title: Cerebral artery blood flow velocity changes following rapid release of lower body negative pressure.
abstract: BACKGROUND
  Circulatory changes occur during exposure to Lower Body Negative Pressure (LBNP). These changes may have some similarities to exposure to moderately and slowly increased G-loads in a relaxed subject without anti-G suit.
  
  
  OBJECTIVE
  Changes will also occur in cerebral blood circulation during a rapid release of LBNP.
  
  
  METHODS
  Transcranial Doppler ultrasound (TCD) was used to measure middle cerebral artery blood flow velocity (CBFV) in 14 human subjects following rapid release of a ramped lower body negative pressure (LBNP) (0.33 mm Hg.s) to presyncope (mean peak negative pressure of -124 mm Hg).
  
  
  RESULTS
  The mean CBFV decreased to an average of 60% (p < 0.05) of the baseline value at peak LBNP. Mean CBFV was still decreased to 65% and 84% of the baseline value (p < 0.05) at the third heart beat and 30 s, respectively, after pressure release. The systolic CBFV decreased similarly to 57% (p < 0.05) of baseline during peak LBNP, and was still 63% (p < 0.05) at the third heart beat after pressure release. Heart rate increased by a mean of 51% (p < 0.001) and systolic heart level blood pressure decreased by 28% (p < 0.001) during peak negative pressure. Both heart rate and blood pressure returned to baseline levels within 30 s after pressure release.
  
  
  CONCLUSIONS
  Following a presyncopal LBNP, the CBFV is not fully restored up to 30 s after the release of the negative pressure. This delayed returning of cerebral circulation following orthostatic stress may have some similarities to what occurs after the release of a gradual onset G-load in a relaxed subject without anti-G suit.
authors.affiliation: Armstrong Laboratory, Brooks AFB, TX 78235-5104, USA.
authors.forename: U I
authors.initials: UI
authors.lastname: Balldin
authors.forename: L P
authors.initials: LP
authors.lastname: Krock
authors.forename: N L
authors.initials: NL
authors.lastname: Hopper
authors.forename: W G
authors.initials: WG
authors.lastname: Squires
mesh_terms: D000328:Adult; D001359:Aviation; D001783:Blood Flow Velocity; D001794:Blood Pressure; D002536:Cerebral Arteries / Q000000981:diagnostic imaging / Q000502:physiology*; D002560:Cerebrovascular Circulation / Q000502:physiology*; D004562:Electrocardiography; D006339:Heart Rate; D006801:Humans; D018471:Hypergravity / Q000009:adverse effects*; D008150:Lower Body Negative Pressure; D008297:Male; D013997:Time Factors; D017585:Ultrasonography, Doppler, Transcranial