Effect of posture on arterial baroreflex control of heart rate in humans

Summary Altered baroreflex function may contribute to the cardiovascular changes associated with weightlessness. Since central blood volume (CBV) increases during simulated weightlessness, we have examined the possibility that acute changes in CBV may modify baroreceptor function. We used graded head-up tilt (HUT) and head-down tilt (HDT) to induce changes in CBV, and neck suction to stimulte carotid baroreceptors, in 6 subjects. The increase in pulse interval induced by a negative pressure of 8.2 kPa (62 mm Hg) imposed for 10 s while supine was compared with the increase while tilted for 8 min at ± 15, ± 30 and ± 45. During HDT at 15 the pulse interval over the first 5 cardiac cycles following suction onset was 51 ± (SEM) 18 ms longer (p<0.05), at 30 it was 61±20 ms longer (p<0.05), and at 45 it was 74±35 ms longer (p<0.01), compared with supine. During HUT at 15 the pulse interval was 25±9 ms shorter (p<0.05) than when supine, but was not significantly different at 30 and 45. These responses occurred independently of changes in brachial blood pressure. Attenuation was also observed after 5 min (56±17 ms; <0.05), and after 40 min (25±9 ms; p<0.05) of 60 HUT compared with supine. We conclude that posture does modify arterial baroreflex control of heart rate. If this occurs primarily as a result of a change in CBV, then the acute effect of weightlessness may be an accentuation, not an attenuation, of baroreflex function.M. H. Harrison was a National Research Council postdoctoral research fellow on leave from the Ministry of Defence, UK