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Repetitive Implantable Cardioverter Defibrillator Shocks: What is the Mechanism?

A 66-year-old man with permanent atrial fibrillation (AF) and hypertrophic cardiomyopathy underwent a single chamber implantable cardioverter defibrillator (ICD; Ellipse, St. Jude Medical [SJM], St. Paul, MN, USA) implantation with a single passive fixation lead (Durata, SJM) for secondary prevention of ventricular fibrillation (VF). Testing of the ventricular lead 1 week after the implantation revealed normal and stable values for the lead impedance, ventricular pacing threshold, and R-wave sensing. Ten days later, the patient received five repetitive ICD shocks while sleeping (Fig. 1), and came to the emergency room on foot. The ambulatory electrocardiogram (ECG) revealed an AF rhythm with a moderate ventricular response. A few minutes later, he received another shock even though the 12-lead ECG showed AF. Subsequently, VF was induced by the inappropriate shock, which required external cardioversion. The stored electrograms suggested oversensing of atrial signals; however, no shocks were delivered for 15 minutes during the oversensing (Fig. 2). What is the mechanism to explain the clinical course?