title: Transcranial Doppler is valid for determination of the lower limit of cerebral blood flow autoregulation.
abstract: OBJECTIVE
  This study validates transcranial Doppler sonography (TCD) for determination of the lower limit of cerebral blood flow (CBF) autoregulation and establishes a relation between global CBF and mean flow velocity (Vmean) in the middle cerebral artery.
  
  
  METHODS
  Relative changes in CBF and in Vmean were compared in 12 normal volunteers (2 women and 10 men; median age, 30 years [range, 21 to 61 years]). Catheters was placed in the left radial artery and in the bulb of the right internal jugular vein, respectively. Baseline CBF was measured by single-photon emission computed tomography scanning; concomitantly, blood samples were drawn for calculation of the cerebral arteriovenous oxygen difference. Then changes in mean arterial pressure (MAP) were induced, and relative changes in global CBF were calculated according to Fick's principle assuming a constant cerebral oxygen metabolism. MAP was increased 30 mm Hg by norepinephrine infusion and was decreased by lower body negative pressure. Vmean was measured in the right middle cerebral artery by a 2-MHz probe, and blood samples were drawn at intervals of 5 mm Hg.
  
  
  RESULTS
  MAP values between 122 (range, 110 to 140) and 48 (range, 34 to 75) mm Hg were measured. The lower limit of autoregulation (the blood pressure under which autoregulation is off) as determined by Vmean did not differ significantly from that determined by relative changes in global CBF: 91 (range, 41 to 108) and 79 (range, 53 to 113) mm Hg, respectively. A significant correlation between Vmean and relative changes in global CBF was demonstrated below the lower limit of autoregulation (R2 = .73, P < .001; CBF = -6.3 + 1.0.Vmean). Above the lower limit both values were stable.
  
  
  CONCLUSIONS
  TCD is valid for determination of the lower limit of CBF autoregulation, and changes in CBF may be reliably evaluated by TCD during changes in cerebral perfusion pressure in normal subjects.
authors.affiliation: Department of Medicine, Rigshospitalet, University of Copenhagen, Denmark.
authors.forename: F S
authors.initials: FS
authors.lastname: Larsen
authors.forename: K S
authors.initials: KS
authors.lastname: Olsen
authors.forename: B A
authors.initials: BA
authors.lastname: Hansen
authors.forename: O B
authors.initials: OB
authors.lastname: Paulson
authors.forename: G M
authors.initials: GM
authors.lastname: Knudsen
mesh_terms: D000328:Adult; D001783:Blood Flow Velocity / Q000187:drug effects / Q000502:physiology; D001794:Blood Pressure / Q000187:drug effects / Q000502:physiology; D002536:Cerebral Arteries / Q000000981:diagnostic imaging* / Q000502:physiology*; D002560:Cerebrovascular Circulation / Q000187:drug effects / Q000502:physiology*; D005260:Female; D006706:Homeostasis / Q000187:drug effects / Q000502:physiology*; D006801:Humans; D007741:Labetalol / Q000494:pharmacology; D008297:Male; D008875:Middle Aged; D009638:Norepinephrine / Q000494:pharmacology; D012039:Regional Blood Flow / Q000187:drug effects / Q000502:physiology; D014294:Trimethaphan / Q000494:pharmacology; D017585:Ultrasonography, Doppler, Transcranial; D014655:Vascular Resistance / Q000187:drug effects / Q000502:physiology
chemical_list: D014294:Trimethaphan; D007741:Labetalol; D009638:Norepinephrine