Biological clocks and ventricular tachyarrhythmias.

Sudden cardiac death (SCD) due to sustained ventricular tachyarrhythmias (VTA) remains a major public health problem because of poor understanding of the mechanisms. Initiation of VTA in animal models and in patients usually requires intense metabolic or electrical stimuli not often observed for spontaneous arrhythmias, suggesting that profound but clinically silent electrophysiological changes are responsible for lethal VTA (1). Studies performed in the 1980s and 1990s showing diurnal variations of SCD and VTA generated excitement because they suggested that time-varying factors participate in the initiation of lethal VTA. For example, in a study of more than 24,000 SCDs, Arntz et al. (2) observed a nadir of events between 12 AM and 6 AM with a striking increase in events between 6 AM and 12 PM as well as a nadir on Sundays and greater event rate on Mondays. Similar distributions were reported in patients with implantable cardioverter-defibrillators (ICDs), which provide superior temporal precision and arrhythmia accuracy (3).