title: Cerebrovascular and cardiovascular responses to graded tilt in patients with autonomic failure.
abstract: OBJECTIVE
  Patients with autonomic nervous system failure often experience symptoms of orthostatic intolerance while standing. It is not known whether these episodes are caused primarily by a reduced ability to regulate arterial blood pressure or whether changes in cerebral autoregulation may also be implicated.
  
  
  METHODS
  Eleven patients and eight healthy age- and sex-matched control subjects were studied during a graded-tilt protocol. Changes in their steady state middle cerebral artery mean flow velocities (MFV), measured by transcranial Doppler, brain-level mean arterial blood pressures (MABPbrain), and the relationship between the two were assessed.
  
  
  RESULTS
  Significant differences between patients and control subjects (P < .05) were found in both their MFV and MABPbrain responses to tilt. Patients' MFV dropped from 60 +/- 10.2 cm/s in the supine position to 44 +/- 14.0 cm/s at 60 degrees head-up tilt, whereas MABPbrain fell from 109 +/- 11.7 to 42 +/- 16.9 mm Hg. By comparison, controls' MFV dropped from 54 +/- 7.8 cm/s supine to 51 +/- 8.8 cm/s at 60 degrees, whereas MABPbrain went from 90 +/- 11.2 to 67 +/- 8.2 mm Hg. Linear regression showed no significant difference in the MFV-MABPbrain relationship between patients and control subjects, with slopes of 0.228 +/- 0.09 cm.s-1.mm Hg-1 for patients and 0.136 +/- 0.16 cm.s-1.mm Hg-1 for control subjects.
  
  
  CONCLUSIONS
  The present study found significant differences between patients and control subjects in their MFV and MABPbrain responses to tilt but no difference in the autoregulatory MFV-MABPbrain relationship. These results suggest that patients' decreased orthostatic tolerance may primarily be the result of impaired blood pressure regulation rather than a deficiency in cerebral autoregulation.
authors.affiliation: University of Western Ontario, Faculty of Kinesiology, London, Ontario, Canada.
authors.forename: R L
authors.initials: RL
authors.lastname: Bondar
authors.forename: P T
authors.initials: PT
authors.lastname: Dunphy
authors.forename: P
authors.initials: P
authors.lastname: Moradshahi
authors.forename: M S
authors.initials: MS
authors.lastname: Kassam
authors.forename: A P
authors.initials: AP
authors.lastname: Blaber
authors.forename: F
authors.initials: F
authors.lastname: Stein
authors.forename: R
authors.initials: R
authors.lastname: Freeman
mesh_terms: D000328:Adult; D000368:Aged; D001342:Autonomic Nervous System Diseases / Q000503:physiopathology*; D001783:Blood Flow Velocity; D001794:Blood Pressure / Q000502:physiology; D002319:Cardiovascular System / Q000503:physiopathology*; D002536:Cerebral Arteries / Q000000981:diagnostic imaging / Q000503:physiopathology; D002560:Cerebrovascular Circulation / Q000502:physiology*; D005260:Female; D018475:Head-Down Tilt / Q000502:physiology*; D006706:Homeostasis / Q000502:physiology; D006801:Humans; D008297:Male; D008875:Middle Aged; D012016:Reference Values; D016683:Supine Position / Q000502:physiology; D017585:Ultrasonography, Doppler, Transcranial