Discussion
The tachycardia exhibited the earliest atrial activation at the HB
region with a ventriculo-atrial interval of -6 ms, i.e., an H-A-V
sequence. These features were diagnostic for slow-fast atrioventricular
nodal reentrant tachycardia (AVNRT), excluding orthodromic reciprocating
tachycardia (ORT) via an accessory pathway (AP) and atrial
tachycardia.1, 2 The V-A-V response was seen during RV
overdrive pacing, which was also excluded atrial
tachycardia.2 However, the atrial activation sequence
gradually shifted, suggesting the presence of a left lateral AP (Figure
1). Similar to the atrial activation during RV overdrive pacing, the
earliest atrial activation was seen in the distal CS region with RV
pacing during sinus rhythm, which indicated the presence of a left
lateral AP (Figure 2). The earliest atrial activation shifted to the HB
region as the ventricular extrastimulus coupling interval decreased to
280 ms at a basic pacing CL of 600 ms, which suggested retrograde
conduction over the fast pathway.
In a setting with the presence of retrograde AP conduction, the most
likely diagnosis would be ORT via an AP. In this case, however, a
diagnosis of AVNRT with a bystander left lateral AP was reached based on
the findings described above. We performed a left lateral AP ablation
followed by a slow pathway ablation. After the AP ablation, we attempted
to induce the clinical tachycardia by atrial extrastimulation with and
without an isoproterenol infusion, but failed. After several junctional
beats were seen during the empirical slow pathway ablation, any further
tachycardia was rendered noninducible.
The teaching points in this case were as follows: (1) The presence of an
AP does not necessarily mean that the AP is involved in the tachycardia
circuit, and (2) a bystander AP may hinder the interpretation of the
intracardiac electrograms during diagnostic pacing maneuvers.
When an AP is identified by RV pacing, we assume that the diagnosis may
be AVRT. However, the presence of an AP does not necessarily mean that
the AP is involved in the tachycardia circuit. Even if an AP is present,
the tachycardia circuit should be confirmed by conventional diagnostic
pacing maneuvers. Although several reports have described double
tachycardias such as ORT via an AP and AVNRT with a bystander
AP,3, 4 there are no electrogram-tracings of RV
overdrive pacing during AVNRT with a bystander AP. Diagnostic pacing may
not have been applicable, possibly due to the repeated and spontaneous
transition of the tachycardia form.
Electrophysiologists should be
aware of these observations through our experience. The tachycardia
circuit and location of the bystander AP can quickly be determined by
only one tracing.
There is some controversy over whether bystander APs should be
eliminated.5 In this case, we eliminated the left
lateral AP first for the correct diagnosis of the tachycardia circuit,
which was hindered by the AP, although no evidence of the ORT was
observed. The incidence of complications is related to the number of
ablation targets, i.e., the more substrates, the more
complications.5 Electrophysiologists should carefully
determine the ablation strategy considering the risk and benefit of a
bystander pathway ablation.