Third Heart Sound (S3)
The S3 is a low frequency sound, generally heard from 120-160 milliseconds after the aortic valve closure sound. In order to find an S3, place the patient in the left lateral decubitus position and identify the apical impulse. Place the bell lightly against the skin surface over the palpated apical impulse.
In childhood and physically active young adults, the third heart sound is felt to arise from vigorous elastic recoil of the left ventricle in early diastole, causing the heart to impact the chest wall. Here, the S3 is a normal finding.
In adults, the third heart sound is a physical sign specific to left ventricular failure. It is heard predominantly in dilated hypo contractile left ventricles, but may also be heard in the setting of severe mitral regurgitation.
In left ventricular failure, the origin of the third heart sound is thought to be the sudden cessation of early, rapid left ventricular filling impelled by high left atrial pressure. This low frequency sound approaches the limits of human hearing and may not radiate widely from the apical impulse.
When uncertain about the presence of an S3, a brief period of exercise performed by the patient will augment the amplitude, making detection easier.
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Updated October 2009