Are Stimulation Maneuvers Showing a Bidirectional Block Sufficient to Exclude Gaps in Ablation Lines?

A 71-year-old man presented for repeat ablation due to intermittent episodes of atypical atrial flutter. He had previously undergone pulmonary vein isolation for atrial fibrillation and a re-ablation that included creating an anterior mitral line and a posterior box. The repeat electrophysiological procedure was conducted in sinus rhythm. High-density mapping of the left atrium revealed reconnection at the left superior pulmonary vein (LSPV). The lines appeared blocked on the voltage map (Figure A). After re-isolating the LSPV, a local activation time (LAT) map was performed in sinus rhythm and under stimulation in the left atrial appendage (Figure B) to demonstrate a bidirectional block of the anterior mitral line. Both maps showed activation vectors converging on the line, confirming it as bidirectionally blocked by this maneuver. However, burst stimulation induced atypical atrial flutter with a cycle length of 300 ms, corresponding to the clinical tachycardia. An LAT map during flutter showed perimitral flutter with very slow conduction through a gap in the anterior mitral line (Figure C). Ablation at this location led to conversion into sinus rhythm, and the tachycardia could not be induced again. This case illustrates that in the presence of very slow conduction through a gap in an anterior mitral line, proving a bidirectional block through stimulation maneuvers may not be sufficient to reliably exclude a gap. If there is significant delay such that the wavefront arrives at the other side of the line before it is fully crossed, the line may falsely appear blocked. This underscores the importance of attempting to induce arrhythmias to map them during flutter, thus demonstrating any gaps with very slow conduction.