Passive head-up tilt and actively standing up produce similar overall changes in autonomic balance

The primary objective of this study was to compare, in normal subjects, the average change in autonomic balance during a 5-minute period by using two methods of changing posture: actively standing up and passive head-up tilt. We hypothesized that the average effect of these two methods of changing posture on autonomic balance would not significantly differ. After collecting supine baseline measurements, subjects were first tilted head up to 60 degrees for 15 minutes, then returned to the supine position for 5 minutes, before they stood up for 5 minutes. Comparing the average (over a 5-minute period) autonomic response to head-up tilt with that of standing, there were no significant differences in the decrease in average R-R intervals (-18% vs -18%, p = not significant), the drop in high-frequency power (-73% vs -69%, p = not significant), or the increase in the low-frequency/high-frequency ratio (+252% vs +298%, p = not significant). The changes in autonomic balance that occur during the first 5 minutes of passive 60-degree head-up tilt are nearly identical to the change in autonomic balance that occurs during the first 5 minutes of quiet standing.