While these events are occurring on the left side of the heart, similar changes are occurring on the right side, which involves the right atrium, tricuspid valve, RV, pulmonic valve, and pulmonary arteries. Right ventricular and pulmonary arterial pressures are significantly lower than corresponding pressures on the left side. Note that right-sided cardiac events usually occur slightly later than those on the left. Instead of a hearing a single heart sound for S2, you may hear two discernible components, the first from left-sided aortic valve closure, or A2, and the second from right-sided closure of the pulmonic valve, or P2.
The second heart sound, S2, and its two components, A2 and P2, are caused primarily by closure of the aortic and pulmonic valves, respectively. During inspiration, the right heart filling time is increased, which increases right ventricular stroke volume and the duration of right ventricular ejection compared with the neighboring left ventricle. This delays the closure of the pulmonic valve, P2, splitting S2 into its two audible components. During expiration, these two components fuse into a single sound, S2 (Fig.1). Note that because walls of veins contain less smooth muscle, the venous system has more capacitance than the arterial system and lower systemic pressure. Distensibility and impedance in the pulmonary vascular bed contribute to the "hangout time" that delays P2.
FIGURE 1. Spitting of S2 during inspiration.
Of the two components of the S2, A2 is normally louder, reflecting the high pressure in the aorta. It is heard throughout the precordium. In contrast, P2 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 the splitting of S2.
S1 also has two components, an earlier mitral and a later tricuspid sound. The mitral sound—the principal component of S1—is much louder, again reflecting the higher 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; 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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