Effects of head-down-tilt bed rest on cerebral hemodynamics during orthostatic stress

Rong Zhang, Julie H. Zuckerman, James A. Pawelczyk, Benjamin D. Levine

Research output: Contribution to journalArticle

87 Citations (Scopus)

Abstract

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 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 using the cumulative stress index (CSI; mmHg x 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, pest-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.

Original languageEnglish (US)
Pages (from-to)2139-2145
Number of pages7
JournalJournal of Applied Physiology
Volume83
Issue number6
StatePublished - Dec 1997

Fingerprint

Lower Body Negative Pressure
Head-Down Tilt
Bed Rest
Cerebrovascular Circulation
Hemodynamics
Blood Flow Velocity
Cardiac Output
Homeostasis
Orthostatic Intolerance
Acetylene
Weightlessness
Middle Cerebral Artery
Syncope
Forearm
Vascular Resistance
Healthy Volunteers
Blood Pressure

Keywords

  • Blood flow
  • Doppler
  • Microgravity
  • Orthostasis

ASJC Scopus subject areas

  • Physiology
  • Endocrinology
  • Orthopedics and Sports Medicine
  • Physical Therapy, Sports Therapy and Rehabilitation

Cite this

Effects of head-down-tilt bed rest on cerebral hemodynamics during orthostatic stress. / Zhang, Rong; Zuckerman, Julie H.; Pawelczyk, James A.; Levine, Benjamin D.

In: Journal of Applied Physiology, Vol. 83, No. 6, 12.1997, p. 2139-2145.

Research output: Contribution to journalArticle

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abstract = "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 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 using the cumulative stress index (CSI; mmHg x 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, pest-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.",
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