Define the third (S3) and fourth (S4) heart sounds, including their timing, mechanisms, and associated clinical conditions.

S3 and S4 Heart Sounds: Definitions and Clinical Significance

S3 Heart Sound (Third Heart Sound)

S3 is a low-pitched, mid-diastolic heart sound occurring during the rapid ventricular filling phase, immediately after S2, that indicates elevated left ventricular filling pressures and volume overload in adults. 1

Timing and Mechanism

• S3 occurs shortly after the early peak of transmitral flow (E-wave), during the rapid filling phase of early diastole 2, 3
• The sound is generated by abrupt deceleration of left ventricular inflow, causing cardiohemic vibrations when blood rapidly decelerates against a stiff or volume-overloaded ventricle 2, 3
• The mechanism involves forced oscillations of the cardiohemic system powered by rapid deceleration of transmitral blood flow 3

Pathophysiologic vs. Physiologic S3

Pathologic S3:

• In adults, S3 is a reliable and highly specific indicator of cardiac decompensation and heart failure, associated with decreased ejection fraction 1
• The presence of S3 substantially increases perioperative risk and is an independent predictor of complications during noncardiac surgery 1
• Patients with pathologic S3 demonstrate elevated E/E' ratio on echocardiography, increased LV filling pressures, and abnormal myocardial compliance 2

Physiologic S3 (Normal Variants):

• 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
• The prevalence of S3 in asymptomatic adults is approximately 10%, with higher prevalence in younger individuals 4

Associated Clinical Conditions

• Heart failure with reduced ejection fraction (most common pathologic cause) 1
• Cardiac decompensation with elevated left ventricular filling pressures 1
• Volume overload states 1
• In perimyocarditis, a new S3 indicates myocardial involvement 1

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S4 Heart Sound (Fourth Heart Sound)

S4 is a low-pitched, late-diastolic heart sound occurring during atrial contraction (presystolic), immediately before S1, that reflects decreased ventricular compliance and is most frequently found in patients with coronary heart disease and hypertension. 1, 5

Timing and Mechanism

• S4 occurs during the late diastolic filling phase that follows atrial contraction, immediately before S1 5
• The sound is generated when the atrium contracts forcefully against a stiff, non-compliant ventricle 1
• S4 requires the presence of atrial contraction and therefore cannot occur in atrial fibrillation 5

Clinical Significance

• S4 is most frequently found in patients with coronary heart disease 1, 5
• S4 is a constant finding in patients with hypertension and reflects left ventricular hypertrophy 5
• S4 does not indicate heart failure, unlike S3 1, 5
• S4 does not independently predict heart failure development but may signal increased risk if the underlying condition is not addressed 1

Associated Clinical Conditions

• Left ventricular hypertrophy (from hypertension or aortic stenosis) 1
• Coronary artery disease and myocardial ischemia 1, 5
• Hypertrophic cardiomyopathy 1
• Severe aortic stenosis 6
• The prevalence of S4 in asymptomatic adults is approximately 15.6%, with increasing prevalence with age 4

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Distinguishing S3 from S4

Bedside Differentiation Techniques

• Pressure technique: S4 is eliminated with firm pressure on the stethoscope, while split S1 and ejection sounds are not 5
• Timing with carotid pulse: S3 occurs well after the carotid upstroke (mid-diastole), while S4 occurs just before the carotid upstroke (presystolic) 5
• "Inching" technique: Move the stethoscope from aortic area to apex while keeping S2 as a reference point; sounds occurring after S2 are diastolic (S3 or S4) 5

Combined Gallops

• Both S3 and S4 may be present simultaneously in patients with cardiac decompensation associated with coronary heart disease, hypertensive heart disease, and dilated cardiomyopathy 5
• When S3 and S4 occur in close proximity, they may create a short rumbling murmur that can be confused with valvular lesions 5
• Summation gallop: When S3 and S4 occur exactly simultaneously (typically with tachycardia), they create a single loud sound that can be misinterpreted as a valvular or congenital lesion 5

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Diagnostic Workup When S3 or S4 is Detected

For S3

• Echocardiography is mandatory to assess left ventricular systolic function, ejection fraction, E/E' ratio, and LV filling pressures 1
• Monitor for signs of heart failure including jugular venous pressure elevation, pulmonary rales, peripheral edema, and hepatomegaly 1
• Initiate standard heart failure therapy including diuretics, ACE inhibitors/ARBs, beta-blockers, and aldosterone antagonists 1

For S4

• Echocardiography to assess for left ventricular hypertrophy and diastolic function 1
• Monitor blood pressure control, as improved management may lead to resolution of S4 1
• Consider cardiac MRI in selected cases to evaluate for infiltrative processes or myocardial fibrosis 1

Common Pitfalls

• Do not dismiss S4 in elderly patients as "normal aging"—even though prevalence increases with age, detection should prompt evaluation for underlying cardiac disease 4
• Do not confuse S3 with split S1 or opening snap—use the inching technique and timing relative to S2 for accurate identification 5
• In patients with emphysematous chest or increased anteroposterior diameter, listen over the xiphoid or epigastric area where gallops may be more easily detected 5