TEMPORAL HETEROGENEITY OF THE BLOOD-FLOW VELOCITY AT THE MIDDLE CEREBRAL-ARTERY IN THE NORMAL HUMAN CHARACTERIZED BY FRACTAL ANALYSIS

The objective of this study is to characterize the temporal fluctuatio n of the axial blood flow velocity (BFV) at the middle cerebral artery (MCA). Biological observables such as BFV present complex oscillation s. The irregularity of physiological systems map be assessed by fracta l analysis by computing the fractal dimension (D-gamma) and the corres ponding temporal correlation (r(gamma)) The BFV at the MCA was registe red with transcranial Doppler ultrasonography (TCD) in four adult volu nteers. As fractal processes are assumed to have no absolute time scal e, two time scales were compared. The digitized signal was averaged re spectively at 1-s intervals and for each heart beat. D-gamma. and r(ga mma) were determined using relative dispersion analysis. The results w ere D-gamma = 1.24 +/- 0.09 and r(gamma) = 0.45 +/- 0.19 (mean +/- SD) for the 1-s based time scale and D-r = 1.17 +/- 0.09 and r(gamma) = 0 .57 +/- 0.20 for the heart-beat scale. We conclude that the temporal h eterogeneity of the BFV at the MCA in the normal human has fractal pro perties. Fractal analysis of TCD data may become useful in clinical di agnosis because loss of complexity in physiological systems has been l inked to senescence or disease conditions. Wide variations of the so c alled normal values of BFV measured by TCD have been reported. The phy siological BFV fluctuations may explain, in part, the variability of v alues recorded during routine TCD diagnostic examinations. Our observa tions may also be of value for understanding the interaction of the va scular endothelium and the blood flow stream (shear stress).