Most military aircrew undergo hypoxia training, including experience in decompression chamber, to understand the ill-effects of the silent but deadly hazard of in-flight hypoxia . Symptoms experienced during the classic mask-off hypoxia indoctrination in altitude/decompression chamber correlates well with reported in-flight symptoms . The objectives of hypoxia indoctrination, an effects-based training, is to make aircrew aware of their individual symptom complex, particularly their early specific symptoms a.k.a. ‘Hypoxia Signature’ . This is to help the hypoxic but hypoxia trained aircrew to take prompt corrective action, within the time of useful consciousness, to ensure safe flight .
Frequency of hypoxia training varies from once in a life time to refresher training every 3 to 5 years in different air forces. Depending on the availability and objectives of hypoxia training, decompression chamber is useful for not just simulating hypobaric hypoxia but also demonstration of rapid or explosive decompression, besides simulating pressure changes and its likely effects on semi-closed cavities. However Reduced Oxygen Breathing Devices (ROBD), simulating normobaric hypoxia, have proven to be cost effective for demonstrating hypoxia alone, besides being economical .
Reportedly during incidences of in-flight hypoxia, those aircrew (31-75.9% [5, 6]) who have undegone hypoxia indoctrination are found to recognise their symptoms easily as compared to those who have not [5, 6, 7]. The latter group also has increased suscpetibility to loss of consicousness due to hypoxia [cited in 5], worst of such cases being the tragic loss of Helios Airways Flight 522 on 14 August 2005 .
However there are certain assumptions about successful outcome of hypoxia training. They are :-
- Aircrew shall be able to remember the symptoms they experience when hypoxic.
- Symptoms that they attribute to hypoxia actually reflect hypoxia experienced during training.
- Accuracy of recall of their individual ‘hypoxia signature’ does not decline with passage of time, until the next refresher training.
Moreover, there are caveats that :-
- Spectrum of “amnesia, confusion, sleepiness, and other features of cognitive impairment may limit the accuracy” for recall or knowing about the threat of in-flight hypoxia. Say, an aircrew may develop significant hypoxic symptoms during training or in-flight but if he/she does not remember them in detail, it may not help in recognising the imminent threat of hypoxia.
- Common symptoms elaborated during didactics may be confused with individual’s ‘hypoxia signature’, resulting in recall bias.
Recall of hypoxia signature, the aim of hypoxia training, during in-flight episodes of hypoxia has proven usefulness [3, 5, 6]. It must be remembered though that not all aircrew may show symptoms of hypoxia . Woodrow et.al. undertook an interesting study “to evaluate the correlation of symptoms experienced during hypoxia training and recall of symptoms from the training sessions 5 years previously” . They reported that the commonest symptoms of hypoxia at simulated altitude of 25,000 ft were dizziness, lightheaded/dizzy, mental confusion; tingling in fingers/toes, and visual symptoms. Other frequently reported symptoms at 25,000 ft simulated altitude include hot/cold flushes, air hunger, euphoria, numbness, apprehension, fatigue, nausea, increased breathing, headache, etc. .
Interestingly, Woodrow et. al., though suggesting that “memory of symptoms is somewhat fragile”, concluded that such an effects-based training of symptoms by regular refresher training is an effective method “to train the proper response to a potentially dangerous situation” . This once again proves the necessity for periodic hypoxia training for safe military flying.
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6. Files DS, Webb JT, Pilmanis AA. Depressurization in military aircraft: rates, rapidity, and health effects for 1055 incidents. Aviat Space Environ Med 2005; 76: 523–9.
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8. Helios Airways Flight 522
9. Royal Australian Air Force . Aviation medicine for aircrew, 2nd ed. Adelaide, Australia : Institute of Aviation Medicine; 2002: 41 – 8
10. Woodrow AD, Webb JT, Wier GS. Recollection of hypoxia symptons between training events. Aviat Space Environ Med 2011; 82: 1143-7.
Acknowledgement Image courtesy Wikimedia Commons