Fusion with postpaced return cycle identical to tachycardia cycle length during transient entrainment of ventricular tachycardia and its implications.

In reentrant ventricular tachycardia (VT), the postpaced return cycle (RC) during transient entrainment at a pacing site far from the central common pathway is longer than the VT cycle length (VTCL), when VT is represented by a figure-eight model. However, the reentrant circuit has not been fully elucidated. The purpose of this study was to present VT in which the postpaced RC became identical to VTCL during transient entrainment while fusion is evident in the surface electrocardiogram (ECG). Among 38 patients with inducible reentrant VTs who underwent electrophysiologic study (EPS), 10 VTs of six patients were selected. All patients had underlying heart diseases: dilated cardiomyopathy (n = 2), coronary artery disease (n = 1), postoperative tetralogy of Fallot (TOF; n = 2), and arrhythmogenic right ventricular dysplasia (n = 1). Catheter mapping was performed to demonstrate that the site of origin was distant from the pacing site. The cycle length of induced VT (n = 10 VTs) was 380 +/- 41 msec. Five patients (83%) had two morphological VTs; one a left bundle branch block (LBBB) and the other a right bundle branch block (RBBB) pattern. During rapid pacing, constant fusion was observed in all VTs, but the postpaced RC was identical to VTCL. In 2 patients (4 VTs), the revolution of wavefronts around an anatomical obstacle (scar of myotomy in TOF, and infarction) was demonstrated. The fact that the postpaced RC was identical to VTCL but showed fusion in the surface ECG can be explained by macro-reentry. The pacing site must be located at the preferential route of the macroreentrant circuit.