An atypical mechanism of pseudo mitral isthmus block clarified by the high-resolution mapping system.

Case

A 31-year-old man with prior step-wise ablation for persistent atrial fibrillation presented with atrial tachycardia (AT). Linear lines for the mitral isthmus, left atrial roof, and the cavo-tricuspid isthmus had previously been performed in addition to pulmonary vein (PV) isolation and defragmentation on the left atrium. The patient was in sinus rhythm (SR) at the beginning of the procedure, so the mitral isthmus line was first confirmed. Although bidirectional block was confirmed by the conventional method as shown in Fig. 1, the high-density activation map during left atrial appendage (LAA) pacing with the Orion catheter and Rhythmia system demonstrated that the activation passed to the posterior wall through the underside of left inferior PV with a severe conduction delay, detouring to the proximal site of the coronary sinus (CS). The low voltage area on the posterior wall from prior ablation slowed the direct activation to the distal CS (Fig. 2). As the result, the wave front turning around the septal mitral annulus reached the proximal CS earlier, resulting in proximal to distal CS activation as shown in Fig. 3(a–f). AT (CL = 340 ms) was easily induced by burst pacing, which was diagnosed as an atypical perimitral flutter using the site of conduction delay during the LAA pacing as the critical isthmus (Fig. 3(g–i)). RF application targeting this site (gray dot in Fig. 2) terminated the AT in 20 seconds, and no other ATs were inducible. Finally, neither the sequence of the CS activation nor the duration between pacing and the local activation changed after ablation. However, high-density mapping in LAA pacing demonstrated the line was completely blocked (Fig. 4).

Fig. 1

Bidirectional block of the mitral isthmus was confirmed by the conventional method.

Fig. 2

Slow conduction and break through on the posterior wall during LAA pacing. Inset - electrograms recorded from this area during LAA and AT.

Fig. 3

Activation during LAA pacing before the procedure (a–f), and during AT (g–i).

Fig. 4

Activation during LAA pacing following the ablation.

The pitfalls in the assessment of mitral isthmus linear conduction block have been described by Shah AJ et al. [1] However, the type of confounding in the present case has not been well demonstrated in the published literature. In the present case, the slow conduction to the posterior wall and the functional block between the endocardial lateral atrium and the distal CS, resulting in the earlier activation of CS from the septum. Because of this, the conventional method could not demonstrate the incomplete block of the mitral isthmus. In addition, the brief activation mapping with a high-density mapping system around the block line during LAA pacing was useful to confirm the complete achievement of the block. As ablation strategies for persistent AF have become more complex, subsequent ATs are often also complex due to scar and regions of slow conduction. The high-density mapping system may clearly display the precise mechanism of the AT, which is not possible with conventional mapping [1].

Conflict of interest

Masateru Takigawa is a temporary consultant in 2016 for Boston Scientific Japan.