PACEMAKER/ICD PROBLEM OF THE MONTH Defibrillator discharges in a young patient: What is the underlying arrhythmia? Nitin Kansal, MBBS, Jeff S. Healey, MD, FRCPC, FHRS, Carlos A. Morillo, MD, FRCPC, FHRS, Syamkumar M. Divakara Menon, MBBS From the Arrhythmia Services, Division of Cardiology, McMaster University and Hamilton Health Sciences, Hamilton, Ontario, Canada. Case summary A 20-year-old man received a dual-chamber implantable cardioverter-defibrillator (ICD; Scientific Incepta E163, Boston Scientific, Natick, MA) for nonischemic dilated cardiomyopathy. He presented with multiple ICD therapies after 1 year. The tachycardia therapy settings for the patient were programmed as follows: ventricular fibrillation zone Z240 bpm (r250 ms) with therapy of 41-J shocks  8, ventricular tachycardia (VT) 2 zone Z185 bpm (r324 ms) with therapy of antitachycardia pacing (ATP)  6 @88% followed by 41- J shock  6, and VT 1 zone Z150 bpm (r400 ms) with therapy of ATP  4 @ 84% followed by 41-J shock  4. Interrogation strips of the ICD therapies are shown in Figure 1. Six tachycardia episodes were recorded. Bursts of ATP were unsuccessful in terminating the tachycardia in 4 of these episodes. A shock delivered from the device converted the patient to normal sinus rhythm in each of these episodes. What is the mechanism of the tachycardia? Commentary Initiation of the tachycardia and the first burst of ATP is shown in Figure 1. Three ICD electrogram channels are shown (atrial, ventricular, and far-field shock channels). Tachycardia had a cycle length of 280 ms with a 1:1 ventriculoatrial (VA) relationship. Tachycardia was classi- fied in the VT 2 zone with rhythm ID being a mismatch annotated as “RID–.” After delivery of 6 ATP therapies, the patient received a shock that converted him to normal sinus rhythm. The differential diagnosis of a tachycardia with 1:1 relationship between atrial and ventricular activations include (1) VT with retrograde 1:1 VA conduction, and (2) supraventricular tachycardia (SVT) with 1:1 atrioventricular (AV) conduction, which could be atrioventricular nodal reentrant tachycardia (AVNRT), atrioventricular reentrant tachycardia (AVRT), or atrial tachycardia (AT), with aber- rant conduction resulting in a mismatch of the ventricular electrogram template stored in the ICD. Atrial response to ventricular overdrive pacing (VOP) is a useful tool for understanding the mechanism of tachycardia in the majority of cases. 1 Undisturbed continuation of the tachycardia in the atrium during VOP is indicative of a diagnosis of SVT, either AT or AVNRT, as these forms of SVT do not require ventricular participation for mainte- nance. This finding is rare with AVRT unless the accessory pathway is located far from the pacing site and the duration of pacing is insufficient to capture the ventricular myocar- dium at site of pathway insertion. This response typically excludes VT with retrograde atrial activation, in which case the atrial rates either accelerate to the pacing rate in case of accelerated VA conduction or revert to sinus rate if there is VOP-induced VA block. Postpacing behavior also helps in determining the mechanism of tachycardia, especially in SVTs. V-A-A-V response indicates AT, and V-A-V response in general excludes an AT. The tachycardia has a 1:1 AV relationship with a VA interval of 80–90 ms (measured using the annotation markings at the base of the trace). Close inspection of the atrial electro- gram shows a stable atrial cycle length of 280–285 ms during first 4 beats of ATP, suggesting AT as a possible mechanism. However, close scrutiny of the shock channel during ATP shows that the completely paced morphology started only after the fifth beat, thus entraining the atria only during the last 4 beats during ATP. Late entrainment of AVRT (except path- ways situated far from pacing site) and VT are less likely. 2 Inspection of the response to ventricular entrainment reveals V-A-V response, ruling out AT. The postpacing interval minus KEYWORDS Implantable cardioverter-defibrillator shocks; Supraventricular tachycardia; Ventricular tachycardia; Entrainment ABBREVIATIONS AT ¼ atrial tachycardia; ATP ¼ antitachycardia pacing; AV ¼ atrioventricular; AVNRT ¼ atrioventricular nodal reentrant tachycardia; AVRT ¼ atrioventricular reentrant tachycardia; ICD ¼ implantable cardioverter-defibrillator; SVT ¼ supraventricular tachycardia; VA ¼ ventriculoatrial; VOP ¼ ventricular overdrive pacing (Heart Rhythm 2014;11:2202–2205) Address reprint requests and correspondence: Dr. Syamkumar M. Divakara Menon, HGH McMaster Clinic #518, 237-Barton St East, Hamilton, ON L8L2X2, Canada. E-mail address: divakara@hhsc.ca. 1547-5271/$-see front matter B 2014 Heart Rhythm Society. All rights reserved. http://dx.doi.org/10.1016/j.hrthm.2014.07.036