You are listening to a typical example of the murmur caused by an atrial septal defect. Because the pressure in the left atria initially exceeds that in the right, the blood flows in a left to right shunt. This high volume of blood next passes into the right ventricle, and the ejection of the excess blood through a normal pulmonary valve produces the prominent mid-systolic flow murmur as heard in this sample. This murmur is best heard over the “pulmonic area” of the chest, and may radiate into the back as with the murmur of pulmonary stenosis.

     The most characteristic feature of an atrial septal defect is the fixed split S2. As mentioned in the murmur overview, a split S2 is caused physiologically during inspiration because the increase in venous return overloads the right ventricle and delays the closure of the pulmonary valve. With an atrial septal defect, the right ventricle can be thought of as continuously overloaded because of the left to right shunt, producing a widely split S2. Because the atria are linked via the defect, inspiration produces no net pressure change between them, and has no effect on the splitting of S2. Thus, S2 is split to the same degree during inspiration as expiration, and is said to be “fixed.”