What are the differences between an S3 and an S4 heart sound, and how do they affect diagnosis and management?

S3 versus S4 Heart Sounds: Clinical Differentiation and Management

Fundamental Acoustic and Timing Differences

S3 is a mid-diastolic sound occurring during rapid ventricular filling, while S4 is a late-diastolic sound occurring during atrial contraction. 1

S3 Characteristics

• Occurs during the rapid filling phase of ventricular diastole, associated with elevated left ventricular filling pressures and volume overload 1
• Always occurs close to peak early filling velocity (E) during early flow deceleration 2
• Timing: After S2, during early diastole 3
• When heard over the mitral area, indicates left ventricular dysfunction; over the tricuspid area, suggests right ventricular dysfunction 4

S4 Characteristics

• Occurs during atrial contraction in late diastole, immediately before S1 1
• Most frequently found in patients with coronary heart disease and is a constant finding in hypertension 3
• Commonly associated with left ventricular hypertrophy 1
• Rather common in obstructive hypertrophic cardiomyopathy and valvular aortic stenosis 1

Bedside Differentiation Technique

• Use the "inching" technique: keep S2 in mind as reference and move the stethoscope from aortic area to apex 3
• If the extra sound occurs after S2, it is an S3 (ventricular gallop) 3
• S4 is eliminated with pressure on the stethoscope, but pressure does not eliminate ejection sounds or split S1 3
• A faint gallop may be detected more easily by listening over the xiphoid or epigastric area in patients with emphysematous chest 3

Clinical Significance and Prognostic Implications

S3: High-Risk Indicator

The presence of S3 is a reliable and highly specific indicator of cardiac decompensation that substantially increases perioperative risk. 1

• S3 is an independent predictor of perioperative complications when combined with history of heart failure, pulmonary edema, bilateral rales, or pulmonary vascular redistribution 1
• Phonocardiographic S3 has 92% specificity for elevated LVEDP, 87% specificity for reduced LVEF, and 92% specificity for elevated BNP 5
• However, sensitivity is limited: only 41% for elevated LVEDP, 52% for reduced LVEF, and 32% for elevated BNP 5
• In perimyocarditis, a new S3 indicates myocardial involvement 1

S4: Lower-Risk Marker

S4 does not independently predict heart failure and does not denote cardiac decompensation like S3. 1, 3

• Phonocardiographic S4 has moderate specificity: 80% for elevated LVEDP, 72% for reduced LVEF, and 78% for elevated BNP 5
• Sensitivity is also limited: 46% for elevated LVEDP, 43% for reduced LVEF, and 40% for elevated BNP 5
• S4 appears to be inferior to S3 as a marker of left ventricular dysfunction 5

Important Caveat: Physiologic S3

• Young healthy individuals with hyperdynamic circulation can 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 and cardiac output 1
• Both physiologic and pathologic S3 are related to abnormally rapid deceleration of early diastolic left ventricular inflow 2

Diagnostic Workup Algorithm

When S3 is Detected

Step 1: Determine if physiologic or pathologic

• Assess patient age, pregnancy status, and presence of hyperdynamic circulation 1
• Look for other signs of heart failure: jugular venous pressure elevation, pulmonary rales, peripheral edema, hepatomegaly 1

Step 2: Perform echocardiography

• Echocardiography is the first-line diagnostic test recommended by the American College of Cardiology 4
• Assess left ventricular systolic function and ejection fraction 1
• Measure E/E' ratio to evaluate LV filling pressures 1
• Assess for left ventricular hypertrophy 1

Step 3: Initiate treatment if pathologic

• Treat underlying heart failure with standard therapy including diuretics, ACE inhibitors/ARBs, beta-blockers, and aldosterone antagonists 1
• Patients with S3 require careful perioperative fluid management and hemodynamic monitoring 1

When S4 is Detected

Step 1: Assess for underlying conditions

• Evaluate for coronary heart disease, hypertension, aortic stenosis, or hypertrophic cardiomyopathy 1, 3

Step 2: Perform echocardiography

• Assess for left ventricular hypertrophy and diastolic function 1
• Evaluate for structural abnormalities 1

Step 3: Optimize blood pressure control

• Improved blood pressure management may lead to resolution of S4 1
• Address underlying condition to prevent progression to heart failure 1

Common Pitfalls to Avoid

• Do not confuse S4 with split first sound or ejection sound: use stethoscope pressure to differentiate 3
• Both S3 and S4 may be present simultaneously in cardiac decompensation associated with coronary heart disease, hypertensive heart disease, and dilated cardiomyopathy 3
• When S3 and S4 occur in close proximity, a short rumbling murmur may be heard, causing confusion with valvular or congenital lesions 3
• When both sounds occur exactly simultaneously, a summation gallop results, which can be misinterpreted as a valvular or congenital lesion 3
• Neither phonocardiographic S3 nor S4 is a sensitive marker of left ventricular dysfunction, so absence does not rule out cardiac pathology 5