title: Cerebral vascular response in airmen exposed to hypobaric hypoxia.
abstract: BACKGROUND
  Cerebral vascular response via local and reflex adjustments is part of the integrated response to hypoxia and is coupled with changes in systemic vascular resistances that allow a redistribution of blood flow toward the brain. The cerebral vascular response in airmen exposed to simulated high altitude is not clear, thus we sought to investigate this aspect.
  
  
  METHODS
  Four healthy military airmen were exposed to simulated high altitude in a hypobaric chamber according to a standard training protocol. Blood saturation (SpO2) and blood flow velocity with transcranial Doppler from the left middle cerebral artery (Vm) were continuously recorded. Pulsatility Index (PI), resistance index (RI), and systolic/diastolic ratio (S/D ratio) were computed. Alternate hypoxia-hyperoxia trials for 2 and 1 min, respectively, were used to assess the cerebrovascular response.
  
  
  RESULTS
  Acute hypoxia induced an increase in Vm that promptly recovered when the oxygen supply was restored (mean increase of 5.5% at 18,000 ft and 17.2% at 25,000 ft). Alternate hypoxia-hyperoxia at 25,000 and 18,000 ft elicited changes in both SpO2 and Vm. In hypoxia, PI significantly decreased (mean decrease o" 25.6% at 18,000 ft and 39.5% at 25,000 ft), as did RI (mean decrease of 18.7% at 18,000 ft and 34.4% at 25,000 ft), while S/D ratio increased.
  
  
  CONCLUSIONS
  The standard altitude training protocol induced a transient cerebrovascular response. The response was as expected, with hypoxia-induced vasodilation and opposite changes when breathing pure oxygen.
authors.affiliation: Italian Air Force Logistic Command, Flight Test Center, Aerospace Medicine Department, Pratica di Mare, Rome, Italy.
authors.forename: Anton Giulio
authors.initials: AG
authors.lastname: Guadagno
authors.forename: Fabio
authors.initials: F
authors.lastname: Morgagni
authors.forename: Edoardo
authors.initials: E
authors.lastname: Vicenzini
authors.forename: Leonardo
authors.initials: L
authors.lastname: Davì
authors.forename: G
authors.initials: G
authors.lastname: Ciniglio Appiani
authors.forename: Enrico
authors.initials: E
authors.lastname: Tomao
mesh_terms: D000328:Adult; D000337:Aerospace Medicine; D000531:Altitude; D001273:Atmosphere Exposure Chambers; D001783:Blood Flow Velocity; D001921:Brain / Q000098:blood supply*; D002560:Cerebrovascular Circulation / Q000502:physiology; D006801:Humans; D000860:Hypoxia / Q000503:physiopathology; D002534:Hypoxia, Brain / Q000503:physiopathology; D008297:Male; D008875:Middle Aged; D020768:Middle Cerebral Artery / Q000503:physiopathology; D011673:Pulsatile Flow / Q000502:physiology; D017585:Ultrasonography, Doppler, Transcranial; D014664:Vasodilation / Q000502:physiology