Carotid baroreflex responsiveness during dynamic exercise in humans

We utilized 5-s changes of neck pressure and neck suction (from 40 to -80 Torr) to alter carotid sinus transmural pressure in seven men with peak oxygen uptake (V̇O₂(peak)) of 41.4 ± 3.6 ml O₂ · kg^-1 · min^-1. Peak responses of heart rate (HR) and mean arterial pressure (MAP) to each carotid sinus perturbation were used to construct open-loop baroreflex curves at rest and during exercise at 25.7 ± 1.1 and 47.4 ± 1.9% V̇O₂(peak). The baroreflex curves were fit to a logistic function describing the sigmoidal nature of the carotid sinus baroreceptor reflex. Maximal gain for baroreflex control of HR (-0.31 ± 0.05 beats · min^-1 · mmHg^-1) and MAP (-0.30 ± 0.08 mmHg/mmHg) at rest was the same as during exercise at 25 and 50% V̇O₂(peak) (-0.30 ± 0.05, -0.39 ± 0.13 beats · min^-1 · mmHg^-1 for HR, P = NS; -0.23 ± 0.04, -0.60 ± 0.38 mmHg/mmHg for MAP, P = NS). Resetting of the baroreflex occurred during exercise at 50% V̇O₂(peak). The centering point, threshold, and saturation pressures were significantly increased for baroreflex control of HR (Δpressure = 26.3 ± 6.8, 19.6 ± 10.4, 33.0 ± 5.6 mmHg, P < 0.05) and MAP (Δpressure = 27.1 ± 7.7, 16.1 ± 14.8, 38.2 ± 8.5 mmHg, P < 0.05). The operating point (steady-state HR and MAP) was shifted closer to threshold of the baroreflex during exercise at 50% V̇O₂(peak), as reflected by differences in HR and MAP between the centering and operating points (ΔHR = 12.5 ± 4.7 beats/min, P = 0.10; ΔMAP = 7.6 ± 1.3 mmHg, P < 0.05). These findings suggest a resetting of the carotid baroreflex during exercise with no attenuation in maximal sensitivity. A shift in operating point toward threshold of the baroreflex enables effective buffering of elevations in systemic blood pressure via reflex alterations in HR and MAP.