aortic valve closure, or A2, and the second from right-sided closure of the pulmonic valve, or P2.
Consider the second heart sound and its two components, A2 and P2, which come from closure of the aortic and pulmonic valves respectively. During expiration, these two components are fused into a single sound, S2. During inspiration, however, A2 and P2 separate slightly, and S2 may split into its two audible components.
Current explanations of inspiratory splitting cite increased capacitance in the pulmonary vascular bed during inspiration, which prolongs ejection of blood from the right ventricle, delaying closure of the pulmonic valve, or P2. Ejection of blood from the left ventricle is comparatively shorter, so A2 occurs slightly earlier.
Of the two components of the second heart sound, A2 is normally louder, reflecting the high pressure in the aorta. It is heard throughout the pre-cordium. P2, in contrast, is relatively soft, reflecting the lower pressure in the pulmonary artery. It is heard best in its own area—the 2nd and 3rd left interspaces close to the sternum. It is here that you should search for splitting of the second heart sound.
S1 also has two components, an earlier mitral and a later tricuspid sound. The mitral sound, its principal component, is much louder, again reflecting the high pressures on the left side of the heart. It can be heard throughout the precordium and is loudest at the cardiac apex. The softer tricuspid component is heard best at the lower left sternal border, and it is here that you may hear a split S1. The earlier louder mitral component may mask the tricuspid sound, however, and splitting is not always detectable. Splitting of S1 does not vary with respiration.
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