Intra-atrial Conduction Block During Radiofrequency Ablation of Left-sided Accessory Pathways

Catheter ablation is currently the therapeutic approach of choice for many patients with accessory pathways. Despite the high success rate of radiofrequency ablation of the left lateral accessory pathways, a rather uncommon manifestation is intra-atrial conduction block at the level of the mitral isthmus. We report 2 cases of orthodromic atrioventricular reentrant tachycardia using a concealed left-sided accessory pathway with an abrupt change in the activation of the coronary sinus from an eccentric to concentric sequence after ablation delivery. The electrophysiological characteristics and the underlying mechanism of the intra-atrial conduction block are commented on. Careful mapping and assessment of relative conduction are helpful to document the diagnosis of intra-atrial conduction block. Familiarity with the likelihood of intra-atrial block during left lateral accessory pathway ablation is needed to avoid the erroneous elucidation that a second accessory pathway is present and to identify correctly the ablation site of interest.