Bed Rest Affects Ventricular and Arterial Elastances in Monkeys: Implications for Humans

Methods: Experimental data were obtained from five chronically instrumented rhesus monkeys exposed to 96 h of 10° head-down bed rest (HDBR) and another 96 h of 80° upright control separated by 9 d of ambulatory recovery in a counter-balanced, crossover experiment design to test the hypotheses that: 1) headward and footward fluid shifts would increase systemic arterial (Eart) and left ventricular end-systolic (Ees) elastances; and 2) changes in Eart and Ees would be related in magnitude and direction. Ees and Eart were calculated from measurements taken during five observation periods for initial 2-h and 4-d exposures to HDBR that produced headward volume shifts, and acute exposure to graded levels of lower body negative pressure (LBNP) designed to produce orthostatic volume shifts. Results: There was no effect of HDBR on Ees and Eart for any observation period (initial 2-h, 4-d, of LBNP). Eart increased in a similar pattern during the 4-d exposure to both control and HDBR. Ees increased with increasing LBNP levels for both control and HDBR while Eart remained unchanged. Conclusion: Our data are consistent with the notion that elevated Eart may represent an adaptation to physical inactivity that is associated with cardiovascular deconditioning.