EVIDENCE FOR MULTIPLE ATRIO-AV NODAL INPUTS IN THE NORMAL DOG HEART

Introduction: Complete AV block after combined fast pathway (FP) and s low pathway (SP) ablation is uncommon. The purpose of this study was t o interrupt activation of these and additional inputs by placing a rad iofrequency lesion across the interatrial septum between the FP and SP ablation sites. Methods and Results: In eight anesthetized open chest dogs, FP ablation induced significant A-H prolongation (Delta A-H: 51 +/-14 msec; P < 0.001) and a shift of earliest retrograde atrial activ ation from the anterior septum to the region of the coronary sinus (CS ) os. Subsequently, ablation of the interatrial septum across the foss a ovalis was successful in 5 of 8 dogs, changing the sequence of atria l activation (A) so that A at the His-bundle electrogram, which initia lly preceded A at the CS os (18 +/- 4 msec vs 46 +/- 7 msec, P < 0.01) , now followed CS os A (81 +/- 31 msec vs 59 +/- 20 msec, P < 0.05). A dditional ablation of the SP caused a type II Mobitz AV block or compl ete AV block in 5 of 8 dogs. The four dogs with complete AV block show ed a stable, high junctional escape rhythm at a rate of 64 +/- 16 beat s/min. Pacing between the ablation lesions and the AV node in one dog showed 1:1 AV conduction and Wenckebach-type AV block indicating prese rved AV nodal function. Histology showed necrotic changes in the FP an d SP transitional cell zones and in the atrial tissue of the interatri al septum. However, the compact AV node, His bundle, and adjacent atri a and transitional cells were undamaged. Conclusion: There are additio nal AV nodal inputs in the interatrial septum in addition to the anter ior FP and posterior SP inputs. Ablation of all of these may be requir ed, if the aim is production of complete AV block proximal to the AV n ode with a high junctional escape rhythm.