Circadian patterns in the occurrence of malignant ventricular tachyarrhythmias triggering defibrillator interventions in patients with hypertrophic cardiomyopathy

Background: Sudden death in hypertrophic cardiomyopathy (HCM) has been reported to occur most frequently in the early morning hours, similar to the pattern observed in ischemic heart disease. However, little is known about the circadian pattern of life-threatening arrhythmias in HCM in the contemporary era of the implantable cardioverter-defibrillator (ICD). Objective: The purpose of this study was to determine the time of day when appropriate device interventions occur for ventricular tachycardia (VT)/ventricular fibrillation (VF) in HCM patients. Methods: Among 63 patients with HCM and appropriate device interventions, 126 intracardiac electrograms were assessed for the hourly distribution of VT/VF. Results: One or more arrhythmic episodes occurred in each hour of the day, and a modest pattern of circadian variability was evident. VT/VF episodes were more common in the afternoon and evening hours from noon to midnight (64%) than in the other 12-hour period (36%; P = .008), with the suggestion of a peak at 2 to 4 pm. A sizeable proportion of events (27%) occurred during the potential sleeping hours of 11 pm and 7 am. Conclusion: In high-risk HCM patients, the afternoon and evening circadian periodicity of ventricular tachyarrhythmias (terminated by the ICD) underscores the largely unpredictable nature of the electrophysiologic substrate in this disease, and differs from the pattern of early morning cardiovascular events reported in ischemic heart disease. These observations also suggest that home automatic defibrillator strategies for sudden death prevention are unlikely to be effective in HCM.