Left bundle branch block and mechanical events of the cardiac cycle.

Left bundle branch block (LBBB) is associated with a prolongation of the interval from the QRS onset to the onset of left ventricular (LV) ejection. The locus and prevalence of specific sites of delay were examined in 56 patients with complete LBBB using echocardiography, phonocardiography and external pulse recordings. The results were compared with those in 52 control subjects without LBBB. The onset of the QRS complex was used as the initial reference point of measurement of time intervals. The following abnormalities were found in patients with LBBB: (1) delayed mitral valve closure (Q-MC greater than 0.08 second) was the major site of delay in 23% of patients; (2) prolongation of the LV isovolumetric contraction time (greater than 0.06 second) was the major site of delay in 41%; (3) both Q-MC and LV isovolumetric contraction time were prolonged in 18%; and (4) in 26% of patients the onset of ventricular contraction determined by the onset of the increase of the apex impulse was delayed (Q-VC greater than 0.07 second). The most common cause of delayed ejection was a prolonged LV isovolumetric contraction time, which occurred in 59% of patients. A control group of 20 patients with abnormal LV function but without LBBB had a low incidence of the 3 types of delay in LV ejection (0 to 15%). Thus, the major abnormalities in the cardiac cycle in LBBB are due to the conduction defect and not to LV dysfunction. The results of this study suggest the presence of variable abnormalities of conduction in complete LBBB.