Physiological Equivalence of Normobaric and Hypobaric Exposures of Humans to 25,000 Feet (7620 m)

Abstract

Skepticism exists about whether normobaric and hypobaric hypoxic exposures are equivalent. We have evaluated if physiological differences between the two environments would translate into actual differences in hypoxia symptoms. We exposed 20 subjects to 5-min 25,000-ft (7620-m) equivalent environments in an altitude chami ber and then in a ground-level portable reduced-oxygen training enclosure (PROTE). Heart rate and hemoglobin oxygen saturation (SaO2) were continuously monitored. Alveolar gas samples were collected at 1, 3, and 4 min elapsed time. Subjects completed hypoxia symptom questionnaires at the same time points. Mean fourth minute alveolar oxygen tension (PaO2), alveolar carbon dioxide tension (PaCO2), and respiratory quotient (RQ) differed significantly between the chamber and PROTE. Declines in SaO2 appeared biphasic, with steepest declines seen in the first minute. Rates of SaO2 decline over the 5-min exposure were significantly different. Heart rate was not different, even when indexed to body surface area. Mean number of hypoxia symptoms between hypobaric and normobaric environments after 1 min were significant. However, the temporal pattern of symptom frequencies across subjects between the chamber and PROTE were similar. Alveolar gas composition and arterial hemoglobin oxygen desaturation patterns differed between a ground level and hypobaric exposure. Differences in mean number of hypoxia symptoms between hypobaric and normobaric environments after 1 min, but not at 3 and 4 min, coupled with similar patterns in symptom frequencies, suggest that ground-level hypoxia training may be a sufficiently faithful surrogate for altitude chamber training.