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Effect of head-down-tilt bed rest and hypovolemia on dynamic regulation of heart rate and blood pressureAdaptation to head-down-tilt bed rest leads to an apparent abnormality of baroreflex regulation of cardiac period. We hypothesized that this "deconditioning response" could primarily be a result of hypovolemia, rather than a unique adaptation of the autonomic nervous system to bed rest. To test this hypothesis, nine healthy subjects underwent 2 wk of -6 degrees head-down bed rest. One year later, five of these same subjects underwent acute hypovolemia with furosemide to produce the same reductions in plasma volume observed after bed rest. We took advantage of power spectral and transfer function analysis to examine the dynamic relationship between blood pressure (BP) and R-R interval. We found that 1) there were no significant differences between these two interventions with respect to changes in numerous cardiovascular indices, including cardiac filling pressures, arterial pressure, cardiac output, or stroke volume; 2) normalized high-frequency (0.15-0.25 Hz) power of R-R interval variability decreased significantly after both conditions, consistent with similar degrees of vagal withdrawal; 3) transfer function gain (BP to R-R interval), used as an index of arterial-cardiac baroreflex sensitivity, decreased significantly to a similar extent after both conditions in the high-frequency range; the gain also decreased similarly when expressed as BP to heart rate x stroke volume, which provides an index of the ability of the baroreflex to alter BP by modifying systemic flow; and 4) however, the low-frequency (0.05-0.15 Hz) power of systolic BP variability decreased after bed rest (-22%) compared with an increase (+155%) after acute hypovolemia, suggesting a differential response for the regulation of vascular resistance (interaction, P < 0.05). The similarity of changes in the reflex control of the circulation under both conditions is consistent with the hypothesis that reductions in plasma volume may be largely responsible for the observed changes in cardiac baroreflex control after bed rest. However, changes in vasomotor function associated with these two conditions may be different and may suggest a cardiovascular remodeling after bed rest.
Document ID
20040112684
Acquisition Source
Legacy CDMS
Document Type
Reprint (Version printed in journal)
Authors
Iwasaki, K. I.
(Institute for Exercise and Environmental Medicine, Presbyterian Hospital of Dallas, The University of Texas Southwestern Medical Center at Dallas and Space Medicine Laboratory, Department of Internal Medicine, Dallas, Texas 75235, United States)
Zhang, R.
Zuckerman, J. H.
Pawelczyk, J. A.
Levine, B. D.
Blomqvist, C. G.
Date Acquired
August 21, 2013
Publication Date
December 1, 2000
Publication Information
Publication: American journal of physiology. Regulatory, integrative and comparative physiology
Volume: 279
Issue: 6
ISSN: 0363-6119
Subject Category
Aerospace Medicine
Funding Number(s)
CONTRACT_GRANT: HL-53206-03
Distribution Limits
Public
Copyright
Other
Keywords
Non-NASA Center
NASA Discipline Cardiopulmonary

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