Modulation of arterial baroreflex dynamic response during mild orthostatic stress in humans

Abstract

We tested the hypothesis that in humans, carotid-baroreflex dynamic responses (evaluated by examining the time course of the carotid-baroreflex-induced alterations in muscle sympathetic nerve activity (MSNA), mean arterial blood pressure (MAP) and heart rate (HR)) would be altered during mild orthostatic stress in ways that serve to limit orthostatic hypotension. In 12 healthy subjects (10 male, 2 female), 5-s periods of neck pressure (NP) (50 mmHg) and neck suction (NS) (-60 mmHg) were used to evaluate carotid baroreflex function at rest (CON) and during lower body negative pressure (LBNP) (-15 mmHg). During LBNP (as compared with CON) (a) the augmentations in MSNA and MAP elicited by NP were greater, (b) the NS-induced period of MSNA suppression was, if anything, shorter, (c) the peak decrement in MAP elicited by NS, although not different in amplitude, occurred earlier and recovered to its initial level more quickly after NS, and (d) the HR responses to NP and NS were greater. These results suggest that during mild orthostatic stress, carotid-baroreflex dynamic responses are modulated in ways that should help maintain blood pressure and limit orthostatic hypotension.

Fig. 1. General experimental protocol (A) and sequence used for application of each neck-chamber stimulus (B)

NP-NS, neck-chamber stimuli (neck pressure or suction).

Fig. 2. MSNA response in one subject to application of neck pressure (A) and neck suction (B) in control and LBNP situations

Fig. 3. Averaged reflex alterations (n = 10–12 subjects) in MSNA (A), MAP (B) and HR (C) elicited by neck pressure in control and LBNP situations

Significant difference from value obtained at 3 s prior to application of neck pressure, P < 0.05. †Significant difference from control, P < 0.05.

Fig. 4. Averaged reflex alterations (n = 10–12 subjects) in MSNA (A), MAP (B) and HR (C) elicited by neck suction in control and LBNP situations

Significant difference from value obtained at 3 s prior to application of neck suction, P < 0.05. †Significant difference from control, P < 0.05.