Case report: 28-year-old man with non-ischaemic dilated cardiomyopathy underwent implantation of an cardioverter defibrillator (ICD) for primary prevention of sudden cardiac death. The implanted device employed a single lead with atrial sensing capabilities (BIOTRONIK Iforia 5 VR-T DX and Linox Smart DX single ICD coil). 6 months after implantation patient received ICD shock. The interrogation reveals a tachycardia with electrogram (EGM) characteristics of very short ventriculoatrial (VA) interval and cycle length (CL) of 280 msec. The tachycardia fell into the ventricular fibrillation (VF) zone, resulting in ICD shock. It was fairly clear that it was initiated by an atrial premature contraction (APC) with a prolonged P-R interval initiating supraventricular tachycardia (SVT). A diagnosis of AV nodal reentrant tachycardia (AVNRT) was strongly suspected on the retrospective review of the tachycardia episode, based on the short V-A time, the unchanged ventricular morphology on intracardiac electrograms. At the electrophysiology (EP) study, dual AV nodal physiology was in fact revealed. A narrow complex tachycardia was reproducibly induced with single atrial extra-stimuli. The tachycardia had a 1:1 VA relationship and concentric atrial activation. Entrainment maneuvers were consistent with typical AVNRT. Slow pathway modification was performed, following which tachycardia was no longer inducible. The ICD normally uses heart rate for a given period of time as the criteria for definition of arrhythmia. Any ventricular rate above the programmed cutoff rate is considered to be an arrhythmia and will be treated according to the programmed protocol. Some supraventricular arrhythmias can attain the programmed cutoff rate and thus be inappropriately treated. Despite increasingly sophisticated discrimination algorithms available in modern ICDs, the ability to differentiate SVT from ventricular tachycardia (VT) can be challenging. Our patient received inappropriate shock for AVNRT.

Inappropriate shocks occur in a certain proportion of patients with ICDs and represent one of the most challenging aspects of management for the physician. An EP study may be necessary to determine the appropriate therapeutic course. (1)