Current military aircraft Liquid Oxygen (LOX) systems supply 99.5 pct. gaseous Aviator's Breathing Oxygen (ABO) to aircrew. Newer Molecular Sieve Oxygen Generation Systems (MSOGS) supply breathing gas concentration of 93 to 95 pct. O2. The margin is compared of hypoxia protection afforded by ABO and MSOGS breathing gas after a 5 psi differential rapid decompression (RD) in a hypobaric research chamber. The barometric pressures equivalent to the altitudes of 46000, 52000, 56000, and 60000 ft were achieved from respective base altitudes in 1 to 1.5 s decompressions. During each exposure, subjects remained at the simulated peak altitude breathing either 100 or 94 pct. O2 with positive pressure for 60 s, followed by a rapid descent to 40000 ft. Subjects used the Tactical Life Support System (TLSS) for high altitude protection. Subcritical tracking task performance on the Performance Evaluation Device (PED) provided psychomotor test measures. Overall tracking task performance results showed no differences between the MSOGS breathing O2 concentration of 94 pct. and ABO. Significance RMS error differences were found between the ground level and base altitude trials compared to peak altitude trials. The high positive breathing pressures occurring at the peak altitudes explained the differences

Nesthus, Thomas E.

Oakley, Carolyn J.

Schiflett, Samuel G.

