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The second heart sound is caused by closure of the aortic-and pulmonary valves at the end of ventricular systole. Two components, from the separate closure of the two valves, are usually audible because of slight asynchrony of the two ventricles, aortic closure preceding pulmonary closure in normal subjects (Leatham and Towers, 1951). Potain (1866) observed that the character of the second heart sound altered with respiration. While it might be single during the expiratory phase of continued respiration it became clearly split with inspiration. Such a change in the second sound is found in most normal subjects and is due to prolongation of right ventricular systole during inspiration. This may be explained by the increased filling of the right atrium and ventricle from the great veins which takes place with the inspiratory fall in intra-thoracic pressure.
Ian Gray (Sun,) studied this question.