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Am J Physiol Heart Circ Physiol 273: H2209-H2216, 1997;
0363-6135/97 $5.00
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Vol. 273, Issue 5, H2209-H2216, November 1997

Complexity of middle cerebral artery blood flow velocity: effects of tilt and autonomic failure

A. P. Blaber1, R. L. Bondar1, F. Stein2, P. T. Dunphy2, P. Moradshahi2, M. S. Kassam2, and R. Freeman3

1 School of Kinesiology, Faculty of Health Sciences, The University of Western Ontario, London, Ontario N6A 3K7; 2 Centre for Advanced Technology Education, Ryerson Polytechnic University, Toronto, Ontario, Canada M5B 2K3; and 3 Department of Neurology, Peripheral Nerve and Autonomic Laboratory, Beth Israel Deaconess Medical Center and Harvard Medical School, Boston, Massachusetts 02215

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°, 10°, 30°, 60°, -10°, supine) in eight autonomic failure patients and age- and sex-matched controls. From supine to 60°, 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°, 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°: 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.

spectral analysis; cerebral autoregulation; fractal


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