COMPLEXITY OF MIDDLE CEREBRAL-ARTERY BLOOD-FLOW VELOCITY - EFFECTS OFTILT AND AUTONOMIC FAILURE

We examined spectral fractal characteristics of middle cerebral artery (MCA) mean blood how velocity (MFV) and mean arterial blood pressure adjusted to the level of the brain (MAP(brain)) during graded tilt (5 min supine, -10 degrees, 10 degrees, 30 degrees, 60 degrees, -10 degre es, supine) in eight autonomic failure patients and age-and sex-matche d controls. From supine to 60 degrees, patients had a larger drop in M AP(brain) (62 +/- 4.7 vs. 23 +/- 4.5 mmHg, P < 0.001; means +/- SE) an d 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 p atients and the controls, but only the patients had a significant decr ease 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). T he beta value of MAP(brain) (MAP(brain)-beta; 2.19 +/- 0.05) was not s ignificantly different between patients and controls and did not chang e with tilt. High and low degrees of regulatory complexity are indicat ed 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 sugges ts an increase in the degree of autoregulation in the patients. This m ay be related to the drop in MAP(brain). The different response of MFV -beta compared with that of MAP(brain)-beta also indicates that MFV-be ta is related to the regulation of cerebral vascular resistance and no t systemic blood pressure.