What does the S2 heart sound signify?

The S2 heart sound is primarily associated with the closing of the semilunar valves, which consist of the aortic and pulmonic valves. This sound occurs at the end of ventricular systole when the ventricles have finished ejecting blood into the aorta and pulmonary artery. As the pressure in the ventricles drops below that in the aorta and pulmonary artery, the semilunar valves close, producing the characteristic "dub" of the heartbeat, which is what the S2 sound represents.

Understanding the mechanics of the cardiac cycle helps clarify why this sound is particularly significant. The separation of the aortic and pulmonic components of the S2 can also provide insights into the physiological state of the heart, especially in situations like right or left bundle branch blocks or during inspiration, which can cause a phenomenon known as physiological splitting of S2.

While the closing of the AV valves pertains to the S1 heart sound and can be associated with mechanical issues like stenosis or regurgitation, those issues relate to different clinical contexts not encapsulated by the S2 sound. Similarly, heart murmurs are abnormal sounds caused by turbulent blood flow, which do not define the normal physiological sounds attributed to the closing of the valves.