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

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.

Effect of moderate and Severe Hypoxic exposure coupled with fatigue on psychomotor vigilance testing, muscle tissue oxygenation, and muscular performance

“…The findings of the current study support the hypotheses of the magnitude of hypoxia results in worsening cognition and vigilance ( McMorris et al, 2017 ; Pun et al, 2018 ; Shaw et al, 2021 ). Specifically, in the current study, MH resulted in greater PVT RT while SH exhibited greater PVT RT and PVT E compared to the Normoxia condition.…”

“…In addition, Pun et al (2018) reported that PVT RT was highly correlated to acute mountain sickness symptoms (r = 0.62) which suggested that acute hypoxia reduces cognitive processing speed and/or movement during vigilance tasks increasing the time to complete the task. It has been hypothesized that the magnitude and duration of hypoxia greatly influences cognitive and vigilance outcomes ( Pun et al, 2018 ; Shaw et al, 2021 ).…”

“…The reduction in inspired oxygen results in inefficient oxygen transfer at the lungs, ultimately causing low tissue saturation. Research examining hypoxia most frequently utilize normobaric hypoxia which achieves hypoxia through reduced FiO 2 ( McMorris et al, 2017 ; Millet et al, 2012 ; Shaw et al, 2021 ; Pun et al, 2018 ; Sander, 2016 ). Although there are some reported differences in the ventilatory responses between normoxic and hypobaric hypoxia, the majority of muscular and neural physiological responses as well as cognitive responses have been shown to be similar through the use of equivalent-air-altitude ( McMorris et al, 2017 ; Shaw et al, 2021 ).…”

“…Research examining hypoxia most frequently utilize normobaric hypoxia which achieves hypoxia through reduced FiO 2 ( McMorris et al, 2017 ; Millet et al, 2012 ; Shaw et al, 2021 ; Pun et al, 2018 ; Sander, 2016 ). Although there are some reported differences in the ventilatory responses between normoxic and hypobaric hypoxia, the majority of muscular and neural physiological responses as well as cognitive responses have been shown to be similar through the use of equivalent-air-altitude ( McMorris et al, 2017 ; Shaw et al, 2021 ). Thus, the utilization of normobaric hypoxia can be utilized to examine the influence of hypoxia on many physiological systems within the body as well as cognitive performance under reduced blood oxygenation conditions.…”

“…The simultaneous examination of heart rate (HR), oxygen saturation (SpO 2 ), and peripheral tissue oxygen saturation (StO 2 ) via near-infrared spectroscopy, and cognitive function via psychomotor vigilance testing (PVT) have been shown to be useful for tracking and identifying the physiological and cognitive changes associated with fatigue and hypoxia ( Basner and Dinges, 2012 ; Davis and Barstow, 2013 ; Khitrov et al, 2014 ; Shaw et al, 2021 ). Specifically, near-infrared spectroscopy allows for the measurement StO 2 through the tissue saturation index (TSI) which is the total concentrations ratio indicating the levels of oxygen supply and oxygen consumption to the targeted muscle ( Jones et al, 2016 ; McManus et al, 2018 ).…”