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.