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This Article Full Text Full Text (PDF) All Versions of this Article: 579/3/799    most recent jphysiol.2006.119636v1 Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Iwasaki, K.-i. Articles by Blomqvist, C. G. Search for Related Content PubMed PubMed Citation Articles by Iwasaki, K.-i. Articles by Blomqvist, C. G. Related Collections Cardiovascular

¹ Institute for Exercise and Environmental Medicine, Presbyterian Hospital of Dallas and the University of Texas South western Medical Center at Dallas, Dallas, TX, USA
² Nihon University School of Medicine, Toyko, Japan
³ The Pennsylvania State University, University Park and Hershey, PA, USA
⁴ Vanderbilt University, Nashville, TN, USA
⁵ Hunter Holmes McGuire Veterans Affairs Medical Center and Medical College of Virginia at Virginia Commonwealth University, Richmond, VA, USA
⁶ The University of Texas at San Antonio, San Antonio, TX, USA
⁷ Baylor Radiosurgery Center at Baylor University Medical Center at Dallas, Dallas, TX, USA
⁸ Dartmouth Hitchcock Medical Center, Lebanon, NH, USA
⁹ DLR-Institute for Aerospace Medicine, Cologne, Germany

Exposure to microgravity alters the distribution of body fluids and the degree of distension of cranial blood vessels, and these changes in turn may provoke structural remodelling and altered cerebral autoregulation. Impaired cerebral autoregulation has been documented following weightlessness simulated by head-down bed rest in humans, and is proposed as a mechanism responsible for postspaceflight orthostatic intolerance. In this study, we tested the hypothesis that spaceflight impairs cerebral autoregulation. We studied six astronauts 72 and 23 days before, after 1 and 2 weeks in space (n = 4), on landing day, and 1 day after the 16 day Neurolab space shuttle mission. Beat-by-beat changes of photoplethysmographic mean arterial pressure and transcranial Doppler middle cerebral artery blood flow velocity were measured during 5 min of spontaneous breathing, 30 mmHg lower body suction to simulate standing in space, and 10 min of 60 deg passive upright tilt on Earth. Dynamic cerebral autoregulation was quantified by analysis of the transfer function between spontaneous changes of mean arterial pressure and cerebral artery blood flow velocity, in the very low- (0.02–0.07 Hz), low- (0.07–0.20 Hz) and high-frequency (0.20–0.35 Hz) ranges. Resting middle cerebral artery blood flow velocity did not change significantly from preflight values during or after spaceflight. Reductions of cerebral blood flow velocity during lower body suction were significant before spaceflight (P < 0.05, repeated measures ANOVA), but not during or after spaceflight. Absolute and percentage reductions of mean (± S.E.M.) cerebral blood flow velocity after 10 min upright tilt were smaller after than before spaceflight (absolute, –4 ± 3 cm s^–1 after versus –14 ± 3 cm s^–1 before, P = 0.001; and percentage, –8.0 ± 4.8% after versus –24.8 ± 4.4% before, P < 0.05), consistent with improved rather than impaired cerebral blood flow regulation. Low-frequency gain decreased significantly (P < 0.05) by 26, 23 and 27% after 1 and 2 weeks in space and on landing day, respectively, compared with preflight values, which is also consistent with improved autoregulation. We conclude that human cerebral autoregulation is preserved, and possibly even improved, by short-duration spaceflight.

(Received 22 August 2006; accepted after revision 18 December 2006; first published online 21 December 2006)
Corresponding author B. D. Levine: Institute for Exercise and Environmental Medicine, 7232 Greenville Avenue, Suite 435, Dallas, TX 75231, USA. Email: benjaminlevine{at}texashealth.org

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