Comparison of baroreflex sensitivity gain during mild lower body negative pressure in presence and absence of long duration bed rest

Lower body negative pressure (LBNP) and head down bed rest (HDBR) are protocols used to simulate hypovolemia and cardiovascular deconditioning, causing an alteration of autonomic control of circulation. The objective of this study was to investigate the combined effects of LBNP and bed rest on cardiac baroreflex sensitivity (BRS). RR and systolic blood pressure (SBP) recordings from seven volunteers were analyzed during a mild LBNP protocol consisting of three different levels of LBNP (-10 mmHg, -20 mmHg, -30 mmHg) before (pre-HDBR) and on day 50 of a HDBR study. Spectra of RR and SBP were computed and BRS was assessed in the low frequency (LF) and high frequency (HF) bands through a bivariate model that takes into account the causal relationships between heart rate (HR) and arterial blood pressure. HR significantly increased from BL in HDBR for LBNP≤-20 mmHg. BRS gain decreased significantly in the LF band with increasing levels of LBNP in both conditions. BRS gain was significantly lower on day 50 of HDBR with respect to pre-HDBR at -20 mmHg. These data suggest that BRS in the LF range is reduced in bed rest, and these changes may be due primarily to a reduction in plasma volume associated with bed rest, which impact the physiological responses of autonomic control of circulation.