The second heart sound is produced by the closure of the aortic and pulmonic valves. The sound produced by the closure of the aortic valve is termed A2 and the sound produced by the closure of the pulmonic valve is termed P2. When these sounds are distinguishable from each other a split S2 can be heard. The patterns of splitting of the S2 heart sound include physiologic splitting, paradoxical splitting, widened splitting and fixed splitting.
A fixed split S2 is not common on cardiac exam, however, when present it almost always indicates an atrial septal defect (ASD). A fixed split S2 occurs when there is always a delay in the closure of the pulmonic valve and there is no further delay with inspiration (compare this to a widened split S2 as described above).
To helo understand why an ASD results in a fixed split S2, we must considered the altered cardiac hemodynamics present in this situation which result in a fixed delay in pulmonic valve closure. During inspiration, as usual there is an increase in venous return to the right side of the heart and thus increased flow through the pulmonic valve delaying its closure. Where the alteration occurs in a person with an ASD is during expiration. As the person expires, the pressure in the right atrium decreases (since there is less venous return). The decreased pressure allows more blood to flow abnormally through the ASD from the high pressured left atrium to the right atrium ultimately resulting again in increased flow through the pulmonic valve again delaying its closure.