CIRCADIAN VARIATION IN DEFIBRILLATION ENERGY-REQUIREMENTS

Background Reports have demonstrated a circadian variation in the inci dence of acute myocardial infarction, ventricular arrhythmias, and sud den cardiac death. We tested the hypothesis that a similar circadian v ariation exists for defibrillation energy requirements in humans. Meth ods and Results We reviewed the time of defibrillation threshold (DFT) measurements in 134 patients with implantable cardioverter-defibrilla tors (ICDs) who underwent 345 DFT measurements. The DFT was determined in 130 patients at implantation, in 121 at a 2 months, and in 94 at 6 months. All patients had nonthoracotomy systems. The morning DFT (8 A M to 12 noon) was 15.1 +/- 1.2 J compared with 13.1 +/- 0.9 J in the m idafternoon (12 noon to 4 PM) and 13.0 +/- 0.7 J in the late afternoon (4 to 8 PM), P<.02. In a separate group of 930 patients implanted wit h an ICD system with date and time stamps for each therapy, we reviewe d 1238 episodes of ventricular tachyarrhythmias treated with shock the rapy. To corroborate the hypothesis that energy requirements for arrhy thmia termination vary during the course of the day, we plotted the fa iled first shock frequency for all episodes per hour. There was a sign ificant peak in failed first shocks in the morning compared with other time intervals (P=.02). Conclusion There is a morning peak in DFT and a corresponding morning peak in failed first shock frequency. This mo rning peak resembles the peaks seen in other cardiac events, specifica lly sudden cardiac death. These findings have important implications f or appropriate ICD function, particularly in patients with marginal DF Ts.