Prolonged head down bed rest-induced inactivity impairs tonic autonomic regulation while sparing oscillatory cardiovascular rhythms in healthy humans

Abstract

Background Physical inactivity represents a major risk for cardiovascular disorders, such as hypertension, myocardial infarction or sudden death; however, underlying mechanisms are not clearly elucidated. Clinical and epidemiological investigations suggest, beyond molecular changes, the possibility of an induced impairment in autonomic cardiovascular regulation. However, this hypothesis has not been tested directly.Methods Accordingly, we planned a study with noninvasive, minimally intrusive, techniques on healthy volunteers. Participants were maintained for 90 days strictly in bed, 24 h a day, in head-down (S6-) position (HDBR). Physical activity was thus virtually abolished for the entire period of HDBR. We examined efferent muscle sympathetic nerve activity, as a measure of vascular sympathetic control, baroreceptor reflex sensitivity, heart rate variability (assessing cardiovagal regulation), RR and systolic arterial pressure and low-frequency and high-frequency normalized components (as a window on central oscillatory regulation). Measures were obtained at rest and during simple maneuvers (moderate handgrip, lower body negative pressure and active standing) to assess potential changes in autonomic cardiovascular responsiveness to standard stimuli and the related oscillatory profiles.Results HDBR transiently reduced muscle sympathetic nerve activity, RR, heart rate variability and baroreceptor reflex sensitivity late during HDBR or early during the recovery phase. Conversely, oscillatory profiles of RR and systolic arterial pressure variability were maintained throughout. Responsiveness to test stimuli was also largely maintained.Conclusion Prolonged inactivity as induced by HDBR in healthy volunteers reduces both cardiovagal and vascular sympathetic regulation, while largely maintaining peripheral responsiveness to standardized stimuli and sparing the functional structure of central oscillatory cardiovascular regulation.

Sex differences in blood pressure control during 6° head-down tilt bed rest

“…Although some studies have found that baroreflex sensitivity in flight is decreased (22) or similar to that in the supine position preflight (19,62), most studies have agreed that baroreflex sensitivity is decreased after spaceflight (16,29,30). The reduction in baroreflex sensitivity after spaceflight is consistent with our findings during bed rest and those of other bed rest studies (13,15,28,39,57). Spaceflight has also been shown to cause changes in autonomic function similar to those we found in bed rest, such as diminished vagal activity (2,16,22,62), increased sympathetic activity (11,22,26,30,31), and increased sympathovagal balance (2,30).…”

“…The differences in these findings may be due to different study lengths, stimulus during the measurement, or methodology, where the particular method could be measuring muscle nerve activity, sympathetic tone, or cardiac sympathetic activity. Although the effect of bed rest on sympathetic activity has been controversial, our results agree with the consensus that parasympathetic modulation (17,40,42,52) and baroreflex sensitivity (13,21,28,39,57) decrease with bed rest.…”

Operational point of neural cardiovascular regulation in humans up to 6 months in space

“…The number of effective baroreflex estimates was used as the quantitative output of cardiac baroreflex function. Results of this analysis illustrate that the operational point of neural cardiovascular regulation in space adapts to a level close to what can be expected for the supine position on Earth (15). In particular, our data confirm that vagal-inhibitory effects occurring early in microgravity (41) prevail during prolonged space missions (4), supporting the hypothesis that the cardiovascular system is chronically relaxed in space (31).…”

Is prolonged sitting at work associated with the time course of neck–shoulder pain? A prospective study in Danish blue-collar workers

“…19 Further, inactivity may affect cardiovascular and pain regulatory systems in the central nervous system. [20][21][22][23] Thus, extensive sitting may lead to less effective pain modulation. On the other hand, sitting could also be expected to be associated with a favourable course of neck-shoulder pain in occupations including high physical demands, such as blue-collar work.…”