Whole-body heating slows carotid baroreflex response in human subjects.

Heat stress increases sympathetic activity and decreases parasympathetic activity to the heart. To test the hypothesis that carotid baroreflex responses of heart rate (HR) and systemic blood pressure become slowed with altered autonomic nerve activities during whole-body heat stress, we determined changes in HR and mean arterial pressure (MAP) in response to approximately 5 s of 40 mmHg neck pressure (NP) and of -65 mmHg neck suction (NS) in normothermia and during whole-body heating produced by a hot water-perfused suit. The NP and NS stimuli were triggered by R waves of an ECG during held expiration in the supine position. Whole-body heating did not alter the onset time of the HR and MAP responses during NP and NS. Whole-body heating significantly increased the time from onset of the HR response until peak of the response during NP (2.53 +/- 0.33 s in normothermia and 3.46 +/- 0.28 s during heating, P<0.05) and NS (1.20 +/- 0.23 s and 2.24 +/- 0.29 s, P<0.05). Whole-body heating significantly increased the time from onset of the MAP response until peak of the response during NP (4.31+/-0.46 s in normothermia, 6.67 +/- 0.56 s during heating, P<0.05) but not during NS (5.06 +/- 0.47 s and 4.50 +/- 0.60 s). These findings suggest that heat stress prolongs the response time of carotid-cardiac and carotid-vasomotor baroreflexes.