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 (HR rst ) associated with acute mountain sickness (AMS), and compared the effects of hypobaric hypoxia (HH) and normobaric hypoxia (NH) on HR rst . We also examined the modulating roles of exercise duration and exposure time on HR rst . Each subject participated in 2 of 6 conditions: normobaric normoxia (NN), NH, or HH (4 400 m altitude equivalent) combined with either 10 or 60 min of moderate cycling at the beginning of an 8-h exposure. AMS was associated with a 2 bpm higher HR rst than when not sick, after taking into account the ambient environment, exercise duration, and SpO 2 . In addition, HR rst was elevated in both NH and HH compared to NN with HR rst being 50% higher in HH than in NH. Participating in long duration exercise led to elevated resting HRs (0.8–1.4 bpm higher) compared with short exercise, while short exercise caused a progressive increase in HR rst over the exposure period in both NH and HH (0.77–1.2 bpm/h of exposure). This data suggests that AMS, NH, HH, exercise duration, time of exposure, and SpO 2 have independent effects on HR rst . It further suggests that hypobaria exerts its own effect on HR rst in hypoxia. Thus NH and HH may not be interchangeable environments.