Circadian rhythm in the cardiovascular system: considerations in non-invasive electrophysiology

Most cardiovascular activities show a circadian rhythm, as do electrophysiological phenomenon. Under the influence of both external stimuli and endogenous homoeostatic mechanisms, cardiac electrophysiological properties change diurnally and enable the cardiovascular system adapt to rest-exercise cycles. According to recent reports, almost all non-invasive electrophysiological phenomena, such as electrocardiographic indices, cardiac refractoriness and conduction, pacing and defibrillation threshold, heart rate variability indices, and even Q-T dispersion and T-wave alternans, show diurnal variability. Furthermore, many of these changes are clinically significant and may affect results of some diagnostic studies. These characteristics of the cardiovascular system require us keep in mind the "time" factor any time we analyze electrophysiological results and make clinical decisions.