DISCUSSION
CTI-dependent AT has been considered a single-loop tachycardia on the
TA.1,2 However, previous studies3,4suggested that a portion of CTI-dependent ATs is a dual-loop tachycardia
with an anterior circuit on the TA and a posterior circuit around the
functional block line on the posterior wall. In the present case, the LW
was captured later than the proximal CS during entrainment pacing from
the lateral TA (Fig. 2B). This phenomenon is paradoxical for the
single-loop mechanism because a site close to the pacing site was
captured later than a site remote from the pacing site (paradoxical
delayed capture). The dual-loop mechanism might explain this paradoxical
delayed capture. During entrainment pacing from the lateral TA, pacing
directly captured the anterior circuit on the TA, and then the
activation propagated to the proximal CS through the CTI. Thereafter,
the activation propagated to the posterior circuit and reached the LW
(Fig. 3A).
Moreover, the anterior and posterior circuits should have a common
isthmus. The TA and LW were captured simultaneously during entrainment
pacing from the proximal CS (Fig. 2C). This finding suggests that the
inferior septum around the proximal CS is the common isthmus, where
pacing can capture both the anterior and posterior circuits (Fig. 3B).
Moreover, the CTI should also be the common isthmus of the two circuits
as the radiofrequency application in the gap on the CTI block line
terminated the AT. Therefore, the mechanism of AT was a figure-8
dual-loop macroreentry with the common isthmus on the CTI and inferior
septum.
It is noteworthy that the dual-loop mechanism in the present case cannot
be revealed by activation mapping, as both circuits propagate in the
same direction. Therefore, this case provides proof that conventional
pacing maneuvers are still useful in this high-density mapping era.