Impact of shock energy and ventricular rhythm on the success of first shock therapy: The ALTITUDE first shock study

Background: The efficacy of shock in converting different ventricular tachyarrhythmias has not been well characterized in a large natural-practice setting. Objective: To determine shock success rate by energy and ventricular rhythm in a large cohort of patients with implantable cardioverter- defibrillators. Methods: Two thousand patients with 5279 shock episodes were randomly sampled for analysis from the LATITUDE remote monitoring system. Within an episode, the rhythm preceding therapy (shock or antitachycardia pacing [ATP]) was adjudicated. Patients who died after unsuccessful implantable cardioverter-defibrillator shocks did not transmit final remote monitoring data and were not included in the study. Results: Of 3677 shock episodes for ventricular tachyarrhythmia, 2679 were treated with shock initially and were classified as monomorphic ventricular tachycardia (n = 1544), polymorphic/monomorphic ventricular tachycardia (n = 371), or ventricular fibrillation (n = 764). The success rate after the first, second, and final shock averaged 90.3%, 96.4%, and 99.8%, respectively. After unsuccessful initial ATP (n = 998), the first, second, and final shock was successful in 84.8%, 92.9%, and 100% of the episodes. The success rate after the first or second shock was significantly lower after failed ATP compared to shock as first therapy (both P<.001). Among episodes treated initially with shock, the success rate for monomorphic ventricular tachycardia (89.2%) when treated with energy level≤20 J was significantly higher than that for ventricular fibrillation (80.8%) (P =.04). The level of shock energy was a significant predictor of the success of the first shock (odds ratio 1.16; 95% confidence interval 1.03-1.30; P =.013). Conclusions: The success rate of first shock as first therapy is approximately 90%, but was lower after failed ATP. Programming a higher level of energy after ATP is suggested.