Perimitral conduction time: Check it before creating an empirical mitral isthmus block

Perimitral atrial tachycardia (PMAT) is the most frequently occurring macro-reentrant tachycardia in the left atrium (LA) and is often related to an arrhythmogenic substrate caused by progressive fibrosis, tissue damage by catheter ablation, or surgical intervention in the LA.1, 2 Addressing postablation PMAT has been a longstanding challenge in catheter ablation for atrial fibrillation (AF). Creation of linear lesions in the lateral or anterior mitral isthmus with confirmed bidirectional conduction block can interrupt the reentrant circuit of PMAT and prevent its recurrence.2 It is reasonable to perform mitral isthmus block when PMAT has been clinically observed or is induced during catheter ablation. However, mitral isthmus block is often performed as an empirical adjunctive therapy, especially in catheter ablations for persistent AF. One of the bases for this strategy is the idea that extensive ablation targeting arrhythmogenic substrates existing outside the pulmonary veins (PVs) may improve ablation outcome. Another reason for empirical mitral block is to prevent possible PMAT in the future, but its efficacy on long-term rhythm outcome is unclear. Cavo-tricuspid isthmus-dependent atrial flutter coexists with AF more commonly than does PMAT, but the benefit of empiric cavotricuspid isthmus block is still controversial as well.3, 4 The STAR AF II trial revealed no reduction in the recurrence rate of AF when empirical linear ablation including a mitral isthmus line was performed in addition to PV isolation,5 which was surprising to us. One reason for the lack of benefit might be the difficulty in creating durable mitral isthmus block. The acute success rate of bidirectional mitral block is suboptimal, and reconnection is not uncommon even with the current technological advances in catheter ablation.2 In addition, precisely diagnosing bidirectional mitral isthmus block is often challenging due to epicardial conductions via the coronary sinus.6 In considering better methods to create complete and durable block, vein of the Marshall (VOM) chemical ablation is an important therapeutic option,7 that not only can contribute to complete and durable block but also eliminate triggers from the VOM and modify autonomic tone through ganglionated plexus ablation. In fact, both the Vein of Marshall Ethanol for Untreated Persistent AF (VENUS) trial.8 and Marshall-PLAN,9 which are well-designed prospective studies, revealed that the comprehensive strategies including empirical mitral isthmus block in combination with VOM chemical ablation improved the ablation outcome. The other important reason is the possibility that the benefit of empirical mitral isthmus block may be underestimated due to inappropriate patient selection. We need a guide to select patients who would likely suffer from PMAT. In a study reported in this issue of the Journal of Cardiovascular Electrophysiology, Hara et al. investigated the association between perimitral conduction time (PMCT) and PMAT.10 Among 1523 patients who underwent catheter ablation for AF or AT, those with available data for the calculation of estimated PMCT (E-PMCT) and who underwent induction tests of AT were identified. These identified patients were then divided into the (1) non-AT, (2) PMAT, or (3) non-PMAT AT group according to the occurrence of AT during their clinical course. E-PMCT was correlated with cycle length of PMAT and was related to the occurrence or induction of PMAT. PMAT was never induced nor observed in patients with an E-PMCT less than 176 ms. Receiver operating characteristics curve analysis showed an E-PMCT of 180 ms to be the best cut-off value to predict noninducibility of PMAT. One of the important messages of the work by Hara et al. is that we should not perform empirical mitral isthmus ablation in patients with little or no conduction delay along the mitral annulus. Radiofrequency applications would create iatrogenic arrhythmogenic substrates in patients who originally were at no risk of PMAT. Also, we need to keep in mind that adjunctive strategies other than linear ablation, such as ablation of fractionated signal areas, could prolong PMCT. The method adopted in this study is quite practical because the E-PMCT is easily calculated in this multipolar mapping era. This is helpful to identify patients without the need for mitral isthmus block. However, the identification of patients in need of mitral isthmus block remains a challenge. Although a long PMCT would appear to be a risk and predictive factor of PMAT, further studies are needed to elucidate this issue to improve practical value. While there seems to be consensus that an ablation strategy based on elimination of triggers is reasonable for paroxysmal AF, there is no established optimal ablation strategy for persistent AF. Despite the accumulation of insight from years of basic and clinical research, the mechanism of AF still remains unclear. Because of this, operators often tend to take an empirical strategy for persistent AF. However, in principle, ablation for AF should be performed according to each patient's specific electrophysiological findings. The authors declare no conflict of interest.