On the Causes of the Changes of the Second Heart Sound in Left Bundle Branch Block

Abstract

A dynamic study by right and left cardiac catheterization and phonocardiography was performed in 10 subjects with normal hearts and in 11 patients with complete left bundle branch block (LBBB). Comparison was made between the mechanical events of the 2 groups and, in each, between those of the right and those of the left heart.
The onset of ventricular contraction was found delayed in all cases of LBBB; that of right ventricular contraction in 3 out of 11. This indicated the possible occurrence of a right intraventricular block plus a complete LBBB.
The isovolumic period of the LV was found prolonged for either ventricle (more for the left) or only for the left.
Individual cases showed either delayed LV contraction or delayed biventricular prolongation.
In the majority of cases, LV contraction terminated after RV contraction but there were exceptions (either simultaneous or opposite sequence). In spite of this, in 7 out of 9 cases, the aortic incisura followed the pulmonary incisura and reverse splitting of the second sound took place.
These facts reveal the interrelationship of conduction delays, prolongation of the isovolumic periods, and peripheral factors affecting the timing of the incisuras and of the sound components.