Causes of the Fourth Heart Sound (S4)
The fourth heart sound (S4) is primarily caused by a stiff left ventricle with decreased compliance, most commonly due to conditions such as coronary artery disease, hypertension, hypertrophic cardiomyopathy, and other conditions that increase left ventricular end-diastolic stiffness. 1, 2
Pathophysiology of S4
The S4 is a low-frequency, presystolic sound that occurs during atrial contraction (late diastole) when blood is ejected into a stiff, non-compliant ventricle. This creates vibrations in the cardiohemic system that can be detected as an audible sound. 3
Key physiologic characteristics:
- Results from deceleration of transmitral blood flow during atrial systole
- Occurs when atrial systole leads to a disproportionately high rise in ventricular end-diastolic pressure
- Associated with enhanced atrial systolic blood filling volume or a stiff ventricular wall 3
Common Clinical Conditions Associated with S4
Cardiovascular Conditions
- Coronary artery disease - S4 is a constant finding in patients with coronary heart disease 4
- Hypertension - S4 is consistently present in patients with hypertensive heart disease 4
- Hypertrophic cardiomyopathy (HCM) - S4 is common in HCM patients 1, 5
- Aortic stenosis - S4 may be present in severe cases 1
- Left ventricular outflow tract obstruction - S4 is common in obstructive HCM 1
Other Conditions
- Aging - S4 may be present in older patients but is considered pathologic 2
- Dilated cardiomyopathy - May have both S3 and S4 gallops 4
Clinical Significance and Detection
The presence of an S4 is clinically significant as it indicates:
- Increased left ventricular end-diastolic stiffness 2
- Decreased ventricular compliance
- Potential underlying cardiac pathology
On physical examination:
- Best heard with the bell of the stethoscope at the apex
- Low-frequency sound occurring just before S1
- Can be eliminated with pressure on the stethoscope (unlike ejection sounds or split S1) 4
- May be more easily detected in patients with emphysema by listening over the xiphoid or epigastric area 4
Differentiating Features
Important distinctions:
- S4 vs. Split S1: S4 is eliminated with pressure on the stethoscope; split S1 is not 4
- S4 vs. Ejection Sound: S4 is eliminated with pressure; ejection sound is not 4
- S4 vs. S3: S4 occurs before S1 (presystolic); S3 occurs after S2 (early diastole) 4
- S4 indicates decreased ventricular compliance but not necessarily heart failure, whereas S3 typically denotes heart failure 4
Clinical Pearls
- The presence of S4 in younger patients is more likely to indicate pathology than in elderly patients
- When both S3 and S4 are present simultaneously, they may create a summation gallop that can be louder than S1 or S2 4
- The audibility of S4 correlates with its recorded amplitude, size, and palpability of the presystolic apical impulse 6
- S4 is more specific for cardiac disease when identified by experienced examiners 6
- Less experienced examiners may confuse splitting of S1 with S4 6
In summary, the S4 heart sound is a valuable clinical finding that indicates increased left ventricular stiffness and decreased compliance, most commonly associated with coronary artery disease, hypertension, and hypertrophic cardiomyopathy.