60. What causes the third heart sound (S3)?

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 (LV).

If the left ventricle is not overly compliant (as 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.

S3_heart_sound

A S3 can be an important sign of systolic heart failure, since in this setting the myocardium is usually overly compliant resulting in a dilated LV (see image below).

S3 is a low pitched sound. This is helpful to distinguish 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 while 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.