The Third Heart Sound (S3) is a Clinical Sign of Systolic Heart Failure
The presence of an S3 is highly specific (90-94%) for reduced ejection fraction heart failure (HFrEF), though it has low sensitivity (23-51%), meaning its presence strongly suggests systolic dysfunction but its absence does not rule it out 1, 2.
Pathophysiologic Relationship
The S3 arises from rapid ventricular filling during early diastole when blood decelerates against a stiff, volume-overloaded ventricle. In systolic heart failure:
- Volume overload: The failing left ventricle operates at higher end-diastolic volumes
- Elevated filling pressures: Increased left ventricular end-diastolic pressure (LVEDP) drives rapid early filling
- Ventricular compliance changes: The dilated, dysfunctional ventricle creates conditions for audible vibrations during the rapid filling phase
Importantly, an S3 can be present as a manifestation of volume load and is not necessarily an indication of acute decompensation 3. This distinguishes it from other signs of congestion.
Diagnostic Performance
The S3 demonstrates strong predictive value when present:
- For LVEF <50%: Sensitivity 51%, specificity 90%, positive predictive value 95% 1
- For LVEF <30%: Sensitivity 78%, specificity 88% 1
- For elevated LVEDP and reduced EF: Specificity 92.7%, positive predictive value 90% 4
The correlation extends beyond systolic function—S3 amplitude correlates with diastolic dysfunction markers, specifically E-wave deceleration time (r=-0.86) 5.
Clinical Implications
When S3 is Present:
You can be confident (95% positive predictive value) that systolic dysfunction exists, typically with LVEF <40% 1. The patient warrants immediate initiation of guideline-directed medical therapy for HFrEF, including SGLT2 inhibitors, ACE inhibitors/ARNIs, beta-blockers, and mineralocorticoid receptor antagonists 6, 7.
When S3 is Absent:
Do not be falsely reassured—the negative predictive value is only 32% for LVEF <50% 1. Approximately 49-77% of patients with confirmed systolic dysfunction will not have an audible S3 1, 2. Objective measurement of ejection fraction via echocardiography remains mandatory for diagnosis 8, 7, 6.
Common Pitfall
The critical error is assuming absence of S3 excludes systolic heart failure. The diagnosis of HFrEF requires three conditions: typical symptoms, typical signs, AND reduced LVEF documented by imaging 8. The S3 is a sign that increases diagnostic confidence when present but cannot exclude disease when absent.
Prognostic Value
Beyond diagnosis, S3 carries prognostic significance. Increases in S3 strength between discharge and follow-up predict subsequent cardiac events (death or rehospitalization) in patients with acute decompensated heart failure 9. This suggests S3 monitoring may identify patients at higher risk for clinical deterioration.
The relationship between S3 and systolic failure is one of high specificity but low sensitivity—making it a "rule-in" rather than "rule-out" test for reduced ejection fraction heart failure.