Circadian rhythm of appropriate implantable cardioverter defibrillator discharges in patients with hypertrophic cardiomyopathy

Aims: To characterize the circadian pattern of implantable cardioverter defibrillators (ICD) discharges in patients with hypertrophic cardiomyopathy (HCM) without previous surgical myectomy or percutaneous alcohol septal ablation. Methods and Results: HCM patients without previous surgical myectomy or percutaneous alcohol septal ablation having undergone ICD insertion at Mayo Clinic from 1992 to 2005 were studied. Analysis of appropriate ICD discharges with respect to time of day was performed. During a mean follow-up of 4.4 ± 4 years (P = 0.02), a total of 122 appropriate discharges were recorded. The average event rate for this group of patients was 4.3% per year. The circadian pattern reveals a pronounced peak in ICD discharges for the hours from 14:00 to 16:00. HCM patients receiving antiarrhythmic or β-blocker therapy did not demonstrate a different circadian pattern compared with patients not receiving these medications. Conclusion: Analysis of the temporal distribution of ICD discharges in HCM patients without previous surgical myectomy or percutaneous alcohol septal ablation reveals a peak occurrence of ICD discharges during the period from 12:00 to 18:00 hours with the highest incidence between 14:00 and 16:00 hours. This is different from the well-documented early morning peak observed in ischemic dilated cardiomyopathy patients.