1. INTRODUCTION
The mitral isthmus (MI) is the anatomical area between the ostium of the left inferior pulmonary vein (PV) and the posterolateral part of the mitral valve annulus. Mitral isthmus ablation is an established strategy in the treatment of perimitral atrial flutter (PMF) [1, 2] as well as an adjunct to pulmonary vein isolation (PVI) in the treatment of non-paroxysmal atrial fibrillation (AF) [3,4].
The usefulness of the MI ablation and generally of linear lesions is questionable as an initial strategy in persistent AF ablation [5,6]. However, it is one of the possible options in cases where PVI alone is not considered sufficient, such as in redo cases with well-maintained PVI [7], long-lasting persistent AF [8,9], or diseased myocardium with low-voltage electrical activity [10]. On the other hand, MI ablation is an effective therapy for PMF, however, these reentrant tachycardias can also be treated by ablating critical isthmuses with slow conduction zones [11-13].
If a linear lesion is attempted in the ΜΙ, the creation of complete and bidirectional block is very important. Failure to achieve bidirectional block can be proarrhythmic and therefore if it cannot be achieved it should not even be attempted [14-16]. The anatomical complexity of the MI hinders the creation of transmural lesions [17]. This can lead to the persistence of conduction gaps that allow the maintenance of PMF despite the apparent evidence of complete block. This is a condition often referred to as MI pseudo-block [18-21].
In this prospective study, we investigated clinical and electrophysiological characteristics of patients who had undergone MI ablation, focusing in particular on the group of patients who continued to have PMF, while MI block had been previously demonstrated by evidentiary pacing maneuvers. At the same time, we tried to peruse standard PMF circuits by measuring conduction velocities, in order to understand the specific electrophysiological properties of these reentrant tachycardias and to improve our knowledge in selecting the most appropriate PMF ablation strategy.