The PainFree SST study was a prospective, multicenter clinical trial, which was conducted in two consecutive phases. Phase I assessed safety as determined by assessment of delays in ventricular fibrillation (VF) arrhythmia detection time using new algorithms. Phase II assessed the influence of new algorithms on inappropriate and unnecessary shocks at 1 year of follow-up. Subjects enrolled in Phase I continued in Phase II of the study and data from all enrolled subjects contributed to the analysis of Phase II objectives. A sub-study of Phase II randomized secondary prevention subjects to either 18/24 NID or 30/40 NID in order to assess the safety of prolonged detection in these patients.