ABSTRACT
Cardiac rhythm is controlled by membrane properties of the sinoatrial node and the activity of the neurohormonal and autonomic nervous system (ANS) modulation. There are two extrinsic ANS influences that determine the natural rhythm of the human heart, known as the sympathetic and parasympathetic signals. HRV is concerned only with changes in the time between successive heartbeats. It is noninvasive and easy to obtain from a surface lead III electrocardiogram (ECG) recording for short recording periods or from Holter data if longer recording periods are required as may be the case for analyzing circadian changes. Nonlinear methods include fractal analysis and entropy measures. The Rényi entropy is naturally a multiscale measure, as it generalizes the Shannon entropy and includes the Shannon entropy as a special case. The histogram method estimates the probability that an RR interval assumes a value within a given range.
