Fractal analysis of foetal heart rate variability

Abstract

Current methods for analysing foetal heart rate (FHR) patterns have yet to meet their full potential in the recognition of hypoxia in the foetus. Following the recent suggestion that fractal analysis can be applied to FHR recordings, the current paper describes a method for distinguishing two simultaneous fractal dimensions in FHR variation. An irregular line was plotted from 2500 consecutive foetal heart beat to beat intervals derived from an ultrasound source. A window of 500 intervals was moved along the line in steps of 20 intervals. At each step the Richardson technique was used to make estimates of the length of the line within the window using 40 different ruler lengths. When the estimates were plotted against the ruler lengths on log-log axes the resulting curve exhibited two distinct linear regions, each demonstrating an inverse power relationship. From the two slopes the fractal dimensions were derived for unspecified low- and high-frequency FHR variation in the current window. The values of both fractal dimensions were plotted simultaneously with the irregular FHR line and were found to accord with perceived changes in FHR variation. The method described is simply a measure of the irregularity in a series of foetal heart beat to beat intervals: the existence of fractal properties in the irregular line does not of itself imply underlying deterministic dynamics (e.g. chaos). This new method of observing FHR variability requires no preprocessing of the measured data, which are all taken into account. Not only does it represent a method for studying normal foetal behaviour but also has potential as a sensitive indicator of impending foetal compromise.