Which of the following cardiac conditions does a fourth heart sound (S4) indicate?
A. Dilated aorta
B. Normally functioning heart
C. Decreased myocardial contractility
D. Failure of the ventricle to eject all the blood during systole
Correct Answer: D. Failure of the ventricle to eject all the blood during systole
An S4 occurs as a result of increased resistance to ventricular filling after atrial contraction. This increased resistance is related to decreased compliance of the ventricle.
Option A: A dilated aorta doesn’t cause an extra heart sound, though it does cause a murmur. The aorta is considered pathologically dilated if the diameters of the ascending aorta and the aortic root exceed the norms for a given age and body size. A 50% increase over the normal diameter is considered aneurysmal dilatation.
Option C: Decreased myocardial contractility is heard as a third heart sound. Optimal myocardial contractility is dependent on an optimal filling pressure, afterload, and the presence and availability of inotropic substances (eg, epinephrine, norepinephrine, or calcium). Calcium influx and binding to troponin C is essential for cardiac contraction.
Option B: An S4 isn’t heard in a normally functioning heart. The fourth heart sound is a low-pitched sound coincident with the late diastolic filling of the ventricle due to atrial contraction. It thus occurs shortly before the first heart sound. Although it is also called the atrial sound, and its production requires an effective atrial contraction, the fourth heart sound is the result of vibrations generated within the ventricle.