The utility and safety of hypoxia experiences for rebreather divers

Abstract

Background: Aircrew training often includes an hypoxic experience aimed at improving symptom recognition and selfrescue in a subsequent hypoxic event. Similar training has been advocated for rebreather divers. We investigated the effect of a prior hypoxic experience on actual and perceived cognitive function during subsequent hypoxia and measured the physiological responses to severe progressive hypoxia. Methods: Twenty-five subjects underwent two hypoxic hypoxia experiences (trials one and two) approximately five weeks apart. Subjects breathed 5.5% oxygen whilst performing a playing card recognition test. The primary endpoint was the time taken to make three consecutive errors in the card recognition test (time of useful consciousness, TUC). Secondary endpoints were the total number of errors made, accuracy of error recollection and physiological variables. Results: Mean (SD) TUC was 166 seconds (37) and 169 s (35), and subjects made 8.9 (2.4) and 7.8 (2.0) errors in trials one and two respectively. Error recall was identical between trials with participants failing to recall 6 (3) and 6 (2) errors made in trials one and two respectively. Across both trials mean nadir arterial blood and cerebral oxygen saturations were 52% and 49% respectively. The mean (SD) increase in heart rate was 42 (16) beats•min-1. Conclusion: An hypoxic experience did not improve cognitive performance or subject insight into performance in a second exposure five weeks later. Hypoxia imposes a significant physiological stress which may be hazardous in unscreened, nonmedically supervised subjects. Hypoxia experience training is not recommended for rebreather divers at this time.

Hypoxic Hypoxia and Brain Function in Military Aviation: Basic Physiology and Applied Perspectives

“…Complex and novel cognitive task performance may be impaired between 6,500 and 12,000 ft, which typically invoke an SpO 2 of 70–90% ( Legg et al, 2012 , 2014 ; Petrassi et al, 2012 ; Pilmanis et al, 2016 ). Whereas, simple cognitive task performance (e.g., card naming and/or sorting) may not deteriorate until below an SpO 2 of 65% ( Hoffman et al, 1946 ; Mitchell et al, 2019 ), which typically occurs following exposure above 18,000–25,000 ft. Although the relevance of these cognitive deficits to military aviation is difficult to interpret, operational tasks have been impaired by hypoxia, such as simulated flight performance ( Temme et al, 2010 ; Robinson et al, 2018 ).…”

“…Recognising hypoxia before profound cognitive impairment is critical for implementing emergency recovery procedures. Since hypoxia impairs the ability to identify cognitive impairment within oneself ( Mitchell et al, 2019 ), the capacity to recognise hypoxic symptoms is also compromised ( Asmaro et al, 2013 ; Rice et al, 2019 ). Moreover, hypoxia can be insidious and include pleasant sensations, such as euphoria, decreased inhibitions, and a strong sense of wellbeing, which will attenuate any perception of urgency.…”

“…The validity of the TUC criterion has been debated since its inception ( Hoffman et al, 1946 ; Izraeli et al, 1988 ) as TUC endpoints differ between studies and often fail to reflect the demands of operational environments. These endpoint tasks have included: card sorting ( Hoffman et al, 1946 ), card recognition ( Mitchell et al, 2019 ), single and choice reaction speed ( Hall, 1949 ), two-digit number addition ( Izraeli et al, 1988 ), sequential numeric writing ( Yoneda et al, 2000 ), handwriting ( Yoneda and Watanabe, 1997 ), behavioural disturbances ( Malle et al, 2016 ), and the magnitude of hypoxaemia ( Hoffman et al, 1946 ; Malle et al, 2016 ). Whether these provide an accurate estimate of time to recognise hypoxia and implement emergency recovery procedures is uncertain.…”

“…TUC may also be extended by: (1) oxygen pre-breathing ( Malle et al, 2016 ), which increases oxygen stores in the lungs ( Tanoubi et al, 2009 ); (2) greater haemoglobin oxygen carrying capacity of the blood ( Hall, 1949 ); and (3) avoiding physical activity during exposure to hypoxia ( Busby et al, 1976 ). Nevertheless, TUC does not appear to be extended by previous hypoxia exposures, suggesting it is not trainable ( Izraeli et al, 1988 ; Mitchell et al, 2019 ).…”

Associations between spontaneous electroencephalogram oscillations and oxygen saturation across normobaric and hypobaric hypoxia

“…Alongside HAI, hypoxia research has a wide range of other applications: investigating the pathogenesis of illness characterized or complicated by hypoxia (Grocott et al, 2007 ); examining responses during acute high‐altitude exposure (Grant, 2002 ); enhancing training methods for performance athletes (Millet et al, 2013 ); working in a hypoxic environment for fire protection (Angerer & Nowak, 2003 ); and hypoxia risk in technical and occupational diving (Mitchell et al, 2019 ).…”