Hypoxia Training for Aircrew: CADO vs. Hypobaric Hypoxia

RESEARCH ARTICLE
Hypoxia Awareness Training for Aircrew: A
Comparison of Two Techniques
Bhupinder Singh, Gordon G. Cable, Greg V. Hampson,
Glenn D. Pascoe, Mark Corbett, and Adrian Smith
SINGH B, CABLE GG, HAMPSON GV, PASCOE GD, CORBETT M, SMITH A.
of decompression sickness (DCS) and barotrauma
Hypoxia awareness training for aircrew: a comparison of two tech(11,18).
niques. Aviat Space Environ Med 2010; 81:857–63.
In response to a number of cases of DCS at the Royal
Introduction: Major hazards associated with hypoxia awareness trainAustralian Air Force Institute of Aviation Medicine
ing are the risks of decompression sickness, barotrauma, and loss of consciousness. An alternate method has been developed which combines
(RAAF AVMED), COMCARE Australia, the Commonexposure to a simulated altitude of 10,000 ft (3048 m) with breathing of
wealth Government occupational health and safety aua gas mixture containing 10% oxygen and 90% nitrogen. The paradigm,
thority, prohibited the exposure of Australian Defense
called Combined Altitude and Depleted Oxygen (CADO), places the
Force aircrew to simulated altitude for hypoxia awaresubjects at a physiological altitude of 25,000 ft (7620 m) and provides
demonstration of symptoms of hypoxia and the effects of pressure
ness training (18). Since hypoxia awareness training is
change. CADO is theoretically safer than traditional training at a simuan essential component of aircrew training, RAAF
lated altitude of 25,000 ft (7620 m) due to a much lower risk of decomAVMED developed and implemented an alternative
pression sickness (DCS) and has greater fidelity of training for fast jet
method to impart such training. This alternate method
aircrew (mask-on hypoxia). This study was conducted to validate CADO
by comparing it with hypobaric hypoxia. Methods: There were 43 subof hypoxia awareness training combined exposure to
jects who were exposed to two regimens of hypoxia training: hypobaric
moderate altitude of 10,000 ft (3048 m) in a hypobaric
hypoxia (HH) at a simulated altitude of 25,000 ft (7620 m) and CADO.
chamber with breathing of a gas mixture containing 10%
Subjective, physiological, and performance data of the subjects were
andUser
90% nitrogen. The paradigm, called Comcollected, analyzed, and compared. Results: ThereDelivered
were no signifi
by cant
Ingentaoxygen
to: Guest
differences in the frequency and severityIP:
of the
24 commonly reported
bined
Altitude
and Depleted Oxygen (CADO), was de146.185.201.107
On: Sun,
26 Jun
2016 03:53:08
symptoms, or in the physiological response, between
the two
types of
Copyright:
Aerospace
Medical
Association
signed to
expose the subjects to a physiological altitude
hypoxia exposure. Conclusions: CADO is similar to HH in terms of the
of 10,000 ft (3048 m), and to provide both a demonstratype and severity of symptoms experienced by subjects, and appears to
tion of hypoxia symptoms and the effects of pressure
be an effective, useful, and safe tool for hypoxia training.
Keywords: aerospace medicine, hypoxia training, respiratory physiolchange. CADO was thought by the authors to be safer
ogy, altitude.
than traditional hypoxia awareness training at 25,000 ft
E
XPOSURE TO ACUTE hypoxia results in the appearance of a variety of symptoms, which may vary
from one individual to another in terms of their severity,
speed, and order of appearance. However, there is considerable consistency in the symptom complex experienced by an individual on repetitive exposure to acute
hypoxia (4,7,12). This consistency of one’s ‘hypoxia signature’, as described by Smith (19), is the basis of hypoxia awareness training imparted to military aircrew
around the world. Aircrew exposed to acute hypoxia
under controlled conditions on the ground become familiar with their own unique hypoxia signature, which
is believed to prepare them better to identify and respond to such symptoms in the air.
The Air and Space Interoperability Council Air Standard on aviation medicine training of aircrew (1), to
which Australia is a signatory nation, mandates that initial aviation medicine/physiology training of aircrew
should include demonstration of the subjective symptoms of hypoxia at 25,000 ft (7620 m). Hypoxia awareness training normally involves a controlled exposure to
reduced barometric pressures in a hypobaric chamber.
