Peak Heart Rate Decreases with Increasing Severity of Acute Hypoxia

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% O(2) in N(2). 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 (approximately 3300 and 6300 m), peak HR decreased linearly: peak HR(hypobaria) = peak HR(sea level) - 0.135 x [hypobaria(3100) - hypobaria (mmHg)]; or peak HR(altitude) = peak HR(sea level) - 0.15 x (altitude - 3100 m). This corresponds to approximately 1-beat x min(-1) reduction in peak HR for every 7-mmHg decrease in barometric pressure below 530 mmHg (approximately 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.