Influence of ten-day head-down bedrest on human carotid baroreceptor-cardiac reflex function.

We studied the effects of simulated microgravity on baroreceptor-cardiac reflex mechanisms, with complex pressure changes delivered to neck chambers worn by six healthy young men, before, during (days 1, 3, 6, and 10), and after ten days' 6 degrees head-down bedrest. During held expiration, a computer-driven bellows increased neck pressure to 40 mmHg for about 4 s, and then decreased pressure to -65, by 15 mmHg R-wave triggered decrements. We plotted R-R intervals as functions of carotid distending pressures (systolic less neck chamber pressures). Each experimental session comprised seven stimulus sequences delivered over about 15 min and averaged. Comparisons between results before bedrest and on the tenth day of bedrest showed that average baseline R-R intervals increased from 933 +/- (SEM) 24 to 1077 +/- 35 msec (P less than 0.05, Wilcoxon signed rank test); maximum slopes (R-R interval/carotid distending pressure) decreased from 4.5 +/- 0.4 to 3.6 +/- 0.7 msec/mmHg (P = 0.11); R-R interval response ranges decreased from 263 +/- 31 to 182 +/- 18 msec (P = 0.03); and operational points (positions of R-R intervals at resting pressures on the stimulus-response relation) remained constant. Our results suggest that head-down bedrest progressively impairs baroreceptor-cardiac reflex responses in healthy human subjects.