EP ROUNDS Tachycardia with QRS Morphology Identical to the Normal Sinus Rhythm Electrocardiogram: What is the Mechanism? BIPINPREET NAGRA, M.D., ROHIT MEHTA, M.D., EMILE DAOUD, M.D., and STEVEN KALBFLEISCH, M.D. VT , ablation, bundle branch reentry, left bundle branch tachycardia Case Presentation A 64-year-old man with a history of prior my- ocardial infarction, coronary artery bypass graft- ing, and ischemic cardiomyopathy with a left ventricular ejection fraction of 20% presented with a three-day history of palpitations and near syncopal episodes. The patient had a biventric- ular pacemaker-defibrillator (BiV-ICD) implanted 2 years prior and was taking amiodarone for a history of frequent defibrillator shocks. The de- vice interrogation showed 102 episodes of tachy- cardia at a rate of approximately 160 beats/min over a 72-hour period. All of these episodes were terminated with antitachycardia pacing. Electro- gram analysis of the BiV-ICD’s stored arrhythmia events showed a tachycardia with a 2:1 ventricu- loatrial (VA) relationship, which was initiated by a premature ventricular contraction (PVC) with a long-short coupling interval (Fig. 1A). A spon- taneous episode of tachycardia was captured on hospital telemetry (Fig. 1B). The tachycardia had a left bundle inferior axis morphology and was initiated by PVC of a different morphology than the tachycardia itself. Baseline electrocardiogram (ECG) demonstrated sinus rhythm with biventricu- lar pacing. The ECG with ventricular pacing turned off showed normal sinus rhythm with a left atrial abnormality, prolonged PR interval of 320 ms, and left bundle branch block (LBBB) pattern (Fig. 2A). During the patient’s electrophysiology (EP) study, the baseline intervals were as follows: atrial-His (HA) interval = 100 ms; His-ventricular (HV) in- terval = 95 ms; QRS = 160 ms; Baseline Cycle Length = 890 ms. A LBBB morphology tachycar- dia with a cycle length of 380 ms was easily in- duced with a single ventricular extrastimulus from the right ventricular apex. A 12-lead ECG of the Conflict of interest: None for the purpose of this publication. Address for reprints: Steven Kalbfleisch, M.D., Director, Car- diac Electrophysiology Laboratory, The Ohio State University Medical Center, 473 W. 12th Avenue, Columbus, OH 43210. Received October 15, 2007; revised November 27, 2007; ac- cepted December 16, 2007. tachycardia showed an identical QRS morphology and axis to the baseline nonpaced ECG (Fig. 2B). The intracardiac recordings from the EP study are shown in Figures 3A and B. Figure 3A shows in- duction of the tachycardia with a single ventricu- lar extrastimulus. A His bundle electrogram pre- ceded each ventricular electrogram with an HV interval of 110 ms. There was a 2:1 VA relation- ship in tachycardia with the earliest atrial acti- vation recorded on the His bundle catheter and a septal to high right atrial activation sequence. During tachycardia beats with VA conduction the atrial activation time on the His bundle catheter was nearly coincident with the onset of the sur- face QRS (VA time = 15 ms) and the His to atrial activation time was 125 ms, which was the same as during ventricular pacing when VA conduction was present. Figure 3B shows a period of cycle length variability with cycle lengths ranging from 375 ms to 385 ms. Attempts at ventricular pacing to entrain the tachycardia resulted in tachycardia termination. Spontaneous or induced PVCs coin- cident with His bundle activation were also able to terminate the tachycardia. What is the tachycardia mechanism? Discussion The differential diagnosis of this tachycar- dia includes: (1) Supraventricular tachycardia with native LBBB conduction, (2) Bundle Branch Re-Entrant Venticular Tachycardia (BBRVT), or (3) Intramyocardial VT with a proximal exit site near or including the normal Purkinje exit site. 1 Supraventricular tachycardia with preex- isting LBBB was felt to be unlikely due to the presence of a 2:1 VA relationship during tachycardia. However, junctional tachycardias, AV node reentrant tachycardia (AVNRT), and ortho- dromic reciprocating tachycardia (ORT) utilizing a nodoventricular or nodofasicular fiber can have variable degrees of VA conduction or VA dissocia- tion during tachycardia. 2,3 The fact that the tachy- cardia terminated with PVCs coincident with the His deflection rules out AVNRT since the PVC C 2008, The Authors. Journal compilation C 2008, Blackwell Publishing, Inc. 604 May 2008 PACE, Vol. 31