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.