Slow pathway ablation
Catheter ablation for AVNRT targets discrete slow pathway potentials at the mid to posterior septum adjacent to the tricuspid annulus7, 29. The established procedural endpoint is slow pathway block or modification with a single AV nodal echo beat39. To date there have been few prospective studies evaluating the characteristics of RF applications required to achieve slow pathway modification or block. Retrospective studies have not identified a clear relationship between the characteristics of JR and success although have been limited by a smaller sample size than the present study and an absence of rigorous testing after each ablation25-27 reported no relationship between the cycle length of JR and outcome. Hence the current approach is a variable duration of RF determined by the electrophysiologist generally aiming for junctional rhythm without causing AV block and periodic subjective testing to determine if acute success has been obtained. The duration of RF required is highly variable as demonstrated in the present study where RF applications as short as 5-10 seconds resulting in junctional rhythm were successful in some patients. Junctional rhythm during RF in the region of the slow pathway has long been accepted as a requirement for successful ablation for AVNRT7, 13, 26, 40. The mechanism responsible for junctional automaticity during RF applications is likely related to direct heating of specialized conducting tissue within the transitional zone41. The slow pathway has not been anatomically defined as a discrete AV connection but is rather housed within a transitional zone of conduction with electrophysiologic properties of both atrial cells and nodal tissue6.
The presence of JA block during faster junctional rhythm was associated with slow pathway block or modification in 74% in the present study. As per the study protocol the occurrence of JA block during RF led to the immediate termination of RF delivery and prompt repeat testing for AVNRT. JA block was considered to be functional in all but 1 patient as it was not accompanied by any increase in the immediate post termination AH interval. It was therefore generally not an indicator of impending heart block. Acute JA block during RF may be rate related or represent ablation at a location within the transitional zone in close proximity to the retrograde fast pathway26, 42.