title: Dynamic cerebral autoregulation after mild dehydration to simulate microgravity effects.
abstract: BACKGROUND
  During and immediately following a 2-wk spaceflight, dynamic cerebral autoregulation is improved, leading to stable cerebral blood flow velocity (CBFV). However, the factors inducing this improved cerebral autoregulation remain unclear. Although mild reduction of plasma volume (one of the key adaptations to spaceflight) may lead to improved cerebral autoregulation, there have been no reports estimating the alteration of dynamic cerebral autoregulation during mild dehydration. We, therefore, investigated the effect of mild intravascular dehydration on dynamic cerebral autoregulation.
  
  
  METHODS
  Furosemide, 0.2 mg x kg(-1), was administered to 14 men. Dynamic cerebral autoregulation was estimated before and approximately 45 min after furosemide administration using spectral and transfer function analyses between mean blood pressure (MBP) variability and mean CBFV variability. We then assessed the degree of dehydration as plasma volume changes, and/or central venous pressure (CVP).
  
  
  RESULTS
  Furosemide administration caused an approximate 10% reduction in plasma volume, with CVP decreasing by 1.2 mmHg. Steady state MBP and mean CBFV did not change. However, transfer function gain in the low-frequency range significantly decreased, implying a decrease in the magnitude of transfer from MBP oscillations to mean CBFV fluctuations.
  
  
  CONCLUSIONS
  The present degree of dehydration was similar to the alteration observed after short-term spaceflight, and enhanced dynamic cerebral autoregulation in the low-frequency range with unchanged steady state mean CBFV. Consequently, these results on cerebral circulation were consistent with those of a previous spaceflight study. Thus, it is suggested that mild intravascular dehydration partly explains the improved dynamic cerebral autoregulation observed during and immediately after spaceflight.
authors.affiliation: Division of Hygiene, Department of Social Medicine, Nihon University School of Medicine, Tokyo, Japan.
authors.forename: Yojiro
authors.initials: Y
authors.lastname: Ogawa
authors.forename: Ken-Ichi
authors.initials: K
authors.lastname: Iwasaki
authors.forename: Ken
authors.initials: K
authors.lastname: Aoki
authors.forename: Takashi
authors.initials: T
authors.lastname: Saitoh
authors.forename: Jitsu
authors.initials: J
authors.lastname: Kato
authors.forename: Setsuro
authors.initials: S
authors.lastname: Ogawa
mesh_terms: D001783:Blood Flow Velocity / Q000502:physiology; D001794:Blood Pressure / Q000502:physiology; D002496:Central Venous Pressure / Q000502:physiology; D002536:Cerebral Arteries / Q000503:physiopathology*; D002560:Cerebrovascular Circulation / Q000502:physiology*; D015331:Cohort Studies; D003681:Dehydration / Q000503:physiopathology*; D006801:Humans; D020896:Hypovolemia / Q000503:physiopathology*; D008297:Male; D018474:Weightlessness Simulation; D055815:Young Adult