Effect of acute normobaric hypoxia on passively simulated illusions: A double-blind randomized study

Introduction: Number of accidents in the past four decades in rotary wing flying in high altitude areas in the Indian Air Force have been attributed to spatial disorientation (SD) or hypoxia or both. Although the two aviation stressors; hypoxia and SD, have been studied independently, literature is scant on the combined effects of the two notorious factors in military aviation. Material and Methods: In a double-blind randomized control design, 32 healthy volunteers divided into two groups (hypoxia group and normoxia group) of 16 subjects each, participated in the study. Subjects in the hypoxia group were exposed to normobaric hypoxia with pre-mixed gases in cylinders with nitrox gas (simulating altitude of 22,000 ft) and the normoxia group was exposed to normal air. Autokinesis time (AT) and vestibular adaptation time (VAT) during acceleration and deceleration, in both clockwise and counter-clockwise turns, were studied as surrogates for SD in both hypoxia group and normoxia group in Disorientation Simulator. Results: Mean AT showed a statistically significant decrease (t = −2.2, P = 0.039) in hypoxia group compared to normoxia group. Similarly, a statistically significant reduction (F = 5.989, P = 0.016) in mean VAT was observed in in hypoxia group compared to normoxia group. There was no significant difference in the VAT in clockwise and counter-clockwise yaw rotation in both the groups. Conclusion: A significant reduction in AT indicates that hypoxia may increase the onset of autokinesis early. The changes in VAT in hypoxic conditions bring out a possible effect of hypoxia on the adaptability of the vestibular system in the angular motion environment.