The difference in arterial baroreflex sensitivity between the supine and standing positions in healthy subjects.

The spectral analysis of heart rate variability (HRV) has long been considered a practical, noninvasive tool to assess autonomic nervous functions that regulate the cardiovascular system. The conventional method, however, has limitations in characterizing transient changes of HRV. We have overcome this problem by adopting the time-frequency analysis method. Using this method, we attempted to clarify differences in the arterial baroreflex (ABR) activity during a transient change in the heart rate between the supine and standing positions in healthy subjects. We found that the ABR gain was significantly greater in the supine position compared to standing, and this gain increase was due to transient increases in 0.15 to 0.20 Hz components of HRV spectral powers caused by enhanced cardiac vagal outflow. Based on these findings, we conclude that the orthostatic stress induced by the postural change from supine to standing markedly reduces the baroreflex gain by suppressing high-frequency cardiac vagal outflow.