An adolescent presents with symptoms of heart murmurs and potential heart residue. What likely defect could cause a fixed split S2?

A fixed split S2 is typically caused by an increase in blood flow through the right heart structures, which can occur in the presence of an atrial septal defect (ASD). In an ASD, there is an abnormal communication between the left and right atria that allows blood to flow from the high-pressure left atrium to the lower pressure right atrium during both inspiration and expiration. This left-to-right shunting leads to increased blood flow through the pulmonary valve, which causes the right heart to have greater volume and results in a fixed split S2—meaning the splitting does not vary with respiration.

In a healthy individual, the second heart sound (S2) consists of two components: the aortic component (A2) and the pulmonic component (P2). During inspiration, these components are separated due to increased venous return to the right heart, but in the case of an ASD, the shunted blood ensures that the pulmonary valve closure is consistently delayed, thus creating a fixed splitting of S2 regardless of the respiratory cycle.

Other defects might not result in a fixed split S2; for example, a ventricular septal defect (VSD) could cause a different heart sound pattern due to its effects on the left to right