Which heart sound is related to atrial contraction that is more forceful than normal?

• A. S1
• B. S2
• C. S3
• D. S4

Answer: (D)

S4 is generated by a strong atrial contraction that pushes blood into a ventricle that is stiff or noncompliant. This atrial kick happens late in diastole, just before the first heart sound, and it produces a low-pitched, late diastolic sound best heard with the bell at the apex in the left lateral position. It reflects diastolic dysfunction or left ventricular hypertrophy, where the ventricle resists filling and the atrium must contract harder to move blood forward. That explains why this sound is described as an atrial contraction more forceful than normal. S4 is different from S1 and S2, which originate from valve closures at other times, and from S3, which comes from rapid ventricular filling in early diastole rather than atrial contraction against a stiff ventricle.