What type of endocardial defect is typically characterized by a fixed split S2 sound?

Atrial septal defect is typically characterized by a fixed split S2 sound due to the hemodynamic changes it creates within the heart. In an atrial septal defect, there is an abnormal connection between the left and right atria, which allows blood to flow from the left atrium (where pressure is higher) to the right atrium. This increased volume of blood in the right atrium leads to increased right ventricular volume and subsequently more blood being ejected into the pulmonary artery.

The fixed split S2 occurs because the timing of closure of the pulmonary valve is delayed due to the increased blood flow in the right heart, while the aortic valve closure remains essentially unchanged. Unlike a usual physiological split that varies with respiration, the split in the case of an atrial septal defect remains constant throughout the breathing cycle, hence the term "fixed."

Other conditions like pulmonary stenosis, ventricular septal defect, or mitral regurgitation may lead to changes in heart sounds, but they do not typically produce the characteristic fixed split S2 sound seen with atrial septal defects. For instance, pulmonary stenosis could cause a variable split of S2 and other additional murmurs, while ventricular septal defects do not typically affect S