Entropy, pattern entropy, and related methods for the analysis of data on the time intervals between heartbeats from 24-h electrocardiograms

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Abstract
Sequences of the time intervals between heartbeats—medically termed RR intervals—extracted from 24-h electrocardiogram recordings are examined as three-dimensional return map images. The recordings were made in humans by means of the medically widely used portable electrocardiograph (Holter system). A time window measured in the number of heartbeats is used and different types of behavior are classified. Bifurcations between the types of dynamics of the heart are noted and a form of intermittency is found. An alternative quantitative measure—a form pattern entropy of the return map image—is defined that characterizes the dynamics of the RR interval sequence. It is shown that this is a measure of the degree of ordering of the RR interval sequence and as such it is a good novel medical diagnostic tool for analyzing heart rate variability which distinguishes between illness and health where other diagnostics fail.