What is S3 in heart sounds?

• A. Closure of AV valves
• B. Closure of semilunar valves
• C. An abnormal heart sound
• D. Turbulent blood flow

Answer: (C)

S3 is the third heart sound, a low-pitched vibratory sound heard after the second heart sound during early diastole. It results from the sudden filling of a ventricle that is compliant enough to rapidly accept blood, causing the ventricular walls to vibrate. In children and young, healthy individuals, S3 can be a normal finding, but in adults it typically reflects increased filling pressures or a dilated, less compliant ventricle—often due to volume overload or heart failure—so it is generally described as an abnormal heart sound in that population. It’s best heard with the bell at the apex in the left lateral decubitus position, shortly after S2. This helps distinguish it from S1 (closure of the AV valves) and S2 (closure of the semilunar valves).