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. 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