During the cardiac cycle, the most common site of occurrence of the third heart sound (S3) in healthy individuals is at which phase?

Explanation

## Timing and Origin of S3 **Key Point:** The third heart sound (S3) occurs in early diastole, specifically during the rapid ventricular filling phase, approximately 120–150 ms after the aortic component of the second heart sound (A2). ### Physiological Basis S3 is produced by: 1. Abrupt deceleration of blood flow as the ventricle reaches its elastic limit 2. Vibration of the ventricular wall and mitral apparatus 3. Occurs when the rate of ventricular filling is maximal **High-Yield:** S3 is a normal finding in children and young adults (< 30 years) due to high compliance and rapid early diastolic filling. It becomes pathological in older adults when associated with ventricular dysfunction or volume overload. ### Cardiac Cycle Phases | Phase | Timing | S3 Occurrence? | |-------|--------|----------------| | Early systole (isovolumetric contraction) | 0–40 ms | No | | **Early diastole (rapid filling)** | **120–150 ms after A2** | **Yes (S3)** | | Mid-to-late diastole | Slow filling phase | No | | Late diastole (atrial systole) | Produces S4 if abnormal | No | **Clinical Pearl:** A physiologic S3 is best heard at the apex with the patient in the left lateral decubitus position using the bell of the stethoscope. It is a low-frequency sound (20–50 Hz). **Mnemonic:** **S3 = Systole + 3** (occurs ~3 phases after S1: isovolumetric contraction → ejection → isovolumetric relaxation → rapid filling). ### Pathologic S3 (Gallop Rhythm) When S3 is audible in adults > 40 years, it indicates: - Ventricular dysfunction (systolic or diastolic) - Mitral regurgitation - Aortic regurgitation - High-output states (anemia, thyrotoxicosis, pregnancy)