Epicardial portions of the circuit of endocardial re-entrant tachycardias are well established phenomena in the ventricles, but little is known about a similar phenomenon in post lesion atrial tachycardia (AT).

Higher spatial and voltage mapping capabilities may help identify extra-endocardial bridging (EEB) during tachycardia.

We retrospectively analysed our database of 119 pts with AT cases with ultra-high density mapping between March 2015 and March 2018. Epi-to-endo breakthrough sites were diagnosed when endocardial activation started at a discrete point before spreading radially in every available direction. PPI after transient entrainment at these sites confirmed local participation in AT.

In 20 out of the 119 pts (mean age 70±9 yrs, always with ≥1 prior procedures), 2 types of EEB were found. Type 1 is a “myocardial” EEB (n=16). Subtype 1A (Fig. 1) with a visible antidromic wavefront back to a line of block (n=10) whereas in subtype 1B activation starts immediately adjacent to a usually large line of block. Type 2 implied an extra-atrial “venous” bridging, either as a “shunt” over a line of block (subtype 2A, n=6) or by using an epicardial vein as a major part of a circuit (subtype 2B, n=1). In presence of EEB, AT termination was 17/22 (77%), significantly lower than our historical cohort of nonEEB AT (97%, P=0.03). The most successful strategy was targeting epi-endo breakthrough site.

Atrial EEB using either epicardial layers of the atrial wall, or extra-atrial venous structures is confirmed. Successful ablation at epi-to-endo breakthrough site proves the validity of the concept but results of ablation are inferior to nonEEB AT.