4.4 Conduction velocities and implications for PMF ablation 
In our study we showed that PMF in patients with MI pseudo-block is an arrhythmia with special electrophysiological features. The evaluation of the activation maps revealed a very slow conduction through a narrow excitable channel in the area of the previous lesions. On the contrary, measuring the CV with high-density mapping in a large number of PMF that occur after AF ablation, we found that the CV does not show large differences throughout the course of the circuit. This also applies to patients with PMF who have had previous ablation in LA beyond PVI. In addition, the presence of conduction channels was found in a very small percentage of PMF circuits. Therefore, it appears that in the majority of PMFs after AF ablation there are areas where the CV is slower, but usually there is no area with marked conduction delay. This reasonably raises the question of what the most secure strategy for PMF ablation could be to ensure no recurrence. If we target the area of the lowest CV or the narrowest corridor, we can temporarily stop the circuit but there may still be pathways throughout the extent of the perimitral area that may allow its re-initiation. Nevertheless, it is doubtful whether the area around a channel in a propagation map is truly unexcitable or could become excitable in a different waveform direction. Thus, it seems reasonable that the most secure way to prevent PMF is to interrupt the circuit in the MI. Perhaps a useful example is the ablation of CTI-dependent flutter, where the arrhythmia can be terminated even with the onset of ablation, if a critical area is affected, but there is no doubt that the circuit can relapse unless complete and bidirectional CTI block is eventually achieved.
In several recent series with high-density mapping, targeting the functional rather than the anatomical isthmus has been the main ablation strategy [11-13]. It should be considered that many of these studies [12,13] have been performed with the Rhythmia system, which actually creates high density maps, but probably does not facilitate the linear ablation with complete and transmural lesions, as this system did not have catheters with contact force technology. According to our previous observation in a longitudinal patient cohort with ATs after AF ablation, targeting the macro-reentrant circuits of the LA with ablation of the anatomical isthmuses has better results in maintaining SR [31]. However, we must recognize that all these views are based on empirical observations. Besides, the question of whether we should ablate the anatomical or the functional isthmus is difficult to answer even by a randomized study, as each case has specific and unique features and so it is difficult to implement a preplanned ablation strategy.