Background: A morning peak in occurrence of sudden cardiac death has been identified in epidemiological studies, but the studies are subject to selection bias, with the exclusion of unwitnessed deaths, which are more likely to occur at night. The recent availability of implantable cardioverter/defibrillators that record the time of ventricular tachyarrhythmias requiring either pacing or shock therapy provides an opportunity to clarify the timing of ventricular tachyarrhythmias predisposing to sudden cardiac death. Analysis of the timing of arrhythmias in different patient subgroups, such as patients with poor left ventricular function, may provide further insight into the mechanism of onset of sudden cardiac death.
Methods and results: We studied patients in whom a cardioverter/defibrillator (Ventak PRx) was implanted between September 1990 and September 1993 in US centers. Events that could be timed occurred in 483 patients. With an RR cycle length of 240 ms as a cutoff, corresponding to a heart rate of 250 beats per minute, episodes were categorized as rapid (n = 1217) or less rapid (n = 9266) ventricular tachyarrhythmias. A higher proportion of both rapid and less rapid ventricular tachyarrhythmias began in the late morning compared with other times of the day. The subgroup of patients with ejection fraction < 20% at the time of implantation demonstrated a more uniform 24-hour distribution of tachycardias < or = 250 beats per minute than patients with higher left ventricular ejection fraction.
Conclusions: Further investigation of the late morning peak and of precipitants of ventricular tachyarrhythmias by use of data from the implantable cardioverter/defibrillator may provide insight into the pathophysiological mechanisms causing sudden cardiac death.