How often does an ICD fail to work?

Long-term monitoring of ICD performance has shown that defibrillating a patient in sudden cardiac arrest is 98% effective in stopping the Sudden Cardiac Arrest (SCA).1,2

1- Zipes DP, Roberts D, for the Pacemaker-Cardioverter-Defibrillator investigators. Results of the International Study of the Implantable Pacemaker Cardioverter-Defibrillator: A Comparison of Epicardial and Endocardial Lead Systems. Circulation. July 1, 1995;92(1):59-65.
2- Volosin, et. al. Virtual ICD: A Model to Evaluate Shock Reduction Strategies. Heart Rhythm. 2010;May Suppl:PO3-125.