Autonomic cardiovascular changes during and after 14 days of head-down bed rest

A 14-day, 6° head-down bed rest (HDBR) study was conducted with 12 healthy young men to determine whether there are transient responses of the cardiovascular autonomic regulatory system including cardiovascular, autonomic nervous, and cardiac baroreceptor reflex functions in the acute phases of HDBR and post-HDBR. Compared with the supine position before bed rest, the high-frequency band power (HF_RRI) of RR intervals (RRIs) decreased significantly at 3, 6, and 24 h of HDBR. This tendency went on until 24 h post-HDBR. Three kinds of cardiac baroreceptor reflex sensitivity (BRS) were estimated from closed-loop approaches to simultaneously recorded spontaneous RRI and systolic arterial pressure (SAP) fluctuations. BRS_sequence is based on the simultaneous changes between RRI and SAP. α_LF and α_HF are based on a cross-spectrum analysis for low- and high-frequency bands of RRI and SAP. Although BRS_sequence decreased significantly at acute phases of both HDBR and post-HDBR, neither α_LF nor α_HF decreased significantly at any of the acute phases of HDBR and post-HDBR. Our results suggest that HF _RRI and BRS_sequence can be used effectively to reveal reductions in cardiac vagal nervous modulation on the sinus node and cardiac BRS within 24 h of both HDBR and post-HDBR.