Objectives. The purpose of this study was to determine the precise incidence, therapeutic options and prognostic implications of electrical storm in patients with transvenous implantable cardioverter-defibrillator (ICD) systems.

Background. Approximately 50% to 70% of patients treated with an ICD receive appropriate device-based therapy within the first 2 years. Most arrhythmic events require only one appropriate ICD firing for termination. However, some patients receive multiple appropriate shocks during a short period of time, a condition referred to as “arrhythmic or electrical storm.”

Methods. This prospectively designed observational study comprised 136 recipients of transvenous ICDs who were followed for 403 ± 242 days. Electrical storm was defined as ventricular tachycardia or fibrillation resulting in device intervention ≥3 times during a single 24-h period.

Results. During follow-up, 57/136 patients (42%) received appropriate ICD therapy. Electrical storm occurred in 14/136 patients (10%) at an average of 133 ± 135 days after ICD implantation. The mean number of arrhythmic episodes constituting electrical storm was 17 ± 17 (range: 3 to 50; median 8) per patient. In 12 patients, electrical storm required hospital admission. The arrhythmia cluster could be terminated by a combined therapy with β-blockers and intravenous amiodarone whereas class I antiarrhythmic drugs were only occasionally successful. The cumulative probability of survival as estimated by the Kaplan-Meier method showed that patients with an episode of electrical storm did not have a worse outcome compared to those without such an event.

Conclusions. Electrical storm represents a frequent event in patients treated with modern ICDs. It occurs most commonly late after ICD implantation and can be managed by combined therapy with β-blockers and amiodarone. Electrical storm does not independently confer increased mortality.