Abstract
Acute hypobaric hypoxia (HH) is a major physiological threat during high-altitude flight and operations. In military aviation, although hypoxia-related fatalities are rare, incidences are common and are likely underreported. Hypoxia is a reduction in oxygen availability, which can impair brain function and performance of operational and safety-critical tasks. HH occurs at high altitude, due to the reduction in atmospheric oxygen pressure. This physiological state is also partially simulated in normobaric environments for training and research, by reducing the fraction of inspired oxygen to achieve comparable tissue oxygen saturation [normobaric hypoxia (NH)]. Hypoxia can occur in susceptible individuals below 10,000 ft (3,048 m) in unpressurised aircrafts and at higher altitudes in pressurised environments when life support systems malfunction or due to improper equipment use. Between 10,000 ft and 15,000 ft (4,572 m), brain function is mildly impaired and hypoxic symptoms are common, although both are often difficult to accurately quantify, which may partly be due to the effects of hypocapnia. Above 15,000 ft, brain function exponentially deteriorates with increasing altitude until loss of consciousness. The period of effective and safe performance of operational tasks following exposure to hypoxia is termed the time-of-useful-consciousness (TUC). Recovery of brain function following hypoxia may also lag beyond arterial reoxygenation and could be exacerbated by repeated hypoxic exposures or hyperoxic recovery. This review provides an overview of the basic physiology and implications of hypoxia for military aviation and discusses the utility of hypoxia recognition training.
Highlights
Acute hypoxia is a major physiological threat during high-altitude flight and operations in military aviation
For each EAA, only PiO2 is equivalent between hypobaric hypoxia (HH) and normobaric hypoxia (NH); whereas, other factors contributing to the alveolar gas equation (AGE) may differ, such as gas flow distribution, diffusivity of gases and nitrogen kinetics, and the magnitude of hyperventilation-induced hypocapnia
Breathing air comprising more than 21% oxygen to accelerate recovery from hypoxia is a common practice in military aviation
Summary
Acute hypoxia is a major physiological threat during high-altitude flight and operations in military aviation. The human brain requires a continuous oxygen supply to function effectively. Whilst it is acknowledged that some military aircraft can elicit loading in the +Gz axis (i.e., commonly referred to as pulling Gs) to impair cerebral perfusion and cause stagnant hypoxia, such as high-performance jets, the physiological effects and mitigating factors markedly differ from hypoxic hypoxia and are considered outside the scope of the current review. The aim of this review is to summarise the basic physiology of hypoxic hypoxia on brain function and recovery and to discuss the implications for military aviation, including the utility of hypoxia recognition training (HRT) for improving emergency responses to hypoxic incidences
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