DiPasquale, D. M., Strangman, G. E., & Muza, S. R. (2013). Altitude vs. hypoxia: Comparing the effects of exercise in hypobaric and normobaric hypoxia on resting heart rate. Medicine & Science in Sports & Exercise, 45(5), Supplement abstract number 2706.

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This study compared the effects of exercising in hypobaric hypoxia and normobaric hypoxia on resting heart rate. Ss (N = 13) completed two of six possible conditions: exposure to low altitude (300 m), hypobaric hypoxia (4,400 m), or normobaric hypoxia (ambient PO2 = 91 mmHg), and cycling at 50%HRreserve for either 10 or 60 minutes. Exposures lasted eight hours and exercising was completed by 1.25 hours into the exposure. Resting heart rate, oxygen saturation (SpO2), and end tidal CO2 (PetCO2) were measured before exposure, 1.5 hours, 4 hours, and 6.5 hours into exposure, and one hour post-exposure for 5-10 minutes with means calculated from the final two minutes. Ss also completed the Environmental Symptoms Questionnaire at those times. An AMS-C (Acute Mountain Sickness Cerebral) score of 0.7 or greater indicated clinical sickness. Analysis consisted of fitting a multiple linear regression model with cluster adjusted standard errors on the three exposure time points, using normobaric hypoxia, hypobaric hypoxia, and clinical Acute Mountain Sickness at any time during exposure as indicator variables, and exercise duration as a continuous predictor.

Significant increases in resting heart rate were observed for: i) normobaric hypoxia compared to low altitude, ii) hypobaric hypoxia compared to low altitude, iii) 60 vs. 10 minutes of exercise, and iv) for the presence vs. absence of Acute Mountain Sickness during the exposure. Similar results were found when eliminating the time point closest to the completion of exercise, as well as when considering only the last exposure time point. That suggested the observed effects on resting heart rate were not due solely to recovery from exercise. The addition of SpO2 and PetCO2 to analysis models did not contribute to elevated resting heart rate.

Implication. Normobaric hypoxia alone had a similar effect on resting heart rate as 50 minutes of added exercise. The effect of hypobaric hypoxia on resting heart rate was double that recorded for normobaric hypoxia and exercise. The presence of Acute Mountain Sickness had the largest single effect and it appeared to be independent of the other effects. Elevated resting heart rate in Acute Mountain Sickness is not solely due to compensation for exposure to hypoxia or previous exercise. What relates Acute Mountain Sickness to elevated resting heart rate needs to be determined.

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