Major hazards associated with such training are the risks
(7620 m) in a hypobaric chamber because subjects were
exposed to altitudes well below the threshold of 18,000 ft
(5486 m) considered to be a risk for DCS (8). It also has
greater fidelity of training for fast jet aircrew as hypoxia
is generated while wearing oxygen equipment (“maskon” hypoxia). Reduced oxygen breathing techniques
have been reported as safe, effective, and the preferred
means of training for military jet aviators by other
authors (2).
Despite the perceived advantages detailed above, the
validity of CADO as a hypoxia awareness training tool
for aircrew and its equivalence with the traditional
method of training in a hypobaric chamber with hypobaric hypoxia (HH) were unknown. Studies comparing
reduced oxygen breathing mixtures at sea level have
From the Royal Australian Air Force Institute of Aviation Medicine,
Edinburgh, Australia.
This manuscript was received for review in August 2009. It was accepted for publication in June 2010.
Address correspondence and reprint requests to: Bhupinder Singh,
Royal Australian Air Force Institute of Aviation Medicine, Edinburgh
SA 5111, Australia; [email protected].
Reprint & Copyright © by the Aerospace Medical Association,
Alexandria, VA.
DOI: 10.3357/ASEM.2640.2010
Aviation, Space, and Environmental Medicine x Vol. 81, No. 9 x September 2010
857
COMPARISON OF HYPOXIA AWARENESS TRAINING—SINGH ET AL.
supported the validity of this type of training (15). Controlled double-blind studies of physiological data and
training experiences in hypobaric chambers versus 10%
oxygen and mild hypobaria have not to date been conducted or published. Further research was, therefore,
needed to document the operational and physiological
validity of this new method. Consequently, a study was
commissioned to validate CADO as a tool for hypoxia
awareness training by comparing it with the traditional
method of training with HH. The study was conducted
at RAAF AVMED during 2006 and 2007.
The aim of the study was to compare CADO and HH
as hypoxia awareness training tools. The objective was
to collect, analyze, and compare subjective and objective data from the same cohort of subjects during exposure to HH and CADO. The subjective data comprised
a record of signs and symptoms experienced by the
subjects, while the objective data comprised a noninvasive record of the subjects’ heart rate, arterial oxygen
saturation, psychomotor vigilance task (PVT), and a
paper-based mathematical processing task (MPT) during each exposure.
outside the chamber, were aware of the type of hypoxia.
The research protocols were approved by the Australian
Defense Human Research Ethics Committee as Protocols
303/02 and 439/06. The null hypothesis of the study
was that there are no subjective (symptoms) or objective (psychophysiological) differences between CADO
and HH.
Hypobaric Chamber Configuration
A 10-place hypobaric chamber (Thompsons, Sydney,
Australia) was modified to incorporate a facility for
breathing different gas mixes through panel-mounted
regulators while at simulated altitude. A detailed description of the development of the CADO system and
the chamber modifications installed is beyond the scope
of this paper, but can be found in a previously published
paper (3). However, some explanation of the breathing
gas composition is required here. Two G-size gas cylinders were mounted external to the chamber, one of which
contained dry breathing air (21% O2, 79% N2), while the
other contained a mixture of 10.3% O2 and 89.7% N2
(hereafter referred to as the ‘hypoxia gas’). This mixture
was selected because breathing an inspired fraction of
METHODS
oxygen (FIo2) of approximately 10% at 10,000 ft (3048 m)
Subjects
gives an inspired partial pressure of oxygen (PIo2)
equivalent to breathing air at 25,000 ft (7620 m). This is
There were 43 healthy serving aircrew members of the
calculated to be 49 mmHg using the equivalent air altiRoyal Australian Air Force who volunteered to particitude model based on Dalton’s law of partial pressures and
pate in the study. The aim and methodology of the
correcting
water vapor pressure (47 mmHg at 37°C):
Delivered
by
Ingenta
to: Guestfor
User
research, their role in it, and the potential risks of particiIP:Their
146.185.201.107
On: Sun, 26 Jun 2016 03:53:08
pation were explained to them.
fully informed
PI O 2 (tracheal,
fully humidified) = FI O 2 (PB - 47) Eq. 1
Copyright:
Aerospace
Medical
Association
consent was obtained. All of the subjects
were
questioned about their current health status and were examTo achieve the desired PIo2, or hypoxic dose, of 49 mmHg
ined for medical fitness to undergo exposure to simulated
at 3048 m, where the barometric pressure is 523 mmHg,
altitude and hypoxia in a hypobaric chamber. Those
and rearranging Eq. 1:
with any history of illness, particularly in relation to siFI O 2(10K) = 49 ÷ (523-47) = 0.103, or 10.3%.
