Comparison of hypobaric hypoxia symptoms between a recalled exposure and a current exposure

BACKGROUND: Aircrew members are required to attend hypoxia awareness training regularly to strengthen their memory of their personal hypoxia symptoms by undergoing training inside a hypobaric chamber. The aim of this study was to examine the association between hypoxia symptoms experienced during two training sessions that were 4 years apart. METHODS: This was a crossover study to compare hypoxia symptoms and self-reported physiological effects of trapped gas between a previous training session and a current training session in an altitude chamber. The subjects were military crew members who undertook a 25,000-feet refresher training course in 2018. We used a structured questionnaire to obtain the target information before and during hypoxia exposure. Data were analyzed using SPSS software. RESULTS: A total of 341 trainees participated in this survey and completely filled out the questionnaire. Gastrointestinal tract discomfort caused by the expansion of trapped gas was the main physiological reaction during the previous and current training sessions. Frequently reported symptoms were poor concentration (30.5%), impaired cognitive function (20.5%), visual disturbances (16.4%), hot flashes (15.8%), and paresthesia (12.6%) during both exposures. However, the proportions of participants reporting poor concentration (P = 0.378) and visual disturbances (P = 0.594) were not significantly different between the recalled and current training sessions. The five most common symptoms among the subjects with less than 1,000 flight hours were poor concentration (29.8%), visual disturbance (27.3%), impaired cognitive function (14.9%), dizziness/lightheadedness (11.6%), and hot flashes (9.9%), which overlapped substantially with the symptoms reported by other subjects. The occurrence of those five most common symptoms in the group with more than 1,000 flight hours did not significantly differ between the recalled training session and the current training session. CONCLUSIONS: The most common hypoxia symptoms reported were similar between the recalled and current training sessions in an environment with a low oxygen concentration. This finding was also clearly affected by the duration of flight experience. Moreover, GI effects of the expansion of trapped gas were commonly observed at low atmospheric pressure.