Professor Emeritus, Division of Cardiovascular Disease, Department
of Medicine, University of Alabama at Birmingham
Address for Correspondence:
G. Neal Kay MD
2432 Henrietta Road
Birmingham, Alabama 35223
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).
Sugumar and colleagues (8) prospectively analyzed the effects of each RF
application on antegrade and retrograde AV nodal conduction in 67
consecutive patients with typical slow-fast AVNRT referred for slow
pathway ablation to determine predictors of success. Every RF
application was analyzed if the duration was at least 5 seconds or if at
least one accelerated junctional beat was induced. An RF application was
discontinued if no junctional rhythm occurred after 30 seconds. RF
applications were also immediately discontinued if any degree of AV or
accelerated junctional to atrial (JA) block occurred or when the
accelerated junctional rhythm cycle length was <350 msec. An
accelerated junctional rhythm was observed during 178 of 301 total RF
applications (59%). Successful slow pathway modification, defined as no
more than one AV nodal echo beat in response to programmed atrial
stimulation, was achieved in 66 of 67 patients (99%). Among these,
complete elimination of antegrade slow pathway conduction was achieved
in 30 patients (46%). These authors found that an accelerated
junctional rhythm was observed in all 66 patients for whom successful
slow pathway ablation was achieved. However, like previous studies, the
presence of an accelerated junctional rhythm was not a specific
indicator of successful slow pathway ablation (1). However, the number
of junctional beats observed was significantly greater for successful
than unsuccessful RF applications (26.0 ± 26.7 vs 13.4 ± 16.1,
p< 0.001). This may have been related, in part, to the fact
that successful RF applications were of longer duration than
unsuccessful applications (38.5 ± 25.6 vs 26.9 ± 18.4 sec, p=0.002). The
cycle length of the induced junctional rhythm did not predict RF
application success. Importantly, junctional-atrial block was observed
during 19 RF applications, 14 of which resulted in successful slow
pathway ablation and 5 of which did not. The authors conclude: 1) that
an accelerated junctional rhythm during RF is a requirement for
successful slow pathway ablation; and 2) junctional-atrial block is a
strong predictor of an application that eliminates or modifies slow
pathway conduction.
The present study, while prospectively and carefully analyzing each and
every RF application, largely confirms the accumulated wisdom of
electrophysiologists who have performed slow pathway ablation for many
years. Electrophysiologists have long recognized that an accelerated
junctional rhythm is necessary to successfully modify slow pathway
conduction. Most operators will discontinue an RF application if an
accelerated junctional rhythm is not observed within 10 seconds. While
being necessary for successful slow pathway modification, the presence
of an accelerated junctional rhythm during RF is not sufficient for a
successful procedure. Thus, the sensitivity of an accelerated junctional
rhythm is high, but the specificity of this finding is relatively low.
And while the presence of junctional-atrial block during RF predicts
slow pathway modification or block, this finding should prompt immediate
cessation of an RF application due to the risk of causing both antegrade
fast and slow AV nodal block. The presence of junctional-atrial block is
likely an indicator that the compact AV node is in jeopardy of thermal
injury. It should also be emphasized that these observations may not
apply to atypical forms of AVNRT that utilize slowly conducting pathways
which may connect to either the left or the right atria relatively
remote from the compact AV node (9,10). Therefore, an accelerated
junctional rhythm is likely not a reliable predictor of ablation success
for these slowly conducting pathways underlying atypical forms of AVNRT.
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