Hypoxia, Hypobaria, and Exercise Duration Affect Acute Mountain Sickness

INTRODUCTION: This study simultaneously quantified the effects of normobaric hypoxia (NH), hypobaric hypoxia (HH), exercise duration, and exposure time on acute mountain sickness severity (AMS-C). METHODS: Thirty-six subjects (27.7 +/- 7.8 yr) participated in a partial repeated measures study, completing two of six conditions: normobaric normoxia (NN: 300 m/984 ft equivalent), NH or HH (PO2 = 91 mmHg; 4400 m/14,436 ft equivalent), combined with moderate intensity cycling for 10 or 60 min. Subjects completed the Environmental Symptoms Questionnaire and oxygen saturation (SpO2) was measured before, 1.5 h, 4 h, and 6.5 h into an 8-h exposure, and 1.5 h post-exposure. We fit multiple linear regression models with cluster adjusted standard errors on the exposure times using NH, HH, and long exercise as indicator variables, and AMS-C as the outcome variable. The SpO2 and pre-exposure AMS-C score were used as covariates. RESULTS: NH and HH led to substantial and progressively increasing AMS-C, but NN did not. The effect of HH on AMS-C was significantly different from NH, with AMS-C in HH being 1.6 times higher than in NH. HH led to significantly increasing AMS-C, regardless of the exercise duration, while NH only did so in combination with longer exercise. DISCUSSION: Increases in AMS-C were each independently related to NH, HH, and long duration exercise, with HH affecting AMS-C more severely. This suggests that hypobaria may affect AMS development above the level induced by hypoxia alone. This further suggests that NH and HH may not be interchangeable for studying AMS and that exercise duration may impact physiological responses.