Characterization of baroreflex response under head-up tilting

Head-up tilting has been used to assess the autonomic functions in the cardiovascular system. It has been believed that the sympathetic regulation becomes enhanced as the tilt angle increases, which is supported by direct and indirect experimental results through spectral analysis and measurement of muscle sympathetic nerve activity. Instead, paradoxically, the baroreflex sensitivity (BRS) estimated through various techniques shows its reduction associated with an increase in the tilt angle. However, intuitionally, regulation maintaining blood pressure level should be augmented under this situation. In this paper, in order to confirm the previous findings, BRS for varied tilt angles is estimated through the sequence method applied to measured heart rate variability and systolic blood pressure and to a time series generated by the multivariate autoregressive (MAR) model driven by the innovation process. In addition, closed-loop impulse responses are calculated to characterize the cardio-vascular response to the orthostatic stress from a novel point of view.