Eq. 2
nuses and ears, blood donation, and scuba diving in the
preceding week, and flying above 10,000 ft (3048 m) in
Instrumentation and Physiological Monitoring
the preceding 24 h, were excluded.
Study Design
Volunteer subjects were each exposed to two different
methods of hypoxia training in a hypobaric chamber at
RAAF Base Edinburgh in South Australia [altitude 65.6 ft
(20 m) AMSL]. One hypoxic exposure consisted of HH
at a simulated altitude of 25,000 ft (7620 m), while the
other consisted of a combination of 10,000 ft (3048 m)
simulated altitude while breathing a gas mixture of 10%
oxygen and 90% nitrogen (CADO). The 25,000-ft altitude is widely employed for hypoxia awareness training as it is considered the most effective for demonstrating
the effects of hypoxia and is consistent with the protocols of other investigators (10,14). The type and severity
of the symptoms experienced by the subjects, and their
physiological and psychomotor performance data during hypoxia exposure, were collected, analyzed, and
compared. It was a double-blinded study, where neither
the subjects, nor the chamber instructor, were aware of
the type of hypoxia in any given session. Only the investigators and the chamber operators, all of whom were
858
Prior to each hypoxia exposure, the subjects were fitted with a standard aviator’s oro-nasal P/Q type oxygen mask assembly supported by a cloth helmet. The
mask was checked for any leakage on a test bench. The
subjects were instrumented for noninvasive recording
of single bipolar ECG lead (CM5) for heart rate and
pulse oximetry (peripheral arterial oxy-hemoglobin saturation, Spo2) via a probe on the middle finger of the
non-dominant hand. The ECG and pulse oximetry leads
were connected to a Siemens SC6002XL bedside monitor, which transmitted the physiological data to a computer outside the chamber via telemetry. The data was
displayed in real time in digital and graphical formats,
and was also captured and saved continuously on another computer with Siemens HL7 software (Siemens,
2005), and converted to Microsoft Excel file format with
customized software (ChamberScan, RAAF AVMED).
The mean minimum Spo2 and mean increase in heart
rate were calculated and analyzed using the paired twotailed t-test, with significance level set at the commonly
accepted value of P , 0.05.
Aviation, Space, and Environmental Medicine x Vol. 81, No. 9 x September 2010
COMPARISON OF HYPOXIA AWARENESS TRAINING—SINGH ET AL.
Symptom Analysis
The order of exposure to the two types of hypoxia was
determined via a Random Latin Square Matrix, with
Subjects were asked to mark the symptoms that they
each group having a different chamber instructor for the
had experienced from a list of 46 symptoms, including
two sessions. For each group of subjects the two hypoxia
24 commonly reported symptoms of hypoxia. They also
sessions were separated by a period of at least 24 h to
marked the severity of each symptom experienced on a
rule out a carry-over effect, and the circadian bias was
10-cm linear visual analog scale (VAS) ranging from
controlled by scheduling both hypoxia sessions for each
“nil” to “extreme,” which was then converted to a digigroup at the same time of the day.
tal scale of 0–10 by simple linear measurement. This repBefore each hypoxia session, the oxygen system was
resented the symptom severity score. The group-mean
fl
ushed
with hypoxia gas and air by selecting each in
symptoms severity scores for HH and CADO were comturn
in
the
control room, and the delivery of the correct
pared using a two-tailed t-test, with significance level
gas
concentrations
at the mask level was confirmed usset at P , 0.05. For each symptom, the score given by
ing
a
portable
oxygen
monitor. Selection was then
each individual during HH was subtracted from the
changed to air. Thus, in the beginning, the subjects
score given for the same symptom during CADO, givbreathed air from the external cylinder. Before coming a mathematical indication of how closely symptoms
mencing hypoxia exposure, the subjects prebreathed
were reported by the same individuals in the two test
100% oxygen for a period of 30 min from a second reguconditions.
lator to minimize the risk of DCS. Although the risk of
DCS is only significant during HH, the subjects prePsychomotor Performance Tests
breathed 100% oxygen for both types of hypoxia expoTwo tools were used to measure cognitive performance
sure to ensure that all actions performed by the subjects
under both types of hypoxia: the PVT on a hand-held comwere the same in both cases to maintain the double
puter (PalmOne, Tungston T5) and the paper-based MPT.
blindedness of the study. The altimeter inside the chamPVT is an accepted objective measure of psychomotor
ber was also obscured from the subjects’ view to achieve
performance (20). The PVT measures visual reaction
this objective.
time by providing a visual stimulus to the subject, who
The subjects had the study protocol explained once
then presses a button in response (9). A 5-min version of
again, then they conducted chamber safety checks, after
the PVT (PalmPVT©, version 2.0.1, Walter Reed Army
which the chamber was decompressed at a rate such that
Institute of Research), specifically designed for use on
the time taken to reach the target altitude (10,000 ft for
Deliveredsystem,
by IngentaCADO
to: Guest
personal digital assistants with a Palm operating
andUser
25,000 ft for HH) was 6 min and 15 s in both
IP: 146.185.201.107
26 Jun 2016 03:53:08
has been shown to be a useful option
for field studiesOn:
of Sun,
cases.
