[Assessment of responsiveness of spontaneous baroreceptor-heart rate reflex under gravitational stress]

Beat by beat arterial blood pressure was measured using Finapres 2300 monitor. Sequences of 3 or more beats with systolic pressure and next beat interval changed in the same direction were identified as baroreflex sequences from 6-min data segments. A linear regression was applied to each sequence and the slope (correlation coefficient > or = 0.85) was taken as a measure of the baroreflex response slope (BRS). The changes in mean BRS during + 65 degrees head-up tilt (HUT), -6.67 kPa lower body negative pressure (LBNP) and 16 days -6 degrees head-down tilt (HDT) bed rest were observed. The results showed that there was a significant decrease in the mean BRS during LBNP or HUT as compared with that of the supine control, though the blood pressure responses differed. The mean BRS was also significantly decreased during bed rest. It suggests that the responsiveness of spontaneous baroreceptor-heart rate reflex can be evaluated in humans under gravitational stress.