Circulatory response to passive and active changes in posture

To understand the sequence of hemodynamic events elicited by active (stand-up) and passive (head-up tilt, HUT) changes in posture, we monitored heart rate and arterial blood pressure continuously in ten healthy volunteers at rest, during rapid tilts (75° HUT over 2 secs), slow tilts (75° HUT over 50 secs), and stand tests. A marked initial transient drop in mean arterial pressure (MAP) and increase in heart rate (HR) seen during rapid tilt, RT, and stand-up, SU, were absent during slow tilt, ST. While the magnitudes of transient heart rate peak and blood pressure minimum for RT and SU, respectively, were different, their relative timing was well preserved. We attribute the timing to the response of homeostatic reflex mechanisms. No statistically significant difference could be detected when comparing steady-state values of MAP and HR for SU and HUT 140 seconds after the onset of the respective stress.