Fractal fluctuations in cardiac time series

Human heart rate, controlled by complex feedback mechanisms, is a vital ind ex of systematic circulation. However, it has been shown that beat-to-beat values of heart rate fluctuate continually over a wide range of time scales . Herein we use the relative dispersion, the ratio of the standard deviatio n to the mean, to show, by systematically aggregating the data, that the co rrelation in the beat-to-beat cardiac time series is a modulated inverse po wer law. This scaling property indicates the existence of long-time memory in the underlying cardiac control process and supports the conclusion that heart rate variability is a temporal fractal. We argue that the cardiac con trol system has allometric properties that enable it to respond to a dynami cal environment through scaling. (C) 1999 Elsevier Science B.V. All rights reserved.