DISCUSSION
To investigate the underlying mechanism, bi-atrial activation was mapped in sinus rhythm (Figures 3A and 3B). Both sides of the interatrial septum were transected by an incomplete line of block between the fossa ovalis and the CS. While one end of this line joined a posterior scar contiguous to the inferior vena cava (at the right side of the septum) and the right pulmonary vein isolation line (at the left side of the septum), a narrow isthmus of residual slow conduction was located at the annular end of the line. Initial PR prolongation was, therefore, due to a conduction slowing through the interatrial septum. As the CTI and roof lines were blocked, the sinus node activation propagated to the left atrium via the Bachman bundle, then in a clockwise direction around the mitral annulus through the lateral mitral isthmus, then to the AV node through the CS which exhibited a distal-to-proximal sequence, and finally collided with the delayed activation of the interatrial septum (Figure 3C).
After VOM ethanol infusion blocked the lateral mitral isthmus, the narrow isthmus at the interatrial septum remained as the only path for electrical propagation from the sinus node to the AV node (Figure 3D). We hypothesized that electrical conduction of this narrow isthmus was fragile enough to be temporarily blocked. Thus, the supra-nodal complete AV block observed in the current procedure was most likely due to a complete isolation of the lower parts of both atria. This isolation resulted in a complete dissociation of the sinus rhythm and the junctional rhythm, which separately activated the LAA and the CS, respectively (Figure 2B).
Although the interatrial septum is a frequent target of CFAE ablation, this will rarely result in an AV block2,3 unless coexisting linear lesions disrupt the alternative conduction paths from the sinus node to the AV node. Given the high incidence of CTI-, roof-, and mitral-dependent flutters during a follow up after AF ablation, linear lesions are frequently created in the recurrent procedure. However, cautious analysis of interatrial conduction is needed before creating linear lesions in the CTI, roof, and mitral isthmus, when extensive CFAE ablation was previously performed at the interatrial septum. Furthermore, extensive ablation of the interatrial septum should probably be avoided to respect physiological atrial activation and to spare associated contractile function.
In the present case, a dual-chamber pacemaker was implanted before patient discharge.