Arterial baroreflex gain for muscle sympathetic nerve activity during head-up tilting in humans.

To clarify the neural mechanisms involved in blood pressure regulation during head-up tilting (HUT), arterial baroreflex controls for heart rate and muscle sympathetic nerve activity (MSNA) were evaluated by applying frequency transfer function and coherence analyses to the equidistant time series data of RR-interval, MSNA and mean blood pressure during graded HUT in six healthy subjects. Coherence analysis indicated that the RR-interval and MSNA were linked linearly with mean blood pressure in the low (0.07-0.12 Hz) and high (0.23-0.27 Hz) frequency domains. Arterial baroreflex gain for the heart rate decreased significantly during HUT, while the baroreflex gain for MSNA showed no significant change. These results indicate that the preserved arterial baroreflex gain for MSNA plays an important role in maintaining blood pressure during orthostasis.