Daytime variability in carotid baroreflex function in healthy human subjects

Variability of blood pressure is limited by arterial baroreceptors, yet blood pressure still shows circadian changes. This study was undertaken to examine if the responses to the carotid baroreflex also change throughout the day. Responses of cardiac interval (RR), mean arterial pressure (MAP) and vascular resistance (VR) to carotid baroreflex stimulation and inhibition using pressures and suction applied to a neck chamber, were measured in 14 healthy, normotensive subjects. Studies were carried out at three hourly intervals between 09:00 and 21:00 hours. Stimulus-response curves were defined and the first differential of the curve was calculated to establish reflex sensitivity (maximal slope) and "operating" point (estimated carotid sinus pressure at point of maximum slope, OP). The principal findings are: (1) baroreflex sensitivity for the control of VR was at its highest at 09:00 (-3.4 +/- 0.6 units) compared to 12:00 (-1.9 +/- 0.4 units), 15:00 (-2.0 +/- 0.4 units) and 18:00 (-1.9 +/- 0.3 units) (all P < 0.05); (2) baroreflex OP for the control of MAP was at its lowest at 09:00 (P < 0.01); (3) baroreflex sensitivity for control of VR was significantly correlated with prevailing mean pressure (P < 0.05) and OP for the control of MAP (P < 0.02); (4) OP for control of RR, MAP and VR are all highly correlated to prevailing MAP (P < 0.0001). Our results suggest that baroreflex function varies throughout the day and this favors higher sensitivity and lower blood pressure in the mornings. We speculate that this may be of importance in long-term blood pressure regulation.