4.3 Technical aspects of MI ablation
In the process of MI ablation, the most prominent demonstration of successful completion of the linear lesion is the clear reversal of the activation sequence in the CS during pacing from the LAA. For the MI block confirmation we used pacing criteria indicative of bidirectional change in the activation sequence of the peri-mitral region and not simply the increase in MI conduction time or the recording of double potentials which are more uncertain criteria [27]. It seems that, in the same manner as in CTI ablation, the activation sequence shifts abruptly [28]. In about half of the cases, epicardial access was required in our study, the sequence in CS was reversed with minimal energy delivery within the CS. The extent of the lesions delivered epicardially through the CS does not anatomically extend to the entire epicardial aspect, obviously because there is no free access to a much wider area, but only to the muscular bundles surrounding the CS. This may create a selective block towards the CS multipolar catheter but not to the entire MI. Presumably, some cardiac bundles may remain intact, and as we have learned, these may be located in the myocardial coverage of the vein of Marshall, the complex anatomical region of the LA ridge, the lateral aspect of the LAA base, or the area between the LAA and the mitral annulus [18-21]. Apparently, the impulse is passing through the remaining MI connections with a very long delay. Due to this fact we have an apparent change in activation sequence as the impulse from pacing beside the line is delayed in its passage through the MI lesions going much faster in the opposite direction (Figure 8 ).
In the study by Barragan et al. [18] it was shown that the breakthroughs are in the majority (8/11) epicardial. In our study all the PMFs with MI pseudo-block were ablated from the endocardial site. This can be explained in three ways: a) the conduction gap is located in the endocardial site, b) more endocardial ablation is needed to create a transmural lesion, or c) that we finally ablate the endocardial breakthroughs of epicardial connections [19], although the critical lesions, in our 5 cases, were located in juxtaposition with the MI line. In the study of Fujisawa et al. [20], the pseudo-block was observed in 33.3% of the cases. In this study, the MI ablation was attempted at a higher level in contrast to our series where the initial choice was the 4 o’clock site of the mitral annulus (LAO view), possibly lower than the position of the ligament of Marshall and posteriorly of the lateral border of the LAA. If the MI line cannot be performed, alternatively other linear lesions were attempted to interrupt MI conduction or PMF circuits [29,30], something that we attempted in our study in case of failure to achieve complete MI block. Probably, one way to avoid residual ΜΙ connections, beyond the re-induction challenge mentioned above, is the detailed mapping in the boundary of the line and the identification of near field potentials representing conduction gaps.