At
altitude,
Copyright:
Aerospace
Association the subjects were asked to toggle
fatigue (9). In this study, the device was
modified
to op- Medical
down their masks to prevent mask leaks and entrainerate for 10 min to ensure that the 5-min maximum
ment of cabin air, and the chamber instructor made apperiod of exposure plus set-up time were fully captured.
propriate selection on the regulators to begin the hypoxia
MPT is a paper-based test, modeled on the Automated
exposure with air or hypoxia gas for HH and CADO,
Neuropsychological Assessment MetricsTM Readiness
respectively. During the period of hypoxia exposure, the
Evaluation System, known as “ARES” (version 2.30, Acsubjects performed the PVT continuously, interrupted
tivity Research Services) (17). In this test the subject was
by the MPT at 1-min intervals. In standard hypoxia
provided with a sheet of paper containing two simple
training the subjects are briefed to recover after they
mathematics problems, each containing three singlehave experienced adequate symptoms of hypoxia. This
digit numbers with either an addition or subtraction
is subjectively determined by each subject based on their
sign between them. The subject was required to calcuown response to hypoxia. As the study was conducted
late the answer and then write it in one of the two boxes,
to simulate training conditions, the same brief was given
depending on whether the answer was less or more than
to each subject. After they had experienced sufficient
five. The subjects performed the MPT immediately besymptoms of hypoxia, but no later than a maximum pefore and at each minute interval during exposure to
riod of 5 min, the subjects recovered themselves, or were
hypoxia.
asked/assisted to recover, an action known as “gang
The subjects practiced the PVT and MPT at least three
loading” in aircrew parlance, which resulted in the detimes prior to their first exposure to hypoxia to minilivery of 100% oxygen to the subject. The time of expomize any learning effect, which is generally extinguished
sure to hypoxia was limited to a maximum of 5 min to
after three trials (6). To minimize any confounding
approximate time of useful consciousness at 7620 m,
caused by recency, subjects again practiced the tests imwhich has been documented to be 270 6 96 s (7).
mediately before each hypoxia session. The results of
Once the recovery from hypoxia was complete in all
the PVT and MPT were analyzed by paired two-tailed
subjects, the chamber was recompressed to ground level.
t-test, with the significance level set at P , 0.05.
During descent, the subjects recorded their symptoms
on the visual analog scale provided. After their second
Study Protocol
hypoxia session, the subjects were asked to identify the
type of hypoxia to which they believed they had been
The subjects were divided into groups of no more
exposed in each session, and also compare their percepthan five, which was the maximum number that could
tion of the rate of onset and severity of the symptoms
be handled by the physiological monitoring equipment.
Aviation, Space, and Environmental Medicine x Vol. 81, No. 9 x September 2010
859
COMPARISON OF HYPOXIA AWARENESS TRAINING—SINGH ET AL.
between the two sessions. It was a concern that novice
subjects, with no prior experience of exposure to hypoxia, or to the hypobaric chamber environment, may
exhibit a higher frequency and severity of symptoms
during their first exposure to hypoxia as compared to
the second, and thus introduce a bias in the results of the
study. To determine this effect, the difference in the
mean hypoxia scores for all the symptoms (composite
score) between the first and the second exposure of the
novice subjects was compared with those of the experienced subjects.
the same symptom during HH, subjects on average
recorded their symptoms during CADO to be 0.3 6 0.5
points (on a 10-point scale) greater than during HH.
After the second hypoxia session, a majority (58.14%) of
the subjects was unable to identify, or identified incorrectly, the type of hypoxic exposure. There were 22 subjects (51%) who reported that the symptoms came on
more quickly with the session that was later determined
to be HH, while 20 (46.5%) reported that the symptoms
were more severe with what was HH.
