Fractal fluctuations in transcranial Doppler signals

Cerebral blood flow (CBF) velocity measured using transcranial Doppler ultr asonography (TCD) is not strictly constant, but has both a systematic and r andom component. This behavior map indicate that the axial blood Bow in the middle cerebral artery is a chaotic process. Herein we use the relative di spersion, the ratio of the standard deviation to the mean, to show by syste matically aggregating the data that the correlation in the beat-to-beat CBF time series is a modulated inverse power law. This scaling of the CBF time series indicates the existence of long-time memory in the underlying contr ol process. We argue herein that the control system has allometric properti es that enable it to maintain a relatively constant brain perfusion. [S1063 -651X(99)11503-3].