EFFECTS OF HEAD-DOWN-TILT BED REST ON CEREBRAL HEMODYNAMICS DURING ORTHOSTATIC STRESS

Our aim was to determine whether the adaptation to simulated micrograv ity (mu G) impairs regulation of cerebral blood flow (CBF) during orth ostatic stress and contributes to orthostatic intolerance. Twelve heal thy subjects (aged 24 +/- 5 yr) underwent 2 wk of -6 degrees head-down -tilt (HDT) bed rest to simulate hemodynamic changes that occur when h umans are exposed to Fc-G. CBF velocity in the middle cerebral artery (transcranial Doppler), blood pressure, cardiac output (acetylene rebr eathing), and forearm blood flow were measured at each level of a ramp ed protocol of lower body negative pressure (LBNP; -15, -30, and -40 m mHg x 5 min, -50 mmHg x 3 min, then -10 mmHg every 3 min to presyncope ) before and after bed rest. Orthostatic tolerance was assessed by usi ng the cumulative stress index (CSI; mmHg x minutes) for the LBNP prot ocol. After bed rest, each individual's orthostatic tolerance was redu ced, with the group CSI decreased by 24% associated with greater decre ases in cardiac output and greater increases in systemic vascular resi stance at each level of LBNP. Before bed rest, mean CBF velocity decre ased by 14, 10, and 45% at -40 mmHg, -50 mmHg, and maximal LBNP, respe ctively. After bed rest, mean velocity decreased by 16% at -30 mmHg an d by 21, 35, and 39% at -40 mmHg, -50 mmHg, and maximal LBNP, respecti vely. Compared with pre-bed rest, post-bed-rest mean velocity was less by 11, 10, and 21% at -30, -40, and -50 mmHg, respectively. However, there was no significant difference at maximal LBNP. We conclude that cerebral autoregulation during orthostatic stress is impaired by adapt ation to simulated mu G as evidenced by an earlier and greater fall in CBF velocity during LBNP. We speculate that impairment of cerebral au toregulation may contribute to the reduced orthostatic tolerance after bed rest.