Acute Hypoxia Decreases Cardiac Response to Catecholamines in Exercising Humans

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Abstract
Cardiac chronotropic response to adrenergic activity at rest and exercise has been studied in 8 sea-level natives on the first two days of exposure to high altitude hypoxia (3823 m, 473 mmHg). Maximal O2 uptake (VO2max) was determined at low altitude (day 0 : DO) and high altitude (day 2 : D2). Submaximal exercise tests were performed at low altitude (day 1 : D1) and high altitude (days 3 and 4 : D3, D4). Plasma venous norepinephrine (NE) and epinephrine (E) concentrations were determined at rest and at the end of submaximal exercise. From DO to D2, maximal heart rate decreased by 7% (p2max decreased by 17% (p-1). In contrast, relative work load decreased at high altitude (% VO2max at D1, D3 and D4 were respectively : 90.2±6.1, 83.3±9.8, 76.9±8.2). Linear relationships were found, both at low and high altitudes, between NE and VO2, NE and % VO2max, and between the increases in NE and heart rate during exercise. Covariance analysis indicates that these relations shifted to the left at high altitude: for the same NE or increase in NE, VO2 or increase in heart rate was lower at high altitude. Variations in E were similar but not significant. We conclude that hypoxia induced a decrease in cardiac chronotropic response to adrenergic activation during submaximal exercise.

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