English

NASA Technical Reports Server

N92-22349
TRACKING PERFORMANCE WITH TWO BREATHING OXYGEN CONCENTRATIONS
AFTER HIGH ALTITUDE RAPID DECOMPRESSION
Thomas E. Nesthus, Ph.D.
KRUG Life Sciences
San Antonio Division
P.O. Box 790644
San Antonio, TX. 78279
Samuel G. Schiflett, Ph.D.
and Carolyn J. Oakley
AL/CFTO
Brooks AFB, TX.
78235-5000
ABSTRACT
Current military aircraft Liquid Oxygen
(LOX) systems supply 99.5% gaseous
Aviator's Breathing Oxygen (ABO) to
aircrew. Newer Molecular Sieve Oxygen
Generation Systems (MSOGS) supply
breathing gas concentrations of 93-95%
oxygen. This study compared the margin
of hypoxia protection afforded by ABO
and MSOGS breathing gas after a 5 psi
differential rapid decompression (RD) in
a hypobaric research chamber. The
barometric pressures equivalent to the
altitudes of 46,000, 52,000, 56,000, and
60,000 ft were achieved from respective
base altitudes in 1-1.5 s
decompressions. During each exposure
subjects remained at the simulated peak
altitude breathing either 100% or 94% O z
with positive pressure for 60 s,
followed by a rapid descent to 40,000
ft. Subjects used the Tactical Life
Support System (TLSS) for high altitude
protection. Subcritical tracking task
performance on the Performance
Evaluation Device (PED) provided
psychomotor test measures. Overall
tracking task performance results showed
no differences between the MSOGS
breathing oxygen concentration of 94%
and ABO. Significant RMS error
differences were found between the
ground level and base altitude trials
compared to peak altitude trials. The
high positive breathing pressures
occurring at the peak altitudes
explained the differences. Considered
with the physiologic data, an acceptable
degree of hypoxia protection was met
with both oxygen concentrations using
TLSS at altitudes <60,000 ft for <60 s
durations.
INTRODUCTION
In both the US Navy and the US Air
Force, there is increasing interest in
Molecular Sieve Oxygen Generation
Systems (MSOGS) for their logistic and
reliability advantages when compared to
liquid oxygen supplied aircraft
breathing systems. A limitation in the
maximum oxygen concentration attainable
with MSOGS, however, has motivated USN
and USAF development communities to
establish laboratory evidence of the
acceptability of using reduced breathing
oxygen throughout the altitude envelope
of current aircraft oxygen systems.
Based upon a fairly well developed
theory of respiratory gas exchange at
altitude, our team of researchers
concluded that there was no reason to
expect adverse effects of MSOGS oxygen
concentrations at normal cabin
pressures. However, after a rapid loss
of cabin pressure while flying at
emergency ceiling altitudes needed
further investigation. Especially, if a
reduction of oxygen concentration is
expected in the breathing gas supplied
to the aircrew. We therefore,
incorporated a rapid decompression (RD)
profile in our study.
The first phase of research employed
the current production oxygen system
including: the CRU-73 dilution-demand
breathing regulator and it's oxygen
delivery/breathing pressure schedule;
the MBU 12/P oxygen mask and HGU 55-P
helmet. The RD profile was across a 5
psi differential, from 20,000 to 50,000
ft, and remained at peak altitude for 60
s. Results of this phase of research
were reported elsewhere (Bomar, et. al,
1988; Holden, et. al, 1987; Nesthus, et.
al, 1988; Nesthus and Schiflett, 1989;
Wright, et. al, 1988; Wright, et. al,
1990).
During the second phase of study we
used a developmental life support system
designed to improve high altitude and
high acceleration protection. The
Tactical Life Support System (TLSS)
included a modified CRU-73/TLSS
dilution-demand oxygen regulator with an
adjusted oxygen delivery and breathing
pressure schedule. Also, a TLSS helmet,
mask, and counterpressure jerkin-vest
590
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system was used to allow breathing gas
delivery at much higher positive
pressures needed for high altitude
protection.
Our altitude profile simulated loss
of cabin pressure while flying at
various potential emergency flight
ceilings. The profile incorporated a 5
psi differential RD similar to Phase I
research but we included 4 different
base-to-peak simulated altitudes seen in
Table I.
Both phases of study were conducted
in the hypobaric research chambers at
the USAF School of Aerospace Medicine
(USAFSAM), Brooks AFB, Texas.
Table I: Four base-to-peak, 5 psi
differential rapid decompression profile
pressures and simulated altitudes.
Pressure (torr) Altitude (ft)
Base peak Base Peak
364.4 - 105.9 = 19,000 - 46,000
340.0 - 79.5 = 20,800 - 52,000
321.3 - 65.7 = 22,000 - 56,000
307.9 - 54.2 = 23,000 - 60,000
METHOD
Our subject population was comprised
of 17 chamber-qualified active duty male
volunteers from the USAFSAM Altitude
Panel. The voluntary fully informed
consent of the subjects used in this
research was obtained as required by AFR
169-3.
In addition to measuring a number of
physiologic parameters, discussed in
detail in the Phase I research
references, a computer-based unstable
tracking task from the Performance
Evaluation Device (PED) provided two
psychomotor measures (Systems Research
Laboratory, 1987). The tracking tasks'
instability was based on an algorithm
similar to that of the subcritical
tracking task (Jex, 1967). RMS offset-
from-center error and the number of
boundary hits were the primary measures
of tracking performance. Subjects were
trained to perform the task while inside
the chamber environment wearing the TLSS
ensemble with most of the physical
distractors in place. Sessions with
high positive breathing pressures were
also included.
Figure 1 shows a generic altitude
profile and time line for one
experimental RD session.
During a 1 hr 100% 02 prebreathe for
decompression sickness prevention, one
performance task warm-up and trials 1
and 2 were conducted. An ear and sinus
check and an abdominal gas check were
made before holding at the base
altitude. Pre-RD physiological
recordings and trials 3 and 4 were
completed. Prior to the RD, the
breathing gas mixture was switched from
100% oxygen to a pre-RD mixture of 02
representative of the CRU-73's scheduled
dilution mixture for each particular
base altitude. Subjects breathed this
mixture for 2-3 minutes for pulmonary
equilibration. The base altitude
breathing gas mixture and the peak
altitude oxygen condition for each
experimental trial was unknown to the
subject. After a final "ready" was
communicated, the subject was cautioned
to breath normally. Then the hypobaric
chamber was rapidly decompressed
(approximately 1 s) to a simulated peak
altitude of either 46,000 ft, 52,000 ft,
56,000 ft, or 60,000 ft. The positive
breathing pressure at 46,000 ft,
irrespective of the O z condition, was 50
