What is an S3 (third heart) sound?

What is the S3 Heart Sound?

The S3 (third heart sound) is a low-pitched, mid-diastolic heart sound that occurs during the rapid ventricular filling phase of early diastole, caused by abrupt deceleration of blood flow into the ventricle. 1

Physiologic Mechanism

The S3 results from cardiohemic vibrations powered by rapid deceleration of transmitral blood flow during early diastolic filling. 2 Specifically:

• The sound occurs when blood rapidly decelerates as it enters the ventricle during the early filling phase, creating oscillations of the cardiohemic system. 2
• The timing is shortly after the peak of the early (E-wave) transmitral flow velocity. 2
• All hearts have these oscillations during E-wave deceleration, but the sound is only audible when there is sufficiently rapid fluid deceleration with adequate cardiohemic coupling. 2

Clinical Significance

Pathologic S3

In most adults, S3 indicates elevated left ventricular filling pressures and volume overload, making it a reliable and highly specific indicator of cardiac decompensation and heart failure. 1 The pathologic S3 is associated with:

• Increased deceleration rate of early mitral inflow 3
• Elevated LV filling pressures 3
• Abnormal myocardial compliance 3
• Decreased ejection fraction 3
• The ratio of early mitral inflow velocity to diastolic mitral annular velocity (E/E') is the most important echocardiographic determinant 3

The presence of S3 substantially increases perioperative risk and is an independent predictor of complications during noncardiac surgery, especially when combined with history of heart failure, pulmonary edema, bilateral rales, or pulmonary vascular redistribution. 1

Physiologic S3 (Normal Variants)

S3 can be a normal finding in specific populations:

• Young healthy individuals with hyperdynamic circulation may have S3 due to rapid early filling velocity without underlying cardiac disease. 1
• In pregnant women, S3 is present in most cases as a normal physiologic finding due to increased circulating blood volume (50% increase) and cardiac output, and does not indicate pathology. 4, 1

Physical Examination Technique

Auscultation Location and Characteristics

• Best heard at the cardiac apex with the bell of the stethoscope 1
• Low-pitched and low-amplitude sound, making detection challenging 1, 5
• Occurs in early diastole during rapid ventricular filling 1

Optimal Patient Positioning

The patient should be positioned in the left lateral decubitus position, which brings the cardiac apex closer to the chest wall—this is critical for detecting low-frequency, low-amplitude sounds like S3. 6 Additionally:

• The patient should be relaxed and quiet during auscultation to minimize artifact 6

Associated Physical Findings

When S3 is pathologic, look for accompanying signs of heart failure:

• Elevated jugular venous pressure 1
• Pulmonary rales 1
• Peripheral edema 1
• Hepatomegaly 1
• Third heart sounds are present in most normal pregnant patients 4

Common Pitfalls

Interobserver agreement for detecting S3 by auscultation is only moderate at best (kappa = 0.40-0.50) and can be as low as slight (kappa = 0.10-0.30), meaning clinicians frequently disagree about whether S3 is present. 7 This highlights the importance of:

• Using echocardiography to confirm clinical suspicion 1
• Not relying solely on auscultation for diagnosis
• Considering the clinical context (age, symptoms, other physical findings)

S3 may be misunderstood as an abnormal second heart sound with fixed split, which requires careful timing assessment—S3 occurs in early diastole during rapid filling, while split S2 occurs at the end of systole. 5

Diagnostic Workup When S3 is Detected

Echocardiography is recommended to assess ventricular function, as S3 correlates with systolic and diastolic dysfunction. 1 The echocardiographic evaluation should include:

• Assessment of left ventricular systolic function and ejection fraction 4
• Measurement of E/E' ratio (early mitral inflow velocity to diastolic mitral annular velocity) 3
• Evaluation of LV filling pressures 3
• Assessment for left ventricular hypertrophy 1

Consider cardiac MRI in selected cases to evaluate for infiltrative processes or myocardial fibrosis. 1

Management Implications

When pathologic S3 is present, treat the underlying heart failure with standard therapy including diuretics, ACE inhibitors/ARBs, beta-blockers, and aldosterone antagonists, as S3 is a reliable indicator of cardiac decompensation. 1

Patients with S3 require careful perioperative fluid management and hemodynamic monitoring due to substantially increased perioperative risk. 1