Acute Mountain Sickness, Hypoxia, Hypobaria and Exercise Duration each Affect Heart Rate

In this study, we quantified the changes in post-exercise resting heart rate (HRrst) associated with acute mountain sickness (AMS), and compared the effects of hypobaric hypoxia (HH) and normobaric hypoxia (NH) on HRrst. We also examined the modulating roles of exercise duration and exposure time on HRrst. Each subject participated in 2 of 6 conditions: normobaric normoxia (NN), NH, or HH (4400m altitude equivalent) combined with either 10 or 60min of moderate cycling at the beginning of an 8-h exposure. AMS was associated with a 2bpm higher HRrst than when not sick, after taking into account the ambient environment, exercise duration, and SpO(2). In addition, HRrst was elevated in both NH and HH compared to NN with HRrst being 50% higher in HH than in NH. Participating in long duration exercise led to elevated resting HRs (0.8-1.4bpm higher) compared with short exercise, while short exercise caused a progressive increase in HRrst over the exposure period in both NH and HH (0.77-1.2bpm/h of exposure). This data suggests that AMS, NH, HH, exercise duration, time of exposure, and SpO(2) have independent effects on HRrst. It further suggests that hypobaria exerts its own effect on HRrst in hypoxia. Thus NH and HH may not be interchangeable environments.