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Vol. 83, Issue 6, 2139-2145, December 1997
Institute for Exercise and Environmental Medicine, Presbyterian Hospital of Dallas, and University of Texas Southwestern Medical Center at Dallas, Dallas, Texas 75231
Received 28 April 1997; accepted in final form 15 August 1997.
Zhang, Rong, Julie H. Zuckerman, James A. Pawelczyk, and
Benjamin D. Levine. Effects of head-down-tilt bed rest on cerebral
hemodynamics during orthostatic stress. J. Appl.
Physiol. 83(6): 2139-2145, 1997.
Our aim was to
determine whether the adaptation to simulated microgravity (µG)
impairs regulation of cerebral blood flow (CBF) during orthostatic
stress and contributes to orthostatic intolerance. Twelve
healthy subjects (aged 24 ± 5 yr) underwent 2 wk of 
6°
head-down-tilt (HDT) bed rest to simulate hemodynamic changes that
occur when humans are exposed to µG. CBF velocity in the middle
cerebral artery (transcranial Doppler), blood pressure, cardiac output
(acetylene rebreathing), and forearm blood flow were measured at each
level of a ramped protocol of lower body negative pressure (LBNP;
15, 30, and 40 mmHg × 5 min, 50 mmHg × 3 min, then 10 mmHg every 3 min to presyncope) before
and after bed rest. Orthostatic tolerance was assessed by using the
cumulative stress index (CSI; mmHg × minutes) for the LBNP
protocol. After bed rest, each individual's orthostatic tolerance was
reduced, with the group CSI decreased by 24% associated with greater
decreases in cardiac output and greater increases in systemic vascular
resistance at each level of LBNP. Before bed rest, mean CBF velocity
decreased by 14, 10, and 45% at 40 mmHg, 50 mmHg, and
maximal LBNP, respectively. After bed rest, mean velocity decreased by
16% at 30 mmHg and by 21, 35, and 39% at 40 mmHg,
50 mmHg, and maximal LBNP, respectively. 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
adaptation to simulated µG as evidenced by an earlier and greater fall in CBF velocity during LBNP. We speculate that impairment of
cerebral autoregulation may contribute to the reduced orthostatic tolerance after bed rest.
microgravity; blood flow; orthostasis; Doppler
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