Which statement best describes the pathophysiology behind an S3 sound in heart failure?

• A. Increased afterload causes early systolic murmur.
• B. Left ventricle becomes stiff and interferes with filling.
• C. Right atrial pressure is decreased.
• D. Pulmonary edema is always present.

Answer: (B)

An S3 in heart failure reflects rapid early diastolic filling of a ventricle that is volume-overloaded and receiving blood at high pressures. After the aortic valve closes, a large amount of blood quickly flows from the left atrium into a dilated, compliant ventricle. The sudden deceleration and tapping of this rapid filling causes a low-pitched sound heard best at the apex with the patient in the left lateral position. This is typical of systolic dysfunction with volume overload and increased LV end-diastolic pressure.

This helps distinguish it from S4, which comes from a stiff, noncompliant ventricle and occurs with atrial kick late in diastole. S3 is a ventricular gallop tied to volume overload, whereas S4 points to reduced ventricular compliance. Also remember that pulmonary edema isn’t required for an S3 to be present, and the other statements don’t describe the mechanism of the S3 sound.