Narrowed sinus rhythm electrograms in the zone of uniform slow conduction are helpful to identify VT isthmus location, shape, and orientation

BACKGROUND: Although the sinus rhythm uniform slow conduction zone is an indicator of the reentrant ventricular tachycardia (VT) sources, it cannot alone be used for delineation. OBJECTIVE: This study aimed to use substrate mapping to determine VT isthmus characteristics. METHODS: In 31 canine postinfarction experiments, an infarct and border zone formed in the anterior left ventricle after left anterior descending coronary artery ligation. In each experiment, 3 to 5 days after infarction, sustained monomorphic reentrant VT was reproducibly inducible via premature electrical stimulation. Epicardial bipolar electrograms were acquired using multielectrode grids. The isthmus location, shape, and orientation were determined from VT activation mapping. Sinus rhythm electrograms were marked for activation time and duration and mapped. The VT isthmus was prognosticated from the sinus rhythm maps via the following paradigm: (1) locate the late activation region, (2) determine and outline the border of the largest area of narrowed electrograms (≲40 ms) nearest to the late activation region, (3) ascertain the direction of uniform slow conduction in the narrowed electrogram region, and (4) draw lateral isthmus boundaries symmetric to the uniform slow conduction direction there. RESULTS: At the VT isthmus vs periphery, sinus rhythm electrograms are narrower (P < .02) and activation time differences trend greater with less variability (P ∼ .1). Overlap of predicted vs actual VT isthmus location was 57.9% ± 23.6%, with a centroid difference of 7.60 ± 6.29 mm and a divergence in exit angle of 18.0° ± 9.5°. CONCLUSION: Narrowed electrogram duration near the sinus rhythm late activation sites, along with uniform slow conduction, is indicative of VT isthmus location, and the activation wavefront exit direction can be discerned from activation maps.