Recurrent perimitral tachycardia using epicardial coronary sinus connection to bypass endocardial conduction block at the mitral isthmus.

Mitral isthmus (MI) ablation is one of the important steps in the context of persistent atrial fibrillation ablation. However, achieving a complete transisthmus conduction block is challenging. Furthermore, incomplete conduction block can increase the risk of recurrent perimitral atrial tachycardia (AT). We describe a case wherein endocardial conduction block across the MI line was established, but a residual left atrial-to-coronary sinus (LA-CS) connection served as an epicardial bypass to maintain recurrent perimitral AT. A 58-year-old man was admitted for catheter ablation of recurrent AT (Figure 1A) after 2 previous persistent atrial fibrillation ablation procedures involving pulmonary vein (PV) isolation and MI ablation. Baseline AT cycle length was 220 ms. A steerable decapolar catheter was inserted into the CS, and an externally irrigated ablation catheter was advanced into the LA through a transseptal puncture. Electrograms on CS 1-2 showed double potentials 74 ms apart (Figure 1B). On activation and entrainment mapping, clockwise perimitral AT was diagnosed. Endocardial mapping with the ablation catheter along the previous MI line showed double potentials separated by 90 ms. The 2nd component of the double potentials corresponded to the 2nd component of the double …