Bundle Branch and Ventricular Activation in Man

His (H), right bundle branch (RB), and left bundle branch (LB) potentials were recorded by electrode catheters passed into the left and right ventricles in seven patients without conduction defects. In another three patients, H and LB were recorded. Initial ventricular activation was considered to coincide with the onset of the QRS (Q). The following intervals were measured in milliseconds: H-Q, LB-Q, RB-Q, H-LB, H-RB, and LB-RB. The average and range of these was as follows: H-Q 46 (39-54), LB-Q 27 (24-30), RB-Q 25 (20-27), H-LB 20 (14-28), H-RB 21 (15-29), and LB-RB 2 (1-10). LB-RB was 3 msec or less in six of seven patients. Coupled atrial pacing produced delays in H-RB without effect on H-LB and LB-Q. The production of incomplete and complete right bundle branch block was related to increase in the LB-RB interval. One patient developed left bundle branch block (LBBB) during coupled pacing with delay of LB while RB-Q remained unchanged. H-Q intervals were measured in three additional patients with rate related LBBB during both control and block. Prolongation of H-Q was less than 3 msec during LBBB. It is concluded that the proximal left and right bundle branches are usually activated simultaneously in man, and that block of either bundle without contralateral bundle delay does not significantly delay the onset of ventricular activation.