Hypoxic Hypoxia at Moderate Altitudes: Review of the State of the Science

This paper presents a systematic literature review that was conducted regarding hypoxic impairment that may affect aviation-related duties at moderate altitude of between 8000 to 15,000 ft/2438 to 4572 m. The findings from the literature regarding hypoxic visual decrements shows that rod photoreceptors are more susceptible to hypoxia, with visual degradation demonstrated at 4000 to 5000 ft (1219 to 1524 m) under scotopic and 10,000 ft (3048 m) under photopic conditions. Under starlight conditions, augmented night vision goggle resolution showed more resilience to mild hypoxic effects than the unaided eye. Hypocapnia enhances visual sensitivity and contrast discrimination. The literature review also indicates that although cognitive and psychomotor deficits related to hypoxia may include learning, reaction time, decision-making, and certain types of memory, results are difficult to quantify and reliably reproduce. Inconsistency of results may be related a variety of factors, including the subtlety of deficits compared to high altitude, heterogeneous neuronal central nervous system response, intra-individual variation and differences in methodology or metrics. Both cognitive/psychomotor and visual experimental results may be confounded by hyperventilation with resulting respiratory alkalosis and cerebral vasoconstriction. Directions for future research are discussed.