Validation of Coronary Sinus Activation Pattern During Left Atrial Appendage Pacing for Beat‐to‐Beat Assessment of Mitral Isthmus Conduction/Block

INTRODUCTION: Mitral isthmus (MI) ablation for treatment of perimitral flutter is often performed during atrial fibrillation (AF) ablation but is technically challenging. Traditional assessment of MI conduction by left atrial activation mapping while pacing from either side of the line is time-consuming, and cannot be performed during ongoing ablation. Analysis of the coronary sinus (CS) activation pattern during left atrial appendage (LAA) pacing has been proposed as a simpler technique for evaluating MI conduction, enabling beat-to-beat assessment of conduction during ablation procedures and prompt identification of conduction block. METHODS: MI conduction was evaluated in 40 patients undergoing MI ablation using both: ((i) endocardial activation mapping and other standard techniques, and (ii) CS activation pattern during LAA pacing (change from distal-to-proximal activation to proximal-to-distal taken to signify the onset of MI block). RESULTS: CS activation sequence was used to assess conduction in 39 of 40 patients (unable to advance CS catheter distally in one case). MI block was achieved in 36 of 39 cases. The mean MI conduction time (LAA to distal CS) was 92.9 +/- 25.9 ms prior to ablation and 178.4 +/- 59.9 ms after MI block was confirmed. The mean step-out in conduction time at point of block was 80.8 +/- 40.6 ms. In all individuals in whom CS activation indicated block, there was concordance with endocardial activation, differential pacing and, where detectable, presence of widely split double potentials. CS lesions were required to achieve block in 24 of 36 (67%) successful cases. Radiofrequency application time and procedure time to achieve MI block were 10.8 +/- 6.0 minutes and 21.1 +/- 15.3 minutes, respectively.