title: Complexity of middle cerebral artery blood flow velocity: effects of tilt and autonomic failure.
abstract: We examined spectral fractal characteristics of middle cerebral artery (MCA) mean blood flow velocity (MFV) and mean arterial blood pressure adjusted to the level of the brain (MAPbrain) during graded tilt (5 min supine, -10 degrees, 10 degrees, 30 degrees, 60 degrees, -10 degrees, supine) in eight autonomic failure patients and age- and sex-matched controls. From supine to 60 degrees, patients had a larger drop in MAPbrain (62 +/- 4.7 vs. 23 +/- 4.5 mmHg, P < 0.001; means +/- SE) and MFV (16.4 +/- 3.8 vs. 7.0 +/- 2.5 cm/s, P < 0.001) than in controls. From supine to 60 degrees, there was a trend toward a decrease in the slope of the fractal component (beta) of MFV (MFV-beta) in both the patients and the controls, but only the patients had a significant decrease in MFV-beta (supine: patient = 2.21 +/- 0.18, control = 1.99 +/- 0.60; 60 degrees: patient = 1.46 +/- 0.24, control = 1.62 +/- 0.19). The beta value of MAPbrain (MAPbrain-beta; 2.19 +/- 0.05) was not significantly different between patients and controls and did not change with tilt. High and low degrees of regulatory complexity are indicated by values of beta close to 1.0 and 2.0, respectively. The increase in fractal complexity of cerebral MFV in the patients with tilt suggests an increase in the degree of autoregulation in the patients. This may be related to the drop in MAPbrain. The different response of MFV-beta compared with that of MAPbrain-beta also indicates that MFV-beta is related to the regulation of cerebral vascular resistance and not systemic blood pressure.
authors.affiliation: School of Kinesiology, Faculty of Health Sciences, University of Western Ontario, London, Canada.
authors.forename: A P
authors.initials: AP
authors.lastname: Blaber
authors.forename: R L
authors.initials: RL
authors.lastname: Bondar
authors.forename: F
authors.initials: F
authors.lastname: Stein
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: R
authors.initials: R
authors.lastname: Freeman
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D001342:Autonomic Nervous System Diseases / Q000503:physiopathology*; D001783:Blood Flow Velocity / Q000502:physiology*; D001794:Blood Pressure / Q000502:physiology*; D002536:Cerebral Arteries / Q000502:physiology* / Q000503:physiopathology*; D003922:Diabetes Mellitus, Type 1 / Q000503:physiopathology; D003929:Diabetic Neuropathies / Q000503:physiopathology; D005260:Female; D018475:Head-Down Tilt / Q000502:physiology*; D006400:Hematocrit; D006801:Humans; D008297:Male; D008875:Middle Aged; D012016:Reference Values; D012791:Shy-Drager Syndrome / Q000503:physiopathology; D016683:Supine Position