HEAD-DOWN BED REST IMPAIRS VAGAL BAROREFLEX RESPONSES AND PROVOKES ORTHOSTATIC HYPOTENSION

We studied vagally mediated carotid baroreceptor-cardiac reflexes in 11 healthy men before, during, and after 30 days of 6° head-down bed rest to test the hypothesis that baroreflex malfunction contributes to orthostatic hypotension in this model of simulated microgravity. Sigmoidal baroreflex response relationships were provoked with ramped neck pressure-suction sequences comprising pressure elevations to 40 mmHg followed by serial R-wave-triggered 15-mmHg reductions to -65 mmHg. Each R-R interval was plotted as a function of systolic pressure minus the neck chamber pressure applied during the interval. Compared with control measurements, base-line R-R intervals and the minimum, maximum, range, and maximum slope of the R-R interval-carotid pressure relationships were reduced (P < 0.05) from bed rest day 12 through recovery day 5. Baroreflex slopes were reduced more in four subjects who fainted during standing after bed rest than in six subjects who did not faint (-1.8 ± 0.7 vs. -0.3 ± 0.3 ms/mmHg, P < 0.05). There was a significant linear correlation (r = 0.70, P < 0.05) between changes of baroreflex slopes from before bed rest to bed rest day 25 and changes of systolic blood pressure during standing after bed rest. Although plasma volume declined by ~ 15% (P < 0.05), there was no significant correlation between reductions of plasma volume and changes of baroreflex responses. There were no significant changes of before and after plasma norepinephrine or epinephrine levels before and after bed rest during supine rest or sitting. We conclude that a model of microgravity, 6° head-down bed rest, significantly reduces the responsiveness and buffer capacity of vagal baroreflex-cardiac reflexes. Impaired baroreflex function appears to reduce the ability of subjects to adjust to transient changes of blood pressure during standing, and this may contribute to orthostatic intolerance.