Left ventricular endocardial activation during right ventricular pacing: Effect of underlying heart disease

Endocardial catheter mapping of the left ventricle was performed in 40 patients during right ventricular pacing to determine the effect of underlying myocardial infarction on endocardial activation. Group I comprised 18 patients without infarction, Group II 12 patients with inferior infarction and Group III 10 patients with anteroseptal infarction. Thirty-nine of the 40 patients had only a single left ventricular breakthrough site located on the midseptum in 33 cases, apical septum in 4 cases and basal septum in 2 cases. The earliest left ventricular local activation time during right ventricular pacing was earlier in Group III (40 ± 11 ms) than in Group I (55 ± 17 ms) and Group II (60 ± 15 ms) (p < 0.01). Total endocardial activation time was significantly longer in Group III (118 ± 30 ms) than in Group I (76 ± 14 ms) and Group II (72 ± 20 ms) (p < 0.001). The latest left ventricular site of activation during right ventricular pacing was the inferoposterior base in 14 (77%) of the 18 Group I patients, and 10 (83%) of the 12 Group II patients. The latest site of activation in Group III patients was variable. It is concluded that: 1) left ventricular endocardial activation patterns and conduction times are influenced by the site of previous infarction. 2) Longer total endocardial activation in Group III suggests that specialized conducting tissue in the septal and anterior walls may play an important role in left ventricular activation during right ventricular pacing. 3) Left ventricular activation times during right ventricular pacing are similar to those previously observed in intrinsic left bundle branch block; thus, right ventricular pacing may serve as a model of left bundle branch block.