Sudden cardiac death usually is caused by malignant ventricular arrhythmias. Malignant ventricular arrhythmias in coronary heart disease are due to an interplay among substrate such as scarred ventricles, triggering events such as spontaneous ventricular arrhythmias, and the autonomic nervous system. Non-invasive methods were needed to evaluate these three components of risk in order to develop comprehensive detection and prevention programs. Non-invasive screening tests for the arrhythmogenic substrate include left ventricular ejection fraction and signal-averaged electrocardiograms, and for triggering events, the 24-hour continuous ECG recordings. Measures of heart rate variability defined as the variability of the instantaneous heart rates or heart period variability defined as variability of the normal R-R intervals may provide the means for non-invasive assessment of autonomic nervous system activity. In previous studies it has been shown that a broad band measure of heart period variability, the standard deviation of all normal R-R intervals in a continuous 24-hour ECG recording made eight to fourteen days after myocardial infarction, predicted mortality in the subsequent two to four years independently of left ventricular dysfunction and spontaneous ventricular arrhythmias.