A 89-year-old woman with dilated cardiomyopathy underwent a second ablation procedure for persistent atrial tachycardia (AT) with repeated worsening of heart failure due to the tachycardia. She had undergone pulmonary vein (PV) isolation by cryoballoon, cavo-tricuspid isthmus, left atrial (LA) roof line, and mitral isthmus linear ablation for symptomatic and drug-resistant persistent atrial fibrillation (AF) in the first procedure 3 months earlier. The starting rhythm in the second procedure was a stable AT with a tachycardia cycle length of 540 ms. No PVs reconnected and the block of LA roof and mitral isthmus lines remained. For detailed analysis of epicardial connection, 2 multipolar catheters were placed at distal and proximal coronary sinus from carotid and femoral veins, respectively. Moreover, an additional 2 Fr hexapolar catheter (EP star, Japan Life Line, Tokyo, Japan) was inserted into vein of Marshall (VOM). Activation mapping using high-resolution 3-dimensional mapping system (Rhythmia, Boston Scientific, Natick, Massachusetts) revealed the both endocardial and epicardial conduction block along the mitral isthmus, and peri-mitral AT in which Marshall bundle (MB) epicardial connection bypassed the scar area (Fig A). Entrainment mapping showed that post-pacing interval from the catheter inside the VOM was equal to the tachycardia cycle length, which further confirmed the diagnosis (Fig A). A 4.8 seconds of an application at the slow conduction area (the connection between LA ridge and distal MB) terminated the AT (Fig A and B). Bi-directional conduction block of MB-LA connections were confirmed, and any ATs were not inducible. No AT recurrence has been observed for 3 months.