Case presentation
A 77-year-old woman with frequent palpitations was referred to our
institution for catheter ablation. Transthoracic echocardiography did
not reveal any abnormalities. The baseline 12-lead electrocardiogram did
not show any pre-excitation. The 12-lead electrocardiogram during the
tachycardia exhibited a narrow complex tachycardia with a long RP
interval (heart rate: 200 bpm). Programmed atrial and ventricular
stimulation did not reveal any distinct dual nodal physiology.
Retrograde ventriculoatrial (VA) conduction revealed a midline and
decremental property. The retrograde atrial earliest site was located in
the postero-septal region. Para-hisian pacing demonstrated a typical
nodal response. The tachycardia was reproducibly induced by atrial
extrastimuli, without a critical atrio-ventricular prolongation. The
atrial activation sequence during the tachycardia was identical to that
during right ventricular pacing. Entrainment from the right ventricular
apex exhibited a V-A-V response. The differences between the corrected
post pacing interval and tachycardia cycle length was 168ms
(> 110ms) (Figure 1A ). The tachycardia was
sustained with a 2:1 AV relationship (Figure 1B ). The
tachycardia was terminated by a His-refractory ventricular premature
depolarization (VPD) from the right ventricular apex (Figure
2A ). Based on these findings, what was the mechanism of the tachycardia
and the response to the pacing maneuver?DiscussionThe differential diagnosis of a long RP supraventricular tachycardia
with the earliest atrial activation near the ostium of the coronary
sinus includes atrial tachycardia originating from the postero-septal
region, orthodromic tachycardia using a slow-conducting accessory
pathway, atypical atrioventricular nodal reentrant tachycardia (AVNRT),
nodofascicular (NF) reentrant tachycardia, and atypical AVNRT with
bystander NF or nodoventricular (NV) bypass tract. The mechanism of a
narrow QRS tachycardia can be diagnosed by combining several standard
pacing maneuvers and observations, as follows: (1) whether the
tachycardia is involved in a nonobligatory 1:1 AV relationship, (2) the
effect of bundle branch block on the VA interval, (3) a V-A-V or V-A-A-V
response to ventricular entrainment; (4) a corrected post-pacing
interval after ventricular entrainment compared to the tachycardia cycle
length (cPPI-TCL), and (5) whether a VPD timed to the His-refractory
period disrupts the tachycardia.1,2In our patients, the V-A-V response to ventricular entrainment excluded
atrial tachycardia (Figure 1A ). The long corrected PPI-TCL (168
ms) after the right ventricular entrainment pacing suggested that the
ventricular pacing site was far away from the tachycardia circuit. The
tachycardia with a 2:1 AV relationship absolutely excluded orthodromic
tachycardia using a slow conducting accessory pathway (Figure
1B ). Based on these findings, the tachycardia was diagnosed as atypical
AVNRT. However, the AVNRT was reproducibly terminated by a
His-refractory VPD, which did not reset the tachycardia cycle length
because of a conflict with the downstream wavefront of the AVNRT at a
site below the His bundle. However, the presence of an NF or NV bypass
tract inserting into the retrograde limb of the AV nodal circuit could
provide a reasonable explanation for the termination of the tachycardia
by a His refractory VPD.3,4 The mechanism is shown by
the laddergram in Figure 3 . Briefly, at the timing of the
His-refractory VPD, the wavefront of the tachycardia had already reached
the retrograde limb of the AVNRT circuit after the lower turnaround
point. The wavefront of the VPD collided with the downstream wavefront
of the AVNRT within the right ventricular. On the other hand, the
wavefront via the NF or NV bypass tract entered the retrograde limb of
the AVNRT circuit ahead of the wavefront of the AVNRT after the lower
turnaround point. Then, the wavefront via the NF or NV bypass tract
allowed the tachycardia to terminate due to collision with the remaining
refractory period of the retrograde slow pathway.
Radiofrequency energy delivered in the posterior septal region where the
earliest activation was recorded during the tachycardia, terminating the
tachycardia within 2 seconds. After the ablation, VA conduction via the
retrograde fast pathway was observed, but no tachycardia was induced.
In summary, we presented a case of an atypical AVNRT with a concealed,
bystander NF or NV bypass tract, that was confirmed by a response to a
His refractory VPD during the tachycardia.