An unusual ICD shock: What is the mechanism?

Case report A 48-year-old man with nonischemic dilated cardiomyopathy, left ventricular ejection fraction r35% status post dual-chamber ICD (Boston Scientific Energen, model E142) since September 2012, history of ventricular tachycardia with antitachycardia pacing (ATP) therapy, paroxysmal atrial fibrillation, and atrial tachycardia on beta-blocker therapy was seen in clinic following an ICD shock. The patient complained of palpitations but denied syncope or presyncope. Physical examination was without any abnormal cardiac findings and a 12-lead electrocardiogram showed atrial-paced rhythm at 60 beats per minute (bpm), normal intervals, and nonspecific inferolateral ST-T abnormalities. Device interrogation showed normally functioning atrial and ventricular leads with the following programmed parameters: Brady programming: DDD 60-115 with paced and sensed atrioventricular (AV) delay of 300 msec; Tachy programming: Three zones—VF zone (200 bpm [300 msec]) with ATP during charging, VT zone (170 bpm [353 msec] with atrial arrhythmia discrimination [Rhythm ID]), and a