Why does splitting if S heart sound occurs with atrial septal defect but not ventricular septal defect?

Splitting of the S2 heart sound is a common finding in patients with atrial septal defect (ASD) but not typically present in those with ventricular septal defect (VSD). Splitting of S2 occurs when the aortic and pulmonary components of the second heart sound are heard separately, usually due to their different timing.

In an ASD, there is an extra pathway for blood flow that bypasses the lungs, meaning that both pulmonary artery flow and aortic artery flow travel through the same valve before circulating throughout the body. When this occurs, it causes asynchronous emptying from the left and right ventricles which leads to slightly different timing between closure of each valve producing two distinct split sounds from each component.

In VSD however, there is often no significant evidence for asynchrony due to lack of pressure differences between both sides on a systemic level since VSD involves interventricular communication (blood flows directly between right and left ventricles). As such, there would be less splitting than what you hear in cases where pressure gradients exist within one side vs another which typically occurs when large ASDs exist causing some diversions.