EP ROUNDS Changing QRS Morphology during Accessory Pathway Ablation RAYMOND W. SY, M.B.B.S., LORNE J. GULA, M.D., M.S., ALLAN C. SKANES, M.D., ANDREW D. KRAHN, M.D., RAYMOND YEE, M.D., and GEORGE J. KLEIN, M.D. From the Division of Cardiology, Department of Medicine, University of Western Ontario, London, Ontario, Canada SVT , electrophysiology, clinical Case Presentation A 30-year-old man was referred for assess- ment of palpitations. The physical examination Address for reprints: Lorne J. Gula, M.D., M.S., F.R.C.P.C., London Health Sciences Center, University of Western Ontario, 339 Windermere Rd., London, Ontario N6A 5A5, Canada. Fax: 519-663-3782; e-mail: lgula@uwo.ca Received September 10, 2009; revised October 22, 2009; accepted November 16, 2009. doi: 10.1111/j.1540-8159.2010.02688.x Figure 1. Baseline 12-lead ECG recorded before and during adenosine administration. was normal and an echocardiogram was similarly unremarkable. Baseline electrocardiogram (ECG) demonstrated sinus rhythm with subtle evidence of preexcitation, which became more obvious dur- ing administration of adenosine (Fig. 1). A baseline electrophysiology study was per- formed under conscious sedation with catheters positioned in the high right atrium, adjacent to the His bundle, in the coronary sinus (CS) and the right ventricle. Baseline intervals were: atrial- His 70 ms, His-ventricular 35 ms, and QRS dura- tion 105 ms. There was progressively increasing C 2010, The Authors. Journal compilation C 2010 Wiley Periodicals, Inc. PACE, Vol. 33 June 2010 747