Physiological Data
RESULTS
Fig. 2 shows the time course of change in mean Spo2
for
all subjects during exposure to HH and CADO. For
A total of 43 serving aircrew members (39 men and 4
CADO
exposures, as compared to HH, there was a dewomen) participated in the study; 21 were pilots while
lay of about 45 s in the onset of hemoglobin desaturation
22 were non-pilot aircrew. Their ages ranged from 20 to
(Fig. 2), with the time to reach lowest Spo2 and maxi45 yr (mean 5 29.4 yr) and six of them were smokers. In
mum heart rate (Fig. 3) also delayed by a similar time
the past, 24 had experienced hypoxia awareness trainperiod. These time differences between HH and CADO
ing while 19 had no prior experience of such training.
were statistically significant (P , 0.05 level).
The point at which recovery was initiated by the subSymptom Frequency
ject (‘gang-loading’) and the time for Spo2 to return to
Fig. 1 shows the number of subjects experiencing each
normal, as measured from the start of hypoxia exposure,
of the 24 commonly reported symptoms of hypoxia durwere also similarly delayed in CADO. Despite this deing HH and CADO exposures. The frequency order of
lay, the mean minimum Spo2 reached during HH and
symptoms is seen to be broadly the same in both CADO
CADO was 56.91 6 11.4% and 58.6 6 12.9%, respecand HH. With the exception that ‘feeling warm’ was retively, with no significant difference between the two
ported more commonly in CADO than HH, the five most
conditions (P . 0.05). A comparison of individuals’ Spo2
frequently reported symptoms are the same in both
minima during CADO and HH indicated that Spo2 durDelivered
byfreIngenta to: Guest User
types of hypoxia and they are reported with
similar
ing26CADO
fell 03:53:08
to within 1.5 6 8.4% of each person’s
IP:
146.185.201.107
On:
Jun 2016
quency. The chamber temperature was lower in HH due Sun,
Spo2 minima
during HH.
Copyright:
Aerospace
Medical
Association
to the lower ambient pressure as compared to the CADO.
The second part of the graph shows the Spo2 response
during the recovery phase. Since the subjects initiated
Symptom Severity
recovery at different times, the start of the recovery
Table I shows the group-mean symptom severity
phase for all subjects was synchronized at the time of
score for each of the 24 hypoxia symptoms on a 10-point
gang-loading to calculate mean recovery time. The time
visual analog scale during HH and CADO, indicating
from gang-loading to reaching Spo2 . 98% was not stathat the mean severity of the symptoms and the rank
tistically different between CADO and HH (P . 0.05).
order for the group was very similar in both conditions.
There was no statistically significant difference in the
However, when comparing each subject’s symptom
mean hypoxia scores for all the symptoms (composite
severity score reported in CADO with their score for
score) between the first and the second exposure of the
Fig. 1. Frequency of hypoxia symptoms during exposure to HH and CADO.
860
Aviation, Space, and Environmental Medicine x Vol. 81, No. 9 x September 2010
COMPARISON OF HYPOXIA AWARENESS TRAINING—SINGH ET AL.
TABLE I. MEAN SEVERITY SCORES FOR 24 COMMONEST SYMPTOMS OF HYPOXIA ON A 10-POINT SCALE, AND THE MEAN OF
DIFFERENCE IN SCORES BETWEEN HH & CADO IN EACH SUBJECT.
Mean Severity Scores for the Entire Group
Symptom
HH
CADO
Light Headed
Reactions Slow
Thinking Slow
Concentration Off
Coordination Off
Dizzy
Faint
Warm
Making Mistakes
Numbness
Mentally Tired
Tingling
Hands Shaking
Heart Pounding
Vision Dim
Short of Breath
Weak
Euphoric
Physically Tired
Nervous
Sleepy
Restless
Headache
Irritable
Overall Mean 6 SD.
4.80
4.51
4.46
4.17
3.36
4.00
3.34
3.02
3.14
2.55
2.54
2.65
2.53
2.33
1.97
2.34
2.06
1.71
1.70
1.87
1.48
1.38
0.81
0.78
4.30
3.87
3.76
3.24
2.49
3.55
2.93
3.97
2.31
1.93
2.23
1.80
1.70
2.18
1.76
3.00
2.10
1.30
2.07
1.27
1.97
0.73
0.75
0.42
Mean of Difference in Severity
Scores Between HH & CADO
in Each Individual
0.5 6 2.3
0.3 6 3.0
0.1 6 2.9
0.9 6 2.6
0.9 6 2.6
0.5 6 3.0
0.4 6 2.6
1.0 6 2.9
0.1 6 3.0
0.6 6 2.1
0.3 6 1.9
0.9 6 3.5
0.8 6 2.8
0.2 6 2.0
0.2 6 2.3
0.7 6 2.1
0.0 6 2.0
0.4 6 1.8
0.4 6 1.9
0.6 6 2.2
0.5 6 2.4
0.7 6 2.1
0.1 6 1.9
0.4 6 1.6
0.3 6 0.5
is considered
novice subjects as compared with those ofDelivered
the experienced
by Ingentaence
to: Guest
User to be too small to be of physiological
signifi
cance.
