Peak Heart Rate Decreases with Increasing Severity of Acute Hypoxia

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Abstract
The purpose of the present study was to investigate the degree to which peak heart rate is reduced during exhaustive exercise in acute hypoxia. Five sea-level lowlanders performed maximal exercise at normobaric normoxia and at three different levels of hypobaric hypoxia (barometric pressures of 518, 459, and 404 mmHg) in a hypobaric chamber and while breathing 9% O2 in N2. These conditions were equivalent to altitudes of 3300, 4300, 5300, and 6300 m above sea level, respectively. At 4300 m, maximal exercise was also repeated after 4 and 8 h. Peak heart rate (HR) decreased from 191 (182-202) (mean and range) at sea level to 189 (179-200), 182 (172-189), 175 (166-183), and 165 (162-169) in the acute hypoxic conditions. Peak HR did not decrease further after 4 and 8 h at 4300 m compared to the acute exposure at this altitude. Between barometric pressures of 518 and 355 mmHg (~3300 and 6300 m), peak HR decreased linearly: peak HRhypobaria = peak HRsea level - 0.135 × [hypobaria3100 - hypobaria (mmHg)]; or peak HRaltitude = peak HRsea level - 0.15 × (altitude - 3100 m). This corresponds to ~1-beat · min-1 reduction in peak HR for every 7-mmHg decrease in barometric pressure below 530 mmHg (~130 m of altitude gained above 3100 m). At termination of exercise, maximal plasma lactate and norepinephrine concentrations were similar to those observed during maximal exercise in normobaric normoxia. This study clearly demonstrates a progressive decrease in peak HR with increasing altitude, despite evidence of similar exercise effort and unchanged sympathetic excitation.

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