Ischemic events and coronary deaths show seasonal variability with a peak during December and January. It remains unclear whether ventricular tachycardias (VT) and ventricular fibrillation (VF) follow a similar pattern. The purpose of this study was to investigate the annual distribution of malignant ventricular arrhythmias.

Over a period of 11 years, all appropriate shock episodes (SE) after VT and VF in patients with an implantable cardioverter defibrillator (lCD) were analyzed with respect to the month of occurrence. An appropriate SE was defined as out-of-hospital VT/VF terminated by lCD shocks. Multiple shocks within 1 week were defined as 1 SE.

Two hundred and thirty-three of 308 patients with an lCD had appropriate SE during follow-up. In these patients the seasonal variation of 753 SE was calculated. Most SE occurred during January (93 SE), and the fewest SE occurred during June (39 SE). The seasonal pattern was statistically significant with a peak during winter (P = .001). The seasonal pattern did not differ between patients with an ischemic and those with a nonischemic underlying cardiac disease.

Appropriate shock episodes due to out-of-hospital VT/VF in patients with an lCD show seasonal variation with a significant peak during winter. The pattern is similar in patients with ischemic and nonischemic cardiac disease.