What effect does left bundle branch block (LBBB) have on the heart sound S1 and S2?

Left bundle branch block (LBBB) typically influences the heart's electrical conduction system, leading to asynchronous contraction of the ventricles. This asynchrony primarily affects the closure of the aortic and pulmonic valves, which are the components of the second heart sound (S2). In individuals with LBBB, the left ventricle contracts later than the right ventricle due to the delay in conduction through the left bundle branch. As a result, the closure of the aortic valve occurs slightly earlier than the closure of the pulmonic valve.

This delay can lead to a split S2, meaning that there is a noticeable interval between the closure of the aortic and pulmonic valves. The splitting of S2 is more pronounced during inspiration due to increased venous return leading to a longer interval between the valve closures. Conversely, the first heart sound (S1) is largely unaffected by LBBB because it primarily reflects the closure of the atrioventricular valves (mitral and tricuspid) and is less influenced by the conduction through the bundle branches.

Thus, the correct choice highlights that LBBB particularly affects S2, resulting in a split, while the first heart sound remains unaltered.