CASE REPORT A Latent Atrioventricular Decrementally Conducting Accessory Pathway Mimicking a Bystander Nodoventricular Fiber YAN HUO, M.D.,* GERHARD HINDRICKS, M.D., PH.D.,* ULRIKE WETZEL, M.D.,* THOMAS GASPAR, M.D.,* and EDUARDO BACK STERNICK, M.D., PH.D., F.H.R.S.† From the *Department of Electrophysiology, Leipzig Heart Center, University of Leipzig, Leipzig, Germany; and †Electrophysiology Unit, Biocor Instituto, Nova Lima, Brazil A 41-year-old woman with recurrent wide-QRS tachycardia is reported. Electrophysiologic findings were consistent with the diagnosis of a preexcited atrioventricular nodal reentrant tachycardia due to a bystander nodo-ventricular fiber. However, slow pathway ablation did not preclude tachycardia recur- rence. A second electrophysiology study shed light on the correct mechanism and eventually a successful ablation was achieved. (PACE 2010; e1–e5) antidromic tachycardia, decrementally conducting accessory pathway Case Presentation A 41-year-old woman with 14-year history of paroxysmal fast palpitations refractory to two antiarrhythmic drugs was referred for catheter ab- lation. A wide QRS-complex tachycardia with a left bundle branch block (LBBB)-like morphology sensitive to verapamil was consistently recorded during palpitations (Fig. 1A). The 12-lead rest- ing electrocardiogram (ECG) was considered nor- mal (Fig. 1B). Noninvasive assessment showed no structural heart abnormalities. Electrophysiologic studies were performed under the fasting nonsedated state. Antiarrhyth- mic drug was discontinued a month before. First Procedure Quadripolar electrode catheters were placed in high right atrium (hRA) and right ventricular apex (RVa); a bipolar electrode catheter was placed in the His bundle. There were normal atrial-His and His-ventricular intervals (80 and 40 ms, re- spectively). There was no “Mahaim” physiology (no ventricular preexcitation during atrial pac- ing at increasing rates). Atrioventricular conduc- tion was discontinuous (dual atrioventricular [AV] nodal pathway). Ventriculoatrial conduction was decremental and midline. An LBBB-like tachycar- dia was induced with premature atrial beats fol- lowing a typical AV nodal echo beat with a nar- Address for reprints: Yan Huo, M.D., Department of Electro- physiology, Leipzig Heart Center, 04289 Leipzig, Germany. Fax: 0049-0341-865-14-60; e-mail: dr.huoyan@googlemail.com Received September 28, 2009; revised October 31, 2009; accepted December 7, 2009. doi: 10.1111/j.1540-8159.2010.02713.x row QRS complex (Fig. 2). There was 1:1 A:V rela- tionship and the His bundle potential was merged inside the ventricular potential. The AV interval during tachycardia was 180 ms. Ventricular acti- vation could not be advanced by late atrial pre- mature beats delivered from the high lateral right atrium. A decision was taken to ablate the slow AV nodal pathway. Atrioventricular conduction became continuous, and tachycardia could not be induced by atrial stimulation anymore. However, tachycardia was still inducible by programmed ventricular stimulation. Second Procedure Decapolar electrode catheters were placed in hRA, His bundle, coronary sinus (proximal dipole -CS 7/8- at the ostium), and a quadripolar catheter in the RVa. During incremental atrial pacing (up to 240 ms), there was no ventricular preexcitation. However, unlike in the first procedure, a critically timed atrial premature beat conducted with a long AV conduction time followed by two preexcited QRS complexes showing the same QRS morphol- ogy was seen during tachycardia. Preexcited tachycardia was only inducible by RVa stimulation. Late atrial premature beats from the hRA did not advance ventricular activation during tachycardia but when delivered from the midseptal region it did advance right ventricu- lar activation by 20 ms (Fig. 3A). Entrainment of the preexcited tachycardia with RVa pacing showed that the H-A interval during tachycar- dia and pacing did not differ (Fig. 3B). Earliest ventricular activation during preexcited tachycar- dia was recorded on the proximal coronary si- nus (CS) (Fig. 3). An accessory pathway poten- tial was recorded along the anatomic course of the C 2010, The Authors. Journal compilation C 2010 Wiley Periodicals, Inc. PACE 2010 e1