title: Critical analysis of cerebrovascular autoregulation during repeated head-up tilt.
abstract: OBJECTIVE
  Cerebrovascular autoregulation has been described with a phase lead of cerebral blood flow preceding changes in cerebral perfusion pressure (CPP), but there has been less focus on the effect of CPP on cerebral vascular resistance. We investigated these relations during spontaneous fluctuations (control) and repeated head-up tilt.
  
  
  METHODS
  Eight healthy adults were studied in supine rest and repeated tilt with 10-second supine, 10 seconds at 45 degrees head-up tilt for a total of 12 cycles. Cerebral blood flow was estimated from mean flow velocity (MFV) by transcranial Doppler ultrasound, CPP was estimated from corrected finger pressure (CPP(F)), and cerebrovascular resistance index (CVRi) was calculated in the supine position from CPP(F)/MFV. Gain and phase relations were assessed by cross-spectral analysis.
  
  
  RESULTS
  In the supine position, MFV preceded CPP(F), but changes in CVRi followed CPP(F). Gain and phase relations for CPP(F) as input and MFV as output were similar in supine and repeated tilt experiments. Thus, changes in cerebrovascular resistance must have had a similar pattern in the supine and tilt experiments.
  
  
  CONCLUSIONS
  Cerebrovascular autoregulation is achieved by changes in resistance in response to modulations in perfusion pressure whether spontaneous or induced by repeated tilt. The phase lead of MFV before CPP(F) is a mathematical and physiological consequence of the relation the input variable (CPP(F)) and the manipulated variable (cerebrovascular resistance) that should not be taken as an indication of independent control of cerebral blood flow.
authors.affiliation: Cardiorespiratory and Vascular Dynamics Laboratory, Faculty of Applied Health Sciences, University of Waterloo, Waterloo, Ontario, Canada. hughson@uwaterloo.ca
authors.forename: R L
authors.initials: RL
authors.lastname: Hughson
authors.forename: M R
authors.initials: MR
authors.lastname: Edwards
authors.forename: D D
authors.initials: DD
authors.lastname: O'Leary
authors.forename: J K
authors.initials: JK
authors.lastname: Shoemaker
mesh_terms: D000328:Adult; D001783:Blood Flow Velocity / Q000502:physiology; D001794:Blood Pressure / Q000502:physiology; D002245:Carbon Dioxide / Q000032:analysis; D002560:Cerebrovascular Circulation / Q000502:physiology*; D004562:Electrocardiography; D005260:Female; D006339:Heart Rate; D006706:Homeostasis / Q000502:physiology*; D006801:Humans; D008297:Male; D020768:Middle Cerebral Artery / Q000000981:diagnostic imaging / Q000502:physiology; D010507:Periodicity; D017156:Photoplethysmography; D011187:Posture / Q000502:physiology*; D011659:Pulmonary Gas Exchange; D012016:Reference Values; D016683:Supine Position / Q000502:physiology; D018667:Tilt-Table Test / Q000379:methods; D017585:Ultrasonography, Doppler, Transcranial; D014655:Vascular Resistance / Q000502:physiology
chemical_list: D002245:Carbon Dioxide