Splitting of the second heart sound is a difficult concept which is stated, rather than explained, in many medical texts. The splitting into the pulmonary valve and aortic valve components occurs in physiology during inspiration, which is due to later closure of the pulmonary valve as well as earlier closure of the aortic valve, determined by changes in intrathoracic pressure and the capacitance of the pulmonary vasculature. Fixed splitting of the second heart sound describes a situation where the second heart sound is split in both inspiration and expiration and is classically described in an atrial septal defect. The evidence behind these mechanisms is discussed in the following article.
Bilton Samuel J and Carande Elliott J