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.