All subjects admitted to the study were submitted to clinical history taking, physical examination, and noninvasive assessment, including blood pressure measurement, resting 12-lead surface electrocardiogram, 24h ambulatory electrocardiogram monitoring, M-Mode/two-dimensional echocardiogram, signal-averaged electrocardiogram in both time and frequency domains. Selected subjects were further submitted to treadmill stress test and coronary angiography to rule out coronary heart disease.