S2 is caused by the closure of the aortic and pulmonary values. Generally, the closure will be simultaneous so that a single S2 is heard, loudest at the base. In some cases, both physiological and pathological, a split S2 is heard. With a split S2 the closure of the two valves does not occur together. Physiological S2 splitting in which the closing of the aortic valve precedes the closing of the pulmonic valve is normal. This physiological S2 splitting is exaggerated by inspiration which lowers intrathorasic pressure causing more blood to be drawn from the superior and inferior vena cava. Because of the increase venous return to the right ventricle, it takes longer to empty, leading to an additional delay in closure of the pulmonic valve. Physiological S2 splitting is best heard in the pulmonic region between the second and third intercostal spaces at the left sternal border.
Pathological split S2 occur in three types:
Pathological split S2 are related to several unlaying causes including:
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(17 October 2006)
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