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Inappropriate Biventricular Implantable Cardioverter Defibrillator Firing Due to Cryptogenic Double Counting

The patient is a 63-year-old man who presented initially with a large anteroseptal myocardial infarction in 2000. Despite urgent revascularization and eventual coronary artery bypass grafting, the patient was left with a severe ischemic cardiomyopathy. In 2003, he underwent implantation of a dual-chamber implantable cardioverter defibrillator (ICD) (model V-235, St. Jude Medical, Minnetonka, MN, USA) for primary prevention of sudden cardiac death. A true bipolar ICD lead (model 1580, St. Jude Medical) was used. The patient was followed uneventfully in device clinic after original ICD implant. By the fall of 2007, the patient had developed significant heart failure symptoms in the setting of chronic left bundle branch block (LBBB) (QRS duration of 198 ms), and underwent revision to a cardiac resynchronization therapy-defibrillator (CRT-D) system (model V-365, St. Jude Medical). The parameters of the chronic right ventricular (RV) lead at this time included a stable lead impedance of 381 , sensed R waves of 24 mV, and an RV capture threshold of 0.5 V. The device was programmed in DDD mode from 50 to 120 beats per minute, with paced and sensed atrioventricular (AV) delays of 200 and 150 ms, respectively, and a left ventricular (LV) pacing offset of 30 ms. Tachy therapies were programmed in a single zone, with a rate cutoff of 200 beats per minute. At the initial device interrogation 6 weeks after CRT-D implant, the patient presented in atrial fibrillation (AF). During interrogation, he was observed to experience an inappropriate 25-J shock for AF with moderate ventricular response rates, with associated RV sensing and double counting (Fig. 1A). The shock restored normal sinus