Using an Accelerated Junctional Rhythm to Predict the Success and Safety of Slow AV Nodal Pathway Ablation

Atrioventricular nodal reentrant tachycardia (AVNRT) has been successfully treated by targeting the slow atrioventricular nodal pathway for ablation going on 30 years (1-4). It has long been recognized that successful ablation of the antegrade slow AV nodal pathway using radiofrequency (RF) current delivered along the tricuspid annulus near, or slightly superior to, the coronary sinus ostium is almost always associated with an accelerated junctional rhythm in patients with typical slow-fast AVNRT (1-4). For example, Yu and colleagues observed an accelerated junctional rhythm with 42 of 43 RF applications that successfully eliminated the slow AV nodal pathway (1). While the sensitivity of an accelerated junctional rhythm as a marker for successful slow pathway ablation is very high (98%), the specificity is rather low, as approximately 45% if unsuccessful RF applications also induce an accelerated junctional rhythm (1). Mapping of the site of earliest atrial activation during the accelerated junctional rhythm induced by RF targeting the slow AV nodal pathway has shown that retrograde conduction occurs at the superior aspect of Koch's triangle where retrograde fast pathway conduction is recorded in response ventricular extrastimuli. This strongly suggests that RF energy applied to the region of the slow pathway stimulates AV nodal cells which conduct antero-superiorly through the fast pathway to the atria at the apex of Koch's triangle. Animal studies using isolated, perfused porcine and rabbit hearts have identified an area in, or close to, the compact AV node that is stimulated by heat (5). Microelectrode recordings have revealed that the accelerated junctional rhythm in response to heating arises in nodal-type cells with increased phase 4 depolarization slope and shortening of the action potential duration (6). Irregularity of the evoked accelerated junctional rhythm was shown to be due to interaction of multiple foci and the presence of conduction block between these foci and the His bundle (3). These findings are compatible with studies of accelerated junctional rhythm during slow pathway ablation in which the retrograde His-atrial conduction interval during RF-induced junctional rhythm is significantly shorter than the His-atrial interval during slow-fast AVNRT (57 + 24 vs 68+ 21 ms, P < 0.01) (7).