What is an S3 Heart Sound?
An S3 (third heart sound) is a low-pitched, early diastolic sound that occurs shortly after the second heart sound (S2) during rapid ventricular filling, caused by abrupt deceleration of blood flow into the ventricle and resulting vibrations of the cardiohemic system.
Timing and Characteristics
The S3 occurs in early to mid-diastole, specifically during the rapid filling phase of the ventricles 1, 2. It is:
- Low-pitched and best heard with the bell of the stethoscope
- Heard shortly after S2, creating a "gallop" rhythm (often described as sounding like "Ken-tuc-ky")
- Most audible at the cardiac apex in the left lateral decubitus position 1
Physiological Mechanism
The generation of S3 involves three key components 3, 4:
- Rapid deceleration of early mitral inflow - The abrupt slowing of blood entering the ventricle during the E-wave of diastolic filling
- Elevated left ventricular filling pressures - Increased pressure in the ventricle during diastole
- Altered ventricular compliance - Decreased ability of the ventricle to expand normally
The sound results from cardiohemic vibrations powered by the kinetic energy of rapid blood flow deceleration 5. When ventricular inflow velocity exceeds local wave velocity, it creates oscillations of the heart-blood system that become audible when amplitude and frequency are sufficient 6, 4.
Clinical Significance
Physiological S3 (Normal)
- Common in children, adolescents, and young adults up to age 40
- Present in most normal pregnant women due to increased blood volume and cardiac output 7, 8
- Reflects vigorous ventricular filling in a compliant ventricle
Pathological S3 (Abnormal)
An S3 in adults over 40 years strongly suggests left ventricular dysfunction and elevated filling pressures 1, 9. It indicates:
- Heart failure - May be the earliest clinical sign of left ventricular failure 9
- Volume overload states - Severe mitral regurgitation, tricuspid regurgitation, ventricular septal defects 10, 11
- Restrictive cardiomyopathy - Impaired ventricular compliance
- Dilated cardiomyopathy 11
Diagnostic Correlation
The presence of a pathological S3 correlates with 3:
- Lower ejection fraction (p=0.0006)
- Increased E/E' ratio on tissue Doppler imaging (p<0.0001) - reflecting elevated filling pressures
- Increased E-wave deceleration rate (p<0.0001)
- Elevated invasively measured LV filling pressures (p<0.0001)
Clinical Utility and Limitations
The S3 predicts high risk for postoperative complications in noncardiac surgery and indicates likely response to digoxin in heart failure patients 9. However, a critical caveat: interobserver variation in S3 detection is high, even among experienced clinicians 9, which limits its reliability as a sole diagnostic tool.
Right-Sided S3
A right ventricular S3 gallop can occur with right ventricular failure and is audible in approximately 23% of patients with pulmonary hypertension 10. It increases with inspiration due to increased venous return to the right heart.
Distinguishing from Other Sounds
Do not confuse S3 with:
- Fixed split S2 - Occurs earlier in diastole, at the time of S2
- Opening snap - Higher pitched, occurs slightly later than S3, associated with mitral stenosis
- S4 (fourth heart sound) - Occurs in late diastole just before S1, creating a "Ten-nes-see" gallop rhythm
The S3 creates a characteristic three-sound cadence in early diastole, while S4 occurs just before the first heart sound in late diastole 1.