Prolonged unloading of the cardiovascular system during bedrest and spaceflight weakens neural coupling between blood pressure and heart rate

Abstract

Impaired baroreflex control following bedrest and spaceflight severely affects rate of recovery and return to ambulatory tasks. Post-bedrest/spaceflight orthostatic intolerance remains a major concern for bedrest confined older patients and for astronauts on long-duration missions. Here, we investigated how baroreflex changes following cardiovascular unloading were associated with autonomic neural coupling between blood pressure and heart rate in returning astronauts and in healthy participants who had undergone prolonged bedrest. Cardiovascular data collected from 27 shuttle astronauts (8–16 days) were compared with 19 head-down tilt bedrest (HDBR, 60 days) volunteers. In each group, beat-by-beat systolic blood pressure (SBP) and heart period (R-to-R intervals, RR) were collected simultaneously during a supine-to-stand test (5–10 min) conducted around 10 days before launch or HDBR and within 120 min of landing or exiting HDBR. Indices characterizing interaction time (fraction time active), response gain (gain), and control directionality (causality) between SBP and RR (SBP→RR: neural reflex, and RR→SBP: mechanical coupling) during standing were derived from the beat-by-beat data. Following spaceflight, cardiac baroreflex fraction time active was reduced along with neural reflex causality, with much larger reductions seen with HDBR. No change in mechanical causality pre-post flight or HDBR was observed; however, comparison of male and female astronauts indicated that neural and mechanical causality were both lower in females. Altogether, these findings suggest that changes in blood pressure regulation after bedrest and spaceflight are primarily reflex/neurally mediated and provides a target for therapeutic approaches to cardiovascular deconditioning.