title: Dynamic pressure--flow velocity relationships in the human cerebral circulation.
abstract: OBJECTIVE
  The pressure-flow velocity relationship in the cerebral circulation is characterized by the critical closing pressure (CCP), which is the pressure at which flow ceases, and the linear slope of a plot between pressure and flow velocity. It has been suggested, but not validated, that CCP can be determined from arterial blood pressure (ABP) and transcranial Doppler (TCD) recordings during the cardiac cycle. We studied a group of patients in whom ventricular fibrillation (VF) was induced. The time interval before defibrillation enabled calculation of CCP from data in which flow approached zero. These estimates were compared with values calculated before and after fibrillation and during regular heartbeats.
  
  
  METHODS
  TCD velocities and ABP in the radial artery were recorded before, during, and after 28 episodes of VF in 13 patients. CCPs were calculated by 3 different methods: (1) linear extrapolation from data during VF (gold standard); (2) linear extrapolation from normal heartbeat data; and (3) first harmonic Fourier filtering of normal heartbeat data.
  
  
  RESULTS
  The CCP during VF calculated from long diastoles was 32.9+/-11 mm Hg (mean+/-SD). The regular heartbeat estimate was 6.0+/-4.3 mm Hg lower (P<0.05). The CCP estimate with the use of a Fourier filter was 1.4+/-3.9 mm Hg (P=NS) lower than during VF. During hyperemia after defibrillation, the CCP decreased by 13.3 mm Hg, while velocity increased by 63%. The decrease in CCP could explain half of the increase in flow velocity during hyperemia.
  
  
  CONCLUSIONS
  CCP can be accurately estimated from regular heartbeat data and is an important factor in regulation of the cerebral circulation.
authors.affiliation: Department of Neurological Surgery, University of Washington School of Medicine, University of Washington Medical Center, Harborview Medical Center, Seattle 98104, USA.
authors.forename: Rune
authors.initials: R
authors.lastname: Aaslid
authors.forename: Stephanie R
authors.initials: SR
authors.lastname: Lash
authors.forename: Gust H
authors.initials: GH
authors.lastname: Bardy
authors.forename: William H
authors.initials: WH
authors.lastname: Gild
authors.forename: David W
authors.initials: DW
authors.lastname: Newell
mesh_terms: D000328:Adult; D000368:Aged; D001783:Blood Flow Velocity / Q000502:physiology; D001794:Blood Pressure / Q000502:physiology; D001921:Brain / Q000098:blood supply* / Q000502:physiology*; D002560:Cerebrovascular Circulation / Q000502:physiology*; D003971:Diastole / Q000502:physiology; D004554:Electric Countershock; D005260:Female; D005542:Forearm / Q000098:blood supply; D005583:Fourier Analysis; D006339:Heart Rate; D006801:Humans; D006940:Hyperemia / Q000503:physiopathology; D016014:Linear Models; D008297:Male; D008875:Middle Aged; D020768:Middle Cerebral Artery / Q000000981:diagnostic imaging / Q000502:physiology / Q000503:physiopathology; D017534:Radial Artery / Q000502:physiology; D015203:Reproducibility of Results; D017585:Ultrasonography, Doppler, Transcranial; D014693:Ventricular Fibrillation / Q000503:physiopathology / Q000628:therapy