Effect of hypoxia on arterial baroreflex control of heart rate and muscle sympathetic nerve activity in humans.

We tested the hypothesis that acute hypoxia would alter the sensitivity of arterial baroreflex control of both heart rate and sympathetic vasoconstrictor outflow. In 16 healthy, nonsmoking, normotensive subjects (8 women, 8 men, age 20-33 yr), we assessed baroreflex control of heart rate and muscle sympathetic nerve activity by using the modified Oxford technique during both normoxia and hypoxia (12% O(2)). Compared with normoxia, hypoxia reduced arterial O(2) saturation levels from 96.8 +/- 0.3 to 80.7 +/- 1.4% (P < 0.001), increased heart rate from 59.8 +/- 2.4 to 79.4 +/- 2.9 beats/min (P < 0.001), increased mean arterial pressure from 96.7 +/- 2.5 to 105.0 +/- 3.3 mmHg (P = 0.002), and increased sympathetic activity 126 +/- 58% (P < 0.05). The sensitivity for baroreflex control of both heart rate and sympathetic activity was not altered by hypoxia (heart rate: -1.02 +/- 0.09 vs. -1.02 +/- 0.11 beats. min(-1). mmHg(-1); nerve activity: -5.6 +/- 0.9 vs. -6.2 +/- 0.9 integrated activity. beat(-1). mmHg(-1); both P > 0.05). Acute exposure to hypoxia reset baroreflex control of both heart rate and sympathetic activity to higher pressures without changes in baroreflex sensitivity.