subjects (P . 0.05). It was thus concluded
that
the
lack
of
IP: 146.185.201.107 On: Sun, 26 Jun 2016 03:53:08
hypoxia/chamber experience of someCopyright:
subjects did
not in- Medical Association
Aerospace
Psychomotor Performance
troduce a significant bias in the results of the study.
Reaction time: There was a gradual increase in reaction
Fig. 3 shows the time course of change in mean HR for
time commensurate with the duration of exposure to
all subjects during exposure to HH and CADO; the secboth types of hypoxia. During HH, the mean reaction
ond part of the graph shows the HR response during the
time increased from 296 6 48 ms before exposure to hyrecovery phase. The mean increase in HR during HH
poxia to 417 6 111 ms at the end of exposure, an increase
and CADO exposure was 37 6 13 bpm and 35 6 12 bpm,
of 41.02%. During CADO, the mean reaction time inrespectively, with no significant difference between the
creased from 292 6 45 ms to 401 6 178 ms, an increase of
two (P . 0.05). A comparison of individuals’ increase in
37.33%. There was no statistically significant difference
HR during CADO and HH reveals that, on average,
in reaction time between HH and CADO, both at baseHH increased a person’s heart rate by only 2 6 8 bpm
line and at 1-min intervals (P . 0.05).
more than they experienced during CADO; this differ-
Fig. 2. Mean SpO2 response during exposure to HH and CADO.
Aviation, Space, and Environmental Medicine x Vol. 81, No. 9 x September 2010
861
COMPARISON OF HYPOXIA AWARENESS TRAINING—SINGH ET AL.
Fig. 3. Mean heart rate response during exposure to HH and CADO.
effect on hypoxia, which is independent of the reduction in inMathematical processing: The MPT involved three
spired oxygen tension, and that the equivalent air altitude model
different attributes of cognition: mathematical calculamay be flawed (5). A similar, but smaller, effect may be at work
tion, selection of the correct box to write the answer in,
during short-term hypoxia in the aviation environment.
2. Prebreathing 100% oxygen for 30 min before each exposure
and the time taken to complete the test. MPT was not a
would result in a higher initial PIo2 at 10,000 ft (3048 m) than at
time-limited test and a subject could take any amount of
25,000 ft (7620 m). Oxyhemoglobin saturation would be maximal
time to complete it. Therefore, the first two attributes of
in both circumstances. The authors, however, believe that alveoMPT showed minimal sensitivity to hypoxia due to unlar washout with inspired gas would occur quickly and extra
amounts of dissolved blood oxygen would not be sufficient to
limited time available to the subject and will not be disexplain a 45-s delay.
cussed further. However, the time taken to complete the
3. Any entrainment of chamber air through mask leakage would
MPT increased with time of exposure to HH and CADO.
have a significant impact on FIo2 in the case of CADO, while havThere was no statistically significant difference in the time
ing no effect at all in the case of HH. Despite thorough leak
checks, mask safety pressure is not delivered below 27,000 ft
taken to complete the MPT during CADO as compared
(8230 m) and mask leaks could have possibly occurred with forto that during HH throughout the period of exposure
ward head tilt during psychomotor testing.
(P . 0.05).
Delivered by Ingenta to: Guest User
various techniques of hypoxia
IP: 146.185.201.107 On: Sun, When
26 Jun evaluating
2016 03:53:08
induction
for the purpose of hypoxia awareness trainDISCUSSION
Copyright: Aerospace Medical
Association
ing, minor physiological differences, if any, are conThe results indicate that subjects exposed to the two
sidered irrelevant. A hypoxia awareness training
types of hypoxia experienced similar symptoms, both in
paradigm should be considered valid so long as it
terms of their frequency and severity. The physiological
helps an individual to experience and become familand psychomotor performance tests revealed no signifiiar with the symptoms of acute hypoxia. Hypoxia
cant difference between the two hypoxia regimens. The
awareness training generally aims to encourage recphysiological data revealed that the degree of hypoxia
ognition of early hypoxia symptoms and start of imas indicated by the mean minimum Spo2 and mean inmediate recovery action, rather than rely on late
crease in heart rate was similar in both types of hypoxia
symptoms. From a training perspective it is, therefore,
exposures. However, there was a delay of about 45 s in
more important to show that these early symptoms
the onset of hemoglobin desaturation and heart rate reare reproducible between training methods, as has been
sponse during CADO as compared to HH, which was
proved by this study. Equivalence with the time-tested
statistically significant (P , 0.05). When asked to comgold standard of HH is considered by the authors to be
pare the two types of hypoxia, a majority of the subjects
a reliable method of validating any new method of hywere unable to identify the type of hypoxia to which
poxia awareness training such as CADO.
they believed they had been exposed in each session.
