An activation map during LAA pacing using the Orion catheter to determine whether there is conduction or block across the posterior mitral isthmus line after ablation of a perimitral AT. Using 0.05 mV as a cut-off to distinguish atrial scar from viable tissue, we cannot be sure of a possible endocardial connection in the posterior mitral isthmus line. However, using 0.03 mV as a cut-off allows us to detect and analyse bipolar EGMs of ultra-low-voltage (0.025 mV) and identify the critical endocardial connection-gap in the posterior mitral isthmus. Differential pacing from both sides of the line in combination with voltage and activation mapping using high-resolution mapping were our endpoints to confirm bidirectional block of the lines and result in durable lesions with long-term successful results. AT, atrial fibrillation; EGM, electrogram.