Inappropriate shock therapy in a heart failure defibrillator.

A 63-year-old male with dilated cardiomyopathy underwent implantation of a "heart failure" defibrillator capable of biventricular pacing. He received an inappropriate shock 5 hours after the procedure. Stored electrograms revealed that during each sinus beat the ventricular channel recorded up to three separate events. These resulted from far-field atrial sensing by the coronary venous lead, appropriate right ventricular sensing, then delayed left ventricular sensing (the result of left bundle branch block). As a consequence of far-field left atrial sensing the two subsequent ventricular electrograms fell within the VF zone. Following an atrial premature beat, VF detection criteria were satisfied and shock therapy delivered. Although coronary venous lead repositioning eliminated far-field atrial sensing, double counting of the widely split right and left ventricular electrograms still occurred during sinus rhythm. Shortening the programmed AV delay resulted in constant biventricular pacing with a single electrogram.