The CADO system is configured to demonstrate
A small number of subjects indicated a perception that
mask-on hypoxia, which is a more realistic simulation of
the symptoms appeared earlier and were more severe
the situation prevailing in aircraft with low differential
in HH as compared to CADO. The reason for this may
or unpressurized cabins (fighter aircraft), where the pihave been the earlier onset of symptoms and oxygen
lot wears the oxygen mask throughout the flight (2). The
de-saturation with HH as compared to CADO menactions required of the pilot to recover from hypoxia
tioned above. The phenomenon is perplexing and its
during CADO training are similar to those required in
exact reasons remain open to debate and further inquiry.
flight. Due to these factors, CADO is considered to be a
However, possible explanations for this might include:
more realistic and higher-fidelity training paradigm for
fast jet aircrew, whereas training in a hypobaric chamber
1. The degree of hypoxia produced by using 10.3% oxygen for CADO
resulted in less severe hypoxia than that produced by HH at 25,000 ft
with HH simulates the transport and maritime patrol
(7620 m). There is a body of literature that discusses physiological
aircraft scenario, in which hypoxia is demonstrated by
differences between hypoxia generated under normobaric and hydropping the mask at 25,000 ft (7620 m), followed by a
pobaric conditions (5,7,16). Conkin and Wessel, in their study of
recovery action of donning the mask.
long-term hypoxia, suggest that barometric pressure per se has an
862
Aviation, Space, and Environmental Medicine x Vol. 81, No. 9 x September 2010
COMPARISON OF HYPOXIA AWARENESS TRAINING—SINGH ET AL.
2. Artino AR Jr, Folga RV, Swan BD. Mask-on hypoxia training for
There are four major advantages of CADO when comtactical jet aviators: evaluation of an alternate instructional
pared with HH. Firstly there is a lower risk of DCS by
paradigm. Aviat Space Environ Med 2006; 77:857–63.
restricting chamber altitude to 10,000 ft (3048 m), well
3. Cable GG, Singh B, Babu S, Smart TL, Pinch I. Combined altitude
depleted oxygen: a new method of aircrew hypoxia training.
below the widely accepted threshold for symptoms (8),
AVMED Report 09A/2006. Adelaide: RAAF Institute of
while still retaining the demonstration of the effects of
Aviation Medicine; 2008.
pressure change. As a consequence, no preoxygenation
4. Civil Aerospace Medical Institute. Hypoxia: the higher you fly…
is required. Secondly, restrictions on air travel after hythe less air in the sky. Pilot Safety Brochure AM-400-90/2.
Retrieved 10 December 2008 from: www.faa.gov/pilots/
pobaric training in order to prevent DCS are not required
safety/pilotsafetybrochures/media/hypoxia.pdf.
(13). Therefore, aircrew members can complete the hy5. Conkin J, Wessel JH. Critique of the Equivalent Air Altitude
poxia awareness training and return to their home base
Model. Aviat Space Environ Med 2008; 79:975–82.
on the same day, saving time and cost. Thirdly, chamber
6. Dinges D, Kribbs NB. Performing while sleepy: effects of
experimentally induced sleepiness. In: Monk TH, ed. Sleep,
instructors can participate in more than one run per 24 h,
sleepiness, and performance. Winchester: Wiley; 1991:97–128.
without risking re-exposures to high altitude. This al7. Gradwell DP. Hypoxia and hyperventilation. In: Rainford DJ,
lows efficiency gains for training organizations. Finally,
Gradwell DP, eds. Ernsting’s aviation medicine, 4th ed. London:
Hodder Arnold; 2006:41–71.
in the case of a medical emergency, simply dropping the
8. Kumar KV, Waligora JM, Calkins DS. Threshold altitude resulting in
mask will terminate the hypoxia session. With HH, the
decompression illness. Aviat Space Environ Med 1990; 61:685–9.
cabin is at 25,000 ft (7620 m) at all times and medical
9. Lamond N, Dawson D, Roach G. Fatigue assessment in the field:
emergencies require administration of 100% oxygen
validation of a hand-held electronic psychomotor vigilance
task. Aviat Space Environ Med 2005; 76:486–9.
through a mask and rapid descent to a safe altitude.
