title: Cerebral autoregulatory responses to head-up tilt in normal subjects and patients with recurrent vasovagal syncope.
abstract: BACKGROUND
  The effect of orthostatic stress on dynamic cerebral autoregulation (CA) in normal subjects and patients with recurrent vasovagal syncope (VVS) is unclear. This study assessed the dynamic CA responses of both groups to head-up tilt.
  
  
  RESULTS
  Seventeen patients with recurrent VVS and 17 pair-matched control subjects underwent 70 degrees head-up tilt for up to 30 minutes. Bilateral middle cerebral artery blood flow velocities (CBFV) were measured with transcranial Doppler ultrasound along with noninvasive beat-to-beat blood pressure (BP), heart rate, and transcutaneous and end-tidal CO(2) concentrations. Indices of dynamic CA were derived for periods before, during, and after tilt. Eight normal subjects who developed VVS in an identical protocol but who had no previous clinical history of syncope were also studied. CBFV and transcutaneous and end-tidal CO(2) levels declined significantly during head-up tilt in all groups (P<0.0001). Dynamic CA indices were unchanged throughout tilt in nonsyncopal control subjects and were initially unchanged in patients but deteriorated significantly in patients and syncopal control subjects in the minutes before (P=0.027 and P=0.012, respectively) and after (P=0.002 and P=0.007, respectively) syncope.
  
  
  CONCLUSIONS
  Dynamic CA is preserved in patients and control subjects initially after head-up tilt. Autoregulatory function remains intact in nonsyncopal control subjects during prolonged orthostasis but deteriorates in patients and syncopal control subjects immediately before and after VVS.
authors.affiliation: Division of Medicine for the Elderly, University of Leicester, Glenfield Hospital, UK. brian.carey@uhl-tr.nhs.uk
authors.forename: B J
authors.initials: BJ
authors.lastname: Carey
authors.forename: B N
authors.initials: BN
authors.lastname: Manktelow
authors.forename: R B
authors.initials: RB
authors.lastname: Panerai
authors.forename: J F
authors.initials: JF
authors.lastname: Potter
mesh_terms: D001783:Blood Flow Velocity; D001785:Blood Gas Monitoring, Transcutaneous; D001794:Blood Pressure; D002560:Cerebrovascular Circulation; D004562:Electrocardiography; D006339:Heart Rate; D006801:Humans; D008875:Middle Aged; D020768:Middle Cerebral Artery / Q000000981:diagnostic imaging / Q000503:physiopathology; D011187:Posture; D011659:Pulmonary Gas Exchange; D012008:Recurrence; D012016:Reference Values; D019462:Syncope, Vasovagal / Q000175:diagnosis / Q000503:physiopathology*; D018667:Tilt-Table Test / Q000379:methods*; D017585:Ultrasonography, Doppler, Transcranial