ELECTROPHYSIOLOGY OF THE A-V NODE IN RELATION TO A-V NODAL REENTRY

During A-V nodal reentry the impulse is supposed to travel through two distinct pathways in the A-V nodal junction, called slow and fast pat hways. Clinically, catheter ablation of these pathways has been very s uccessful in abolishing A-V nodal reentrant tachycardias. So-called do uble potentials have been used as a marker for the slow pathway, and t he occurrence of accelerated junctional rhythms (AJR) following ablati on is an indicator of successful destruction of the slow pathway. In L angendorff, blood-perfused porcine and canine hearts, extensive mappin g of extracellular potentials, combined with microelectrode recordings , was carried out to answer the following questions: 1) what is the or igin of double extracellular potentials? 2) what causes post-ablation AJR? 3) what is the activation pattern of the AV junction during ventr icular echoes? 1) Two types of double potentials were found: a low-fre quency component followed by a high-frequency deflection, the LH poten tial was caused by asynchronous activation of the sinus septum above t he coronary sinus and the region between the coronary sinus orifice an d tricuspid annulus, where the L component is a far field potential. H L potentials (high-frequency deflection followed by a low frequency co mponent) were caused by asynchronous activation of atrial cells and ce lls with AV nodal characteristics at the same location. These cells we re present around the entire tricuspid annulus, and were not part of t he compact node. The proximity of LH potentials to the slow pathway is probably serendipity, HL potentials could represent the slow pathway. 2) Two types of AJR could be initiated both by application of radiofr equency energy and by heat: a regular rhythm that progressively accele rated and an irregular rhythm. The discrete sites where heat applicati on induced AJR did not correlate with areas showing double potentials, nor with exit regions during ventricular pacing. They were close to t he compact node and the underlying mechanism was accelerated phase 4 d epolarization in single or multiple foci, the latter accounting for ir regular AJR. The association between presence of AJR and successful sl ow pathway ablation is probably also serendipity. 3) During ventricula r pacing, two separate areas of earliest atrial activity were found. W hen ventricular echoes were induced by premature stimulation, the retr ograde impulse activated both atrial exit sites and still returned to the ventricles as an echo. Thus, no evidence was found that atrial tis sue forms part of the reentrant circuit.