10. North Atlantic Treaty Organization [NATO]. Aeromedical aspects
Large P-values found in the analysis of symptoms,
of aircrew training. In: Proceedings of the NATO Human
physiological measures of Spo2 and heart rate, and psyFactors and Medicine Panel Workshop; 14-18 October 1998;
San Diego, CA. Neuilly-sur-Seine, France: NATO Research and
chomotor measures have allowed the null hypothesis to
Technology Organization; 1998.
be accepted. The degree of physiological hypoxia
11. Ohrui N, Takeuchi A, Tong A, Ohuchi M, Iwata M, et al.
achieved by both CADO and HH, as measured by the
Physiological incidents during 39 years of hypobaric chamber
mean minimum Spo2 and mean increase in heart rate, is
training in Japan. Aviat Space Environ Med 2002; 73:395–8.
12. Pickard JS. The atmosphere and respiration. In: Dehart RL,
similar. Therefore, it is concluded that CADO is similar
Davis JR, eds. Fundamentals of aerospace medicine, 3rd ed.
to HH, and appears to be an effective and useful tool for
Philadelphia: Lippincott Williams and Wilkins; 2002:19–38.
hypoxia awareness training. However, the rate of onset
13. Rush WL, Wirjosemito SA. The risk of developing decompression
of hypoxia in CADO as compared to HH is slightly desickness during air travel following altitude chamber flight.
Delivered
by Ingenta to: Guest
UserEnviron Med 1990; 61:1028–31.
Aviat Space
layed. The reason for this delay is uncertain
and requires
IP: 146.185.201.107 On: Sun,
Jun 2016
03:53:08
14.26
Sausen
KP, Bower
EA, Stiney ME, Feigl C, Wartman R, Clark
further study. Minor physiological differences,
Copyright:however,
Aerospace MedicalJBAssociation
. A closed-loop reduced oxygen breathing device for
do not seem to impact on the symptom experience or
inducing hypoxia in humans. Aviat Space Environ Med
2003; 74:1190–7.
training value of CADO compared to HH.
ACKNOWLEDGMENTS
The authors wish to thank the aircrew members of the Australian
Defence Force for their participation in this study. Special thanks to the
technical staff of the RAAF Institute of Aviation Medicine for making
this study possible.
Authors and affiliations: Bhupinder Singh, Dip.Av.Med., M.D.,
Gordon G. Cable, M.B.B.S., D.Av.Med., Greg V. Hampson, M.B.B.S.,
D.Av.Med., Glenn D. Pascoe, M.B.B.S., D.Av.Med., Mark Corbett, B.Sc.,
M.Tech.(AvHF), and Adrian Smith, D.Av.Med., M.Aerospace (Hons.),
Royal Australian Air Force Institute of Aviation Medicine, Edinburgh,
Australia.
REFERENCES
1. Air and Space Interoperability Council [ASIC]. Aviation
medicine/physiological training of aircrew. ASIC Air Standard
61/101/03. Washington, DC: Air and Space Interoperability
Council; 1990.
15. Sausen KP, Wallick MT, Slobodnik B, Chimiak JM, Bower EA,
et al. The reduced oxygen breathing paradigm for hypoxia
training: physiological, cognitive and subjective effects. Aviat
Space Environ Med 2001; 72:539–45.
16. Savourey G, Launay J-C, Besnard Y, Guinet A, Travers S. Normoand hypobaric hypoxia: are there any physiological differences?
Eur J Appl Physiol 2003; 89:122–6.
17. Short P, Cernich A, Wilken JA, Kane RL. Initial construct validation
of frequently employed ANAM measures through structural
equation modeling. Arch Clin Neuropsychol 2007; 22(Suppl.
1):S63–77.
18. Smart TL, Cable GG. Australian Defence Force hypobaric chamber
training, 1984-2001. ADF Health 2004; 5:3–10.
19. Smith AM. Hypoxia symptoms in military aircrew: long-term
recall vs. acute experience in training. Aviat Space Environ
Med 2008; 79:54–7.
20. Thorne DR, Johnson DE, Redmond DP, Sing HC, Belenke G. The
Walter Reed palm-held psychomotor vigilance test. Behav Res
Methods 2005; 37:111–8.
Aviation, Space, and Environmental Medicine x Vol. 81, No. 9 x September 2010
863