title: Human cerebral autoregulation before, during and after spaceflight.
abstract: 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 approximately 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.
authors.affiliation: Institute for Exercise and Environmental Medicine, 7232 Greenville Avenue, Suite 435, Dallas, TX 75231, USA.
authors.forename: Ken-ichi
authors.initials: K
authors.lastname: Iwasaki
authors.forename: Benjamin D
authors.initials: BD
authors.lastname: Levine
authors.forename: Rong
authors.initials: R
authors.lastname: Zhang
authors.forename: Julie H
authors.initials: JH
authors.lastname: Zuckerman
authors.forename: James A
authors.initials: JA
authors.lastname: Pawelczyk
authors.forename: André
authors.initials: A
authors.lastname: Diedrich
authors.forename: Andrew C
authors.initials: AC
authors.lastname: Ertl
authors.forename: James F
authors.initials: JF
authors.lastname: Cox
authors.forename: William H
authors.initials: WH
authors.lastname: Cooke
authors.forename: Cole A
authors.initials: CA
authors.lastname: Giller
authors.forename: Chester A
authors.initials: CA
authors.lastname: Ray
authors.forename: Lynda D
authors.initials: LD
authors.lastname: Lane
authors.forename: Jay C
authors.initials: JC
authors.lastname: Buckey
authors.forename: Friedhelm J
authors.initials: FJ
authors.lastname: Baisch
authors.forename: Dwain L
authors.initials: DL
authors.lastname: Eckberg
authors.forename: David
authors.initials: D
authors.lastname: Robertson
authors.forename: Italo
authors.initials: I
authors.lastname: Biaggioni
authors.forename: C Gunnar
authors.initials: CG
authors.lastname: Blomqvist
mesh_terms: D000222:Adaptation, Physiological / Q000502:physiology*; D000328:Adult; D018480:Astronauts; D001783:Blood Flow Velocity / Q000502:physiology; D001794:Blood Pressure / Q000502:physiology; D001921:Brain / Q000098:blood supply* / Q000502:physiology*; D002560:Cerebrovascular Circulation / Q000502:physiology; D006706:Homeostasis / Q000502:physiology*; D006801:Humans; D007024:Hypotension, Orthostatic / Q000503:physiopathology; D008297:Male; D013026:Space Flight; D013312:Stress, Physiological / Q000503:physiopathology; D014893:Weightlessness