Endocardial activation of left bundle branch block.

Endocardial catheter mapping was performed in 18 patients with left bundle branch block (LBBB). Four patients had no organic heart disease (group I), six had cardiomyopathy (group II), and eight had coronary artery disease and previous infarction (group III). Twelve patients had one septal site of left ventricular endocardial breakthrough, while six had two left ventricular endocardial breakthrough sites, with one site always being septal. There was no significant difference among the groups with respect to time of left ventricular breakthrough (group I, 44 msec after the onset of the QRS complex; group II, 58 msec; and group III, 51 msec). Total left ventricular endocardial activation time was significantly longer in group III (119 msec) than group I (81 msec; p less than .05) and group II (61 msec; p less than .001). Duration of total right ventricular endocardial activation was 36 msec (seven patients). The final site of right ventricular activation was at 44 msec after the onset of the QRS complex. We conclude that (1) right ventricular activation occurs before initiation of left ventricular activation in patients with LBBB, (2) left ventricular endocardial activation in patients with LBBB most likely occurs as a result of right-to-left transseptal activation, (3) left ventricular endocardial activation sequence in patients with LBBB is heterogeneous, and (4) patients with coronary artery disease and LBBB have significantly longer total left ventricular endocardial activation times than patients with no organic heart disease or those with cardiomyopathies.