The third heart sound (S3), also known as the “ventricular gallop,” occurs just after S2 when the mitral valve opens, allowing passive filling of the left ventricle. The S3 sound is actually produced by the large amount of blood striking a very compliant left ventricle.
CLINICAL PEARL: A S3 heart sound is produced during passive left ventricular filling when blood strikes a compliant LV.
If the LV is not overly compliant, as is in most adults, a S3 will not be loud enough to be auscultated. A S3 can be a normal finding in children, pregnant females and well-trained athletes; however, a S4 heart sound is almost always abnormal.
CLINICAL PEARL: A S3 heart sound is often a sign of systolic heart failure, however it may sometimes be a normal finding.
A S3 can be an important sign of systolic heart failure because, in this setting, the myocardium is usually overly compliant, resulting in a dilated LV; this can be seen in the image below.
Normal LV vs. Dilated LV (S3 Present)
S3 is a low-pitched sound; this is helpful in distinguishing a S3 from a split S2, which is high pitched. A S3 heart sound should disappear when the diaphragm of the stethoscope is used and should be present while using the bell; the opposite is true for a split S2. Also, the S3 sound is heard best at the cardiac apex, whereas a split S2 is best heard at the pulmonic listening post (left upper sternal border). To best hear a S3, the patient should be in the left lateral decubitus position.