720 Arrhythmia Rounds Section Editor: George J. Klein, M.D. Change in Coronary Sinus Activation Following Catheter Ablation: What Is the Mechanism? PUGAZHENDHI VIJAYARAMAN, M.D., * MARK A. WOOD, M.D.,† RICHARD K. SHEPARD, M.D.,† and KENNETH A. ELLENBOGEN, M.D.† From the * Cardiac Electrophysiology, Geisinger Wyoming Valley Medical Center, Wilkes-Barre, Pennsylvania, USA; and †Division of Cardiac Electrophysiology, Virginia Commonwealth University Health System, Richmond, Virginia, USA Case Presentation A 21-year-old woman presented with recurrent episodes of palpitations and documented supraventricular tachycar- dia (SVT). Quadripolar catheters were placed in the high right atrium, His bundle, and RV apex. A decapolar catheter (2–8–2 mm spacing) was placed in the coronary sinus (CS) with the proximal bipole at the CS ostium and the distal bipole posterolaterally. Baseline sinus cycle length was 810 ms, and the atrial-His (AH) interval and the His-ventricular intervals were 105 and 45 ms, respectively. There was no preexcita- J Cardiovasc Electrophysiol, Vol. 22, pp. 720-722, June 2011. No disclosures. Address for correspondence: Pugazhendhi Vijayaraman, M.D., Direc- tor, Cardiac Electrophysiology, Geisinger Wyoming Valley Medical Center, MC 36-10, 1000 E. Mountain Boulevard, Wilkes-Barre, PA 18702, USA. Fax: 570-821-2306; E-mail: pvijayaraman1@geisinger.edu, pvijayaraman@gmail.com doi: 10.1111/j.1540-8167.2010.01988.x Figure 1. The tracings from the top to bottom are leads I, aVF, and V1 followed by intracardiac electrograms from high right atrium (HRA), proximal His bundle (HIS p), distal His bundle (HIS d), ablation (Abl), coronary sinus proximal (CSp at the ostium) to distal bipoles (CS 12), and the right ventricular apex (RVA) at a sweep speed of 200 mm/sec. The intracardiac electrograms at baseline during the initial SVT (A) and during RV pacing (B) are shown. The ablation catheter is located near the site of initial ablation at the lateral mitral annulus. The earliest atrial activation is at the distal CS with eccentric activation with VA interval of 80 ms. tion during right atrial or CS pacing. Dual atrio ventricular (AV) node physiology was not demonstrable. SVT with right bundle branch block aberrancy (Fig. 1A) was easily and re- producibly inducible with atrial burst pacing or extrastimulus testing. Retrograde atrial activation in the CS during SVT and RV pacing was eccentric consistent with the presence of a left lateral accessory pathway (Figs. 1A and B). Ventricular overdrive pacing during the tachycardia confirmed the mech- anism to be orthodromic AV reentry tachycardia. Following two lesions at the site of early activation in the lateral mitral annulus, the atrial activation in the CS during SVT (Fig. 2A) and RV pacing (Fig. 2B) changed significantly. What is the possible mechanism for the change in the atrial activation following ablation? Commentary The SVT shown in Figure 1A had a tachycardia cycle length (TCL) of 270 ms. The VA intervals during the initial tachycardia were 125, 120, 100, and 80 ms at His, proxi- mal, mid, and distal CS electrodes, respectively. Following 2 radiofrequency lesions at the lateral mitral annulus, the