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A patient with dilated cardiomyopathy and aborted sudden death was admitted for repeated shocks from his Guidant Ventak Prizm dual chamber implantable cardioverter-defibrillator (ICD). Interrogation of stored events over the last six months revealed 46 episodes at a rate of 140/min, interpreted as ventricular tachycardia (VT). These were usually interrupted by programmed overdrive ventricular pacing, but sometimes required shocks. During monitoring, rhythm strips showed repeated episodes of narrow complex tachycardia at 136/min, initiated by an atrial premature beat (APB) with a prolonged PR interval (*), without visible P waves during tachycardia (A). These episodes set off antitachycardia ventricular pacing by the ICD with return to sinus rhythm (B). Interrogation of stored events of the ICD revealed similar episodes initiated by an APB (•), with inconsistent atrial sensing during tachycardia (AS 2033), despite evident 1:1 atrial activity on the electrogram. This was due to the very short ventriculoatrial (VA) interval, with atrial activity falling within the blanking period of 85 ms (during which the atrial channel is “blinded” to avoid far-field sensing of ventricular activity). Thus apparent atrioventricular (AV) dissociation led to misdiagnosis of VT by the device. AV nodal re-entrant tachycardia was suspected and subsequently confirmed by an electrophysiological study, leading to successful catheter radiofrequency ablation of the slow pathway. At eight months’ follow up, interrogation of the ICD showed no relapse of tachycardia. This case illustrates how ICDs may be tricked by a relatively common supraventricular arrhythmia despite dual chamber technology, resulting in inappropriate shocks.
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