Discrepancy between Activation and Postpacing Interval Mapping in Predicting Atrial Tachycardia Foci: What is the Mechanism?

A 67-year-old man with a tachycardiainduced cardiomyopathy developed a drugrefractory incessant atrial tachycardia (AT) after cavo-tricuspid isthmus (CTI)-dependent atrial flutter (AFL) ablation and was referred for electrophysiologic study (EPS) and catheter ablation. At the EPS, persistent CTI conduction block was confirmed by bidirectional pacing maneuvers. AT (cycle length = 260 ms) was induced by burst atrial pacing from the coronary sinus (CS). During the AT, the 12-lead electrocardiogram exhibited a negative saw-tooth pattern in the inferior leads, positive P waves in lead V1, and negative P waves in lead V6. Activation mapping along the tricuspid annulus (TA) revealed the earliest atrial activation in the His bundle region and activation going down the free wall of the right atrium (RA). Entrainment pacing from the CTI on the lateral side of the CTI block line and CS ostium revealed that the postpacing interval (PPI) was 290 and 360 ms, respectively (Fig. 1). Entrainment pacing from several electrode pairs of the TA mapping catheter was then additionally performed. It revealed that the shortest PPI ( = TCL + 7 ms) was obtained from the second distal electrode pair of the TA mapping catheter and the PPI became progressively longer as the pacing site became further away from the second distal electrode pair (Fig. 2). However, even the longest PPI obtained by pacing from the TA mapping catheter was still shorter than that from the CS ostium. Where is the substrate of the tachycardia?