Clockwise "Mitral Isthmus Block" complicating ablation of left free wall accessory pathways - two cases and review of the literature.

BACKGROUND

Mitral isthmus block (MIB) complicating radiofrequency ablation (RFA) of orthodromic reciprocating tachycardia (ORT) using left - sided accessory pathways (APs) is poorly understood.

METHODS

Two cases and a systematic review of the literature of patients (pts) who developed MIB complicating left - sided ORT RFA is presented.

RESULTS

Among 27 pts (34 ± 12 years old, 54% female, 68% concealed AP), 15 (56%) had ≥ 1 failed RFA procedure. One RF lesion caused MIB in 6 (22%) (≤ 3 lesions in 11 (41%)). MIB caused switch from eccentric to pseudo-concentric atrial activation (23/27 (85%)) without increasing septal ventriculo-atrial (VA) intervals/ ORT cycle lengths (17/18 (94%)). Recurrent ORT with "concentric" activation was misdiagnosed as atrio-ventricular nodal reentrant tachycardia (AVNRT) in 3 (11%) - 1 requiring pacemaker implantation after slow pathway (SP) RFA. By targeting earliest retrograde atrial activation on the high mitral annular free wall (1-3 o'clock (17/19 (89%)) above the line of block (LOB), successful AP RFA occurred in 23/23 (100%).

CONCLUSIONS

Left free wall ORTs with RFA - induced MIB are (1) difficult ablations with > 50% requiring > 1 procedure, (2) can masquerade as AVNRT causing unnecessary SP RFA, and (3) are successfully ablated on the high mitral annular free wall predominantly between 1 and 3 o'clock and always superior to the LOB.