second heart sound
The second heart sound is the dub in lub-dub. It is the crisper, higher-pitched, slightly shorter sound that follows the first, marking the close of each contraction.
It is produced when the two outlet valves — the aortic and pulmonary valves — snap shut at the end of systole, as the ventricles relax and pressure inside them falls below the pressure in the great arteries. The recoil of blood against the closing valves sets up the vibration.
S2 marks the start of diastole, so it is the bookend to S1. The aortic component normally closes a touch before the pulmonary; with a breath in, the split widens, a normal finding called physiological splitting that doctors listen for.
Abnormal S2 patterns are clinically telling. A widely fixed split can point to an atrial septal defect, a loud aortic or pulmonary component can suggest high pressure in the aorta or lungs, and a soft or single S2 can occur with stiff, calcified valves.
Listening at the upper left sternal border while the patient breathes in, a clinician can hear S2 split into its aortic and pulmonary components.
Physiological splitting of S2 is best appreciated during inspiration.
S2 has two components: aortic closure (A2) and pulmonary closure (P2). Normal splitting widens on inspiration; a fixed split that does not vary with breathing is abnormal.