S3, the third heart sound in the cardiac cycle. Normally, it is audible only in children and physically active young adults and usually disappears with age. In older people, it is an abnormal finding and usually indicates myocardial failure. It is heard with the bell of a stethoscope placed lightly over the apex of the heart with the patient lying down and facing left. A weak, low-pitched, dull sound, it is thought to be caused by vibrations of the ventricle walls when they are suddenly distended by blood from the atria. Also called physiological third heart sound, ventricular gallop.
