act as a rebreathing bag.
If the mask dogs not provide a good fit to the
face, thereby allowing relatively large areas of
leakage to exist as illustrated in Figs. 5 and 6,
ambient air ‘will be drawn into the mask
through these’ unobstructed leakage sites in-
stead of from the valved reservoir bag. Under
these conditions, the flow dynamics of the sys-
tem approach those of a purely constant-flow
system, since the action of the reservoir bag
is functionally retarded by ambient inboard



AN IMPROVED METHOD FOR DETERMINIING THE
EFFICIENCY OF CREW AND PASSENGER OXYGEN MASKS

ERNEST B, McFADDEN, M.S.
JAMES W. RAEKE, MS.
JOSEPH W. YOUNG, AM.

ABSTRACT

A method of determining oxygen mask leakage as developed under contract F'A-885
between the Federal Aviation Agency and the Pioneer-Central Division of the Bendix Corpo-
ration was evaluated. Measurement of nitrogen concentration within an oxygen mask follow-

ing respirator

nitrogen washout appears to provide a valid index of inboard mask leakage.

Further development of this technique and” its application to a proposed mask design’ is

described.

One of the most significant factors relating
to the efficiency of an oxygen mask is the quan-
titative inboard leakage of nitrogen-rich am-
bient air which dilutes the oxygen supplied to
the wearer. This factor is quite variable, de-
pending upon the design of the mask and the
facial structure of the wearer, In practice, in-
board leakage may be minimized or even con-
verted to outboard leakage by increasing the
volume flow and/or hydrostatic pressure of
oxygen supplied at the mask. Howeyer, com-
pensation for excessive leakage in this manner
results in poor oxygen econdmy.

The ultimate aim of an oxygen mask at alti-
tude is the production of an adequate blood
oxygen saturation, However, in order to evalu-
ate the capabilify of a mask to maintdin an
adequate blood oxygen sdturation in the sub-
ject, the performance of the mask and the re-
sulting saturation must be predicted theoreti-
cally, or the subject must ascend to altitude
and the blood oxygen saturation determined.

#The latter procedure tends to exclude somé of
the subject pogylation and involves large and
expensive facilities. In addition, techniques
for the determination of blood oxygen satura-
tion are quite tedious or limited in accuracy.

’

-1-

The primary purpose of this study was to ex-
tend the investigation and development of tech-
niques of oxygen mask evaluation initiated by
the Pioneer-Central Division of the Bendix
Corporation under Contract FA-885,’ Initial
emphasis is being placed upon developing
methods that may be utilized at ground level,
on relatively large numbers of subjects, widely
distributed in age and physical fitness, Ground
level testing allows the study of subjects typical
of the airline passenger population, many of
which normally could not safely be exposed to
hypoxia and environmental conditions in an al-
titude chamber. In addition, a greater number
oftsubjects may be tested at ground level under
controlled conditions, at a much lower cost than
in an altitude chamber.

Since the pétGentage composition of nitrogen
in the ambient: air at ground level and at alti-
tude (up to 80,000 feet) is quite stable (78.09%),
a reliable standard reference gas of relatively
high concentration exists at all times in the
surrounding ambient environment of the
subject. When the subject is breathing 99.5%
oxygen, any mask inboard leakage results in an
inflow of nitrogen-containing air from the sur-
rounding ambient environment. This produces



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i TABLE 3
; Mean mask nitrogen concentrations obtained by continuous analysis and the tracheal
Po derived by P, = (B- 47) F,
i ; ° °. $
a
4 30 1/m Flow O, +83.3mm/Hg 15 1/m Flow O, +100/Hg
( Cale. Py Pp @N, Cale.P, Py Tidal
Subject %N, oO, O, (Mean) Oo, oO, vol.
j No. (Mean) 40,000 ft. CAR 4b 18,500 ft. CAR 4b (BTPS)cc.
j Fl 10 84.8 +15 20 260 +160 1100
‘ F-2 30 65.9 -174 49 166 + 66 1100
| F-3 ll 83.8 + 0.5 38 201 +101 1100
| F-4 22 73.5 - 98 38 201 +101 1100
F5 11.5 83,3 +0 175 267 +167 1100
| F-6 22 73.5 - 98 45 179 + 79 1100
1 F-7 14 81.0 — 23 21 257 +157 1100 1
‘ F8 12 83.0 -03 36 208 +108 1100 s
| F9 9 86.0 +27 30 227 +197 1000
| F-10 16 79.0 -— 43 29 231 +131 1000
Mean Female
Subjects 15.7 79.4 32.3 219
M-11 8 , 866 + 3.3 18 266 +166 1500
M-12 9 85.7 + 24 27 237 +137 1500
M-13 24 716 -11.7 43 185 + 85 1500
M-14 13 81.9 —14 26 240 +140 1400
| M-15 22 735 - 98 33 218 +118 1500
M-16 12 83.0 — 0.3 22, 253 +153 1400
| M-17 10 84.8 +15 25 244 +144 1200
H M-18 18 77.2 - 61 38 235 +135 1100
M-19 6 88.5 + 52 20 260 +160 1500
M-20 22 73.5%, — 98 26 240 +140 1700
Mean Male ' ‘ ’
Subjects 144 80.6 5 27.8 \ 237
! Mean All y !
Subjects 15.0% 800 | 300% 94  .
| /
1 ¥ . 1



