The Prevent Study is designed to investigate the effect of a rate-smoothing algorithm on the onset of ventricular tachycardia in patients with implanted cardioverter defibrillators (ICD) with dual-chamber pacing and sensing function. Included in the study are patients who require ICD therapy for recurrent ventricular tachycardia or aborted cardiac arrest. After giving informed consent to the study, the patients receive a Ventak AV ICD (CPI-Guidant, St. Paul, MN). The patients are randomized to start either with active or inactive rate-smoothing algorithm. After 3 months, all patients cross over to the opposite group. Questionnaires are used to investigate quality-of-life issues. A total of 240 patients will be enrolled in the study, with a minimum follow-up of 6 months.