The second heart sound ($S_2$) marks the end of systole and the beginning of diastole. While often clinically summarized as the "Dub" in "Lub-Dub," $S_2$ is mechanically complex, serving as the acoustic signature of semilunar valve closure. This paper explores the generation, splitting physiology, and pathological variations of $S_2$. Understanding the nuances of $S_2$—specifically the relative timing of the aortic ($A_2$) and pulmonic ($P_2$) components—provides critical bedside diagnostic information regarding systemic and pulmonary hemodynamics, valvular integrity, and intracardiac shunts.
If the split is —meaning the valves close in reverse order (P2 before A2) and the split narrows on inspiration instead of widening—that could murmur of left bundle branch block or aortic stenosis . The left ventricle struggles to finish its squeeze, so A2 arrives late.
S2 occurs at the beginning of isovolumetric ventricular relaxation. As the ventricles finish contracting, the pressure within them drops below the pressure in the great vessels (the aorta and pulmonary artery). This pressure gradient causes blood to flow back toward the heart, forcing the semilunar valves to snap shut.
The following essay explores the physiological mechanisms, audible components, and clinical significance of S2. 1. Mechanism and Physiology
The behavior of the $A_2$–$P_2$ interval is a powerful diagnostic tool.
Caused by the closure of the pulmonic valve. Due to lower pressures on the right side of the heart (approx. 10 mmHg), P2 is softer and is typically only heard at the left upper sternal border. Physiologic Splitting