SS i

AN IMPROVED METHOD FOR DETERMINING THE
EFFICIENCY OF CREW AND PASSENGER
OXYGEN MASKS

ERNEST B. McFADDEN, M.S.
Chief, Survival Equipment Section

JAMES W. RAEKE, M.S.
Survival Equipment Section

JOSEPH W. YOUNG, A.M.
‘Anthropology Section

Protection and Survival Branch

A PRELIMINARY REPORT

' ' 62.21 ‘

FEDERAL AVIATION AGENCY
AVIATION MEDICAL SERVICE
AEROMEDICAL RESEARCH DIVISION
CIVIL AEROMEDICAL RESEARCH INSTITUTE
OKLAHOMA CITY, OKLAHOMA



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Ficure 9, Nitrogen analysis recording’ of subject F-9
wearing the passenger mask. This subject demonstrated
one of the lowest mean nitrogen levels. idegate in the
lower channel. Oxygen continuous flow 30 I/min., tidal
volume 1100ce, minute volume 14 1/min. Scale 0-100# N,.

volume flow of oxygen to protluce reservoir
bag equilibrium may represent only a fractign
of the total minute volume. Dual channel re-
cording of the nitrogen concentration was con-
tinued throughout the testing procedure.

In erder to evaluate the procedure, ten fe-
male and ten male gubjects wearing the pro-
posed mask design were tested.

In addition, a survey of facial anthropometry
was performed on all test subjects by standard-
iged and new measuring techniques, This sur-

vey was conducted to obtain reliable and realis-
tic dimensional data to aid in evaluating mask
efficiency by establishing significant correla-
tions of facial structure with level of efficiency
attained.

A formal statistical treatment of these data
was not performed due to the small number
of subjects involved.

RESULTS

Adjustment of the tidal volume in female
subjects to 1100ce was readily accomplished.
Tidal volumes of 1100cc were not readily ob-
tained with male subjects, even when the tread-
mill speed was reduced to its lowest level
(18 mph). Apparently, in males (who nor-
mally exhibit a larger basal tidal volume than
females), even the lowest level of exercise is
sufficient to elevate their tidal volumes to val-
ues in excess of 1100cc. However, it may be
noted in Table 1 that the mean nitrogen con-
centration of female and male subjects appears
to be in close agreement. Total leakage as cal-
culated from the nitrogen composition utiliz-
ing the curve developed by Bendix under Con-
tract FA-885° is presented in Table 2, Nitro-
gen concentration and the calculated tracheal
pO, as related to CAR 4b is presented in
Table 3. Nitrogen percentage obtained by
analysis, as compared to the nitrogen percent-
age calculated from the relationship of the
oxygen flow required to maintain reservoir
equilibrium to the minute volume, is shown in
Table 4. Nitrogen concentrations calculated
from flow data are less precise and over-
estimate the values obtained by actual contin-
uous nitwogen analysis.

With the treadmill adjusted to the slowest
speed, the tidal volume could not be reduced
to the 700ce specifié#th CAR 4b (for altitudes
of 10,000-]8,500 ft.). It may be noted in
Table 3 that the specified tracheal pO, is read-
ily obtained and exceeded (even when a tidal
volume of 1100cc is maintained and the oxygen
flow reduced to 15 I/min.). Therefore, reduc-
tion of the tital volume from the 1100cc to
700cc was not deemed necessary, With the
flow rates and tidal volumes specified for the
altitude range of 10,000-18,500 feet, the mask
provides adequate protection.

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