Instantaneous Electrophysiological Changes Characterizing Achievement of Mitral Isthmus Linear Block

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Achievement of complete conduction block across left mitral isthmus (MI) is a challenging endpoint of linear lesion, and recognizing the precise moment of block is important during ongoing ablation. The objective of this study is to evaluate the changes in P wave morphology and local MI potential at the moment of block during ongoing radiofrequency (RF) application. We evaluated 69 patients (procedures) in whom successful MI linear conduction block was achieved during coronary sinus (CS) pacing. P wave morphology and/or local MI potential could be evaluated in 64 (93%) and 69 (100%) procedures, respectively. The achievement of MI block was associated with substantial instantaneous changes in 57/69 (82.6%) procedures. P wave morphology changed in 44 (64%) procedures with the change restricted to lateral leads in 39 (57%). Abrupt prolongation of local conduction delay from 106 ± 24 ms to 167 ± 39 ms (P < 0.0001) was observed on proximal bipole of ablation catheter in 34/69 (49.3%) procedures during ongoing RF application. In addition, prolongation of conduction delay was associated with significant change in the electrogram amplitude and polarity in 11 and 19 procedures, respectively. The substantial change in P wave morphology was not observed in any patients without achievement of complete block. The achievement of conduction block across MI line is associated with recognizable changes in the local MI electrograms and the P wave morphology especially in the lateral leads. These instantaneous critical changes may assist catheter ablation and indicate the requirement for prolonged RF application, if necessary.