Commentary
The differential diagnosis of a long RP tachycardia, with the earliest
atrial activation at the HB region, includes atypical atrioventricular
(AV) nodal reentry tachycardia (AVNRT), atrial tachycardia (AT), and
orthodromic reciprocating tachycardia (ORT) using a slowly conducting AV
accessory pathway or a concealed nodofascicular/nodoventricular (NF/NV)
bypass tract (BT). The persistence of the SVT with a 2:1 AV block
excluded the possibility of an ORT with a slowly conducting AV accessory
pathway and a concealed NF/NV BT. As shown in Figure 2A, ventricular
burst pacing did not accelerate the atrium to the pacing cycle length,
and the SVT terminated without conduction to the atrium after the third
fully-paced QRS complex. This finding is consistent with AVNRT and ruled
out the possibility of an AT. Figure 2B shows the response to
ventricular burst pacing with a delayed timing during the SVT. The SVT
terminated without conduction to the atrium after the first fused paced
QRS complex. The fused-paced QRS complex indicates a His-refractory
timing. Therefore, this response to the ventricular pacing indicates the
presence of a concealed NF/NV BT, but not necessarily its participation
in the tachycardia circuit. The responses to these pacing maneuvers can
be explained as follows: when ventricular burst pacing with a delayed
timing was performed during the SVT (Figure 2B), the first
His-refractory ventricular pacing impulse was conducted over the NF/NV
AP and penetrated the slow pathway (SP). The conduction block in the SP
resulted in tachycardia termination owing to the decremental properties
of the SP. When ventricular burst pacing with an earlier timing was
performed during the SVT (Figure 2A), the connection of the NF/NV BT to
the SP was blocked, but the SVT persisted. The timing of the third
ventricular pacing impulse being earlier than the His-refractory period
resulted in tachycardia termination without conduction to the atrium,
due to the conduction block in the SP via the right bundle branch-His
conduction. These findings established the diagnosis of an atypical
AVNRT, with a bystander NF/NV BT inserting into the retrograde SP.
We performed electroanatomical mapping to identify the earliest site of
atrial activation during the SVT, and radiofrequency ablation was
performed in the noncoronary aortic cusp of Valsalva just superior to
the HB region, thereby eliminating the inducible tachycardias (Figure
3). Kaneko et al. reported that the mechanism of an atypical AVNRT,
incorporated a superior-SP, located above Koch’s triangle as the
retrograde limb.1 Thus, we diagnosed the SVT as an
atypical AVNRT, with a bystander NF/NV BT inserting into the retrograde
superior SP.
NF/NV BTs are rare accessory pathway variants connecting the AV node to
the right bundle branch or right ventricle. Most of NF/NV BTs are
reportedly inserted into the SP; hence, concealed NF/NV BT-associated
supraventricular tachycardias can be eliminated by the ablation of the
right or left inferior extensions of the AV node, regardless of their
mechanism.2,3 In this case, the nodal pathway of the
NF/NV BT is the superior SP, because the successful SP ablation site was
the noncoronary aortic cusp of Valsalva just superior to the HB region.
Termination of the SVT without conduction to the atrium after the fused
paced QRS complex favors an NV BT over an NF BT, because the collision
site between antidromic and orthodromic wavefronts during the QRS fused
period occurs in the ventricular myocardium. The present case
highlighted that the precise mechanism of AVNRT, in which NVBT was
attached to the superior SP, could be elucidated by adjusting the timing
of ventricular burst pacing during tachycardia.