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.