Reversal of the Activation Sequence along the Coronary Sinus during RF Application with a Single Left Lateral Pathway:

A 16-year-old girl with a 4-year history of recurrent supraventricular tachycardia was referred for catheter ablation. A standard 12-lead electrocardiogram (ECG) during sinus rhythm revealed no abnormalities. Invasive electrophysiologic study was performed. Two quadripolar catheters were inserted via femoral venous access and placed in the high right atrium and right ventricle. A decapolar catheter was inserted via the right internal jugular vein into the coronary sinus with the proximal bipole at the os. Programmed atrial stimulation revealed a continuous atrioventricular (AV) node function curve. Right ventricular pacing at different cycle lengths (CLs) showed nondecremental ventriculoatrial (VA) conduction and the earliest atrial activation at the distal coronary sinus (CS). Rapid ventricular stimulation easily and repeatedly induced a regular narrow QRS complex tachycardia with a CL of 300 ms. The activation sequence on the CS leads during tachycardia is distal to proximal (Fig. 1). A diagnosis of orthodromic AV reentrant tachycardia (AVRT) using a left-sided free-wall accessory pathway (AP) was made. The retrograde transaortal approach was used. Due to difficulty in reaching a left anterolateral position beneath the mitral valve, the ablation catheter was positioned supra the mitral valve in a left anterolateral location. Radiofrequency (RF) energy was delivered to the earliest atrial activation site 4 mm above the distal CS catheter electrode at the mitral annulus. After 3 times RF energy application (10 second × 3) during sinus rhythm, another type of tachycardia was induced (Fig. 2). The catheter in CS were placed 1.5-cm distal to the original site. During tachycardia, CS3–4 electrodes showed double atrial potentials with an interval of 40 ms (Fig. 3). What has happened in Figures 2 and 3 and what is the mechanism of the changed tachycardia? Before ablation, tachycardia with a CL of 300 ms and the activation sequence on the CS leads was distal to proximal. After RF energy application, the earliest atrial activation was at the proximal CS electrogram with a reversal of activation sequence along the coronary sinus during tachycardia. The catheter in CS was placed 1.5-cm distal to the original site. CS3–4 electrodes showed double atrial potentials (↑↑) with the earliest atrial activation at the distal CS electrogram (↑) during tachycardia. During left lateral AP ablation, AVRT remained inducible with a change in the retrograde atrial activation sequence after application of radiofrequency energy can suggest a second AP. In our case, a reversal of activation sequence along the coronary sinus with the earliest atrial activation at the proximal CS electrogram was noted after three RF energy deliveries that did not eliminate AVRT. This phenomenon could be explained by the existence of a second septal or right side AP. The arguments against this mechanism are that the tachycardia CL and the VA interval at the ablation catheter remained unchanged during AVRT. This indicated the targeted AP has not been interrupted and there may be conduction block made by RF energy between the lateral AP insertion site and the distal CS electrode. Additionally, the CS3–4 electrodes presented double atrial potentials with the earliest atrial activation at the distal CS electrogram when more lateral positioning of the CS catheter. The double atrial potentials with an interval of 40 ms during AVRT provided direct evidence of intra-atrial conduction block along the mitral valve annulus. Luria et al. reported the phenomenon of intra-atrial conduction block in 11 patients during left AP ablation. They postulated that the area between the left inferior pulmonary vein and the inferolateral mitral annulus might serve as an mitral isthmus.1 Cheng et al. suggested a circumferential bundle of myocardial fibers with the minimum width of 0.8 cm in the inferoposterior left atrium may serve as the underlying anatomic substrate.2 Since the width was in the same order of magnitude as the typical RF lesion size, it is not surprising that a complete mitral isthmus block was made by a limited number of RF energy application, resulting a reversal of activation sequence along the coronary sinus as in this case. The ablation catheter was repositioned lateral to the site of the former attempts with atrial activation 25 ms earlier than the distal CS electrogram. A single application of radio frequency energy interrupted the tachycardia. Ventricular pacing at different CLs showed no VA conduction. Tachycardia could not be induced by programmed electrical stimulation both from the right atrium and ventricle.