mmHg at the mask. Positive breathing
pressure at the remaining peak altitudes
was 70 mmHg. The subject, initiating
the "Peak" performance task trial ten
seconds after the RD, remained at that
altitude for 50 s more, whereupon the
chamber pressure was increased to a
40,000 ft equivalent (141.18 torr).
When the subject completed the unstable
tracking task a descent to ground level
was made. This procedure was repeated
for each 02 condition and peak altitude.
EON5 II RAPID DEC(D_*IPRE%SIOF_J Pf:_OFILE
i
6 I
L
J \L,
4O 6a _ ,_a
Fig%Ire I: A generic altitude profile for
the EONS II rapid decompression study.
A mixed, random and fixed effects
design was followed. The fixed effects
included: two peak altitude oxygen
conditions--100% O_ and 94% Oz; four
peak altitude conditions--46,000,
52,000, 56,000, and 60,000 ft; and three
trial levels--Ground, Base, and Peak.
Measures analyzed for this report
included: Root-Mean-Squared offset from
center (RMS) and boundary hits or
control losses for unstable tracking
performance; and one physiologic
parameter, oxyhemoglobin saturation
(sao2)•
591
RESULTS
Our overall design analysis revealed
3-way interactions (O2-by-Level-by-Peak
Altitude) for RMS tracking error,
Boundary Hits (BHITS), and SaO 2. These
results were anticipated. Separate
analyses for 02 and Peak Altitude were
conducted and resulted in predominant
Level effects for RMS error and SaO 2.
The former was due primarily to the
combined effects of positive breathing
pressures delivered at the peak
altitudes and potential hypoxia. No
positive breathing pressure was
delivered at ground and base levels.
These results can be seen in Figure 2
for the 100% 0 z condition and in Figure
3 for the 94% 02 condition.
EOFJ5 l i- TRACK INO TASK _MS E_F_OR
D2 - _eo ,,
Figure 2: Mean RMS error by Level and
Peak Altitude for the 100% 02 Condition.
EONS ! q TDACI'< I PSG TASK R_,SS ERDO_
_o d
4o 7
,a
o
Figure 3: Mean RMS error by Level and
Peak Altitude for the 94% 02 Condition.
The Level effect for SaO 2 was primarily
due to high oxyhemoglobin saturations
which occured while breathing 100% 02
during the ground and base level trials
(prior to the RD) compared to high
altitude desaturations which occured at
peak altitudes. This effect is seen in
Figure 4.
The Level analysis revealed an O2-by-
Peak Altitude interaction which is
clearly seen in Figure 5. Least Square
mean t-tests showed that boundary hits
for the 94% O z condition were greater at
52,000 ft compared to 56,000 or 60,000
ft.
Figure 4: Mean minimum SaO 2 percentage by
Level and Peak Altitudes for the 100% and
94% 02 Conditions.
i
EONS II -T_ACKIt',IG TASK
\
\
Figure 5: Mean Boundary Hits for O2 by
Peak Altitude interaction
Figures 6 and 7 are examples of
additional physiologic data showing 5 s
mean PETO2 values (with +/- standard
error) i0 s before and 80 s after RDs to
60,000 ft for the 100% and 94% O_
conditions, respectively. The flgures
show a rapid fall in PO 2 at the RD
(verticle line in figures) followed by
relatively stable values before the
descent to 40,000 ft (at time 60 s in
figures) as an increase in barometric
pressure occurred. The values indicated
subjects were exposed to compensatory
levels of hypoxia as described in the
592
USAF Physiological Training Pamphlet
(Tables 4-3 and 4-5). Any performance
deficit assumed at this level of hypoxia
was confounded with the positive
breathing pressures at peak altitudes
and were probably diminished by the
transient exposure (i.e., <60 s). The
relatively high SaO 2 values seen in
Figure 4 at peak altitudes may also
reflect the transient nature of the
exposure.
_]/J ()_]1} f_ PqA_I[) DECFMPl E55 ON
Figure 6: Mean (5 s epoch) End-Tidal pO
before and after rapid decompressions to
60,000 ft for the 100% 02 Condition.
i °i
40
I,,
_J FUll ft RtF} c1 [hPctJrrf,F_C;_, cl *_
-i
I,I
L_
1
/
Figure 7: Mean (5 s epoch) End-Tidal pO 2
before and after rapid decompressions to
60,000 ft for the 94% 02 Condition.
experienced by the subjects in this
phase of research. We feel the increase
in RMS error was not of a magnitude
which would translate into operational
instability.
The O2-by-Peak Altitude interaction
for the boundary hits measure, as
displayed in Figure 5, demonstrated the
only evidence of a performance decrement
with the 94% 02 condition compared to
the 100% 02 condition. The elevated
mean boundary hits found at 52,000 ft
for the 94% condition were not fully
understood. A thorough investigation of
the data and various post-hoc tests did
not help us explain this effect. No
other performance differences were found
between the 100% and 94% conditions.
CONCLUSIONS
We conclude that unstable tracking
performance was not appreciably
different for the two oxygen conditions
compared. The combined effects of
positive breathing pressure and possible
hypoxia during the peak altitude trials
affected unstable tracking performance
by increasing RMS error. High breathing
pressures were necessary for high
altitude protection and were not present
during the ground or base level trials.
Overall, we believe the TLSS provided an
adequate degree of protection against
hypoxia for both oxygen conditions for
durations less than 60 s at altitudes up
to 60,000 ft as were studied in this
phase of research.
ACKNOWLEDGEMENTS
We wish to express our sincere
gratitude toward key members of the Crew
Systems Branch of the Crew Technology
Division including: Cols John Bomar, Jr.
and Roger Stork, Lt Col Roberta Russell,
Mr Ron Holden, Lts Catherine Wright and
Rob O'Connor for all direction and
support provided to the "cognitive
performance" group during the two phases
of "EONS" research. Additional thanks
are extended to Ed Lee and Irma Baker of
KRUG Life Sciences for their assistance
in running most of the subjects during
this second phase of research.
REFERENCES
DISCUSSION
We believe decrements found in
tracking performance for the Level
effect were due primarily to the
anticipated effects of high positive
breathing pressures delivered to the
mask for high altitude protection.
Confounded in these data, however, are
potential effects due to the transient,
compensatory level of hypoxia
Bomar, J.B., Jr., Holden, R.D.,
O'Connor, R.B., Wright, C.S., and
Nesthus, T.E. "Effect of OBOGS Breathing
Gas Concentrations On Crew Performance
At High Altitude" Aviation, Space and
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Holden, R.D., Bomar, J.B., O'Connor,
R.B., Wright, C.S., and Nesthus, T.E.
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593
breathing gear at high altitude" In
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594


Tracking performance with two breathing oxygen concentrations after high altitude rapid decompression

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