Repetition Training - Basic Heart Sounds Third Heart Sound Gallop Lesson Waveform Anatomy play pause Table of Contents 1Third Heart Sound Gallop 2Fourth Heart Sound Gallop 3Aortic Stenosis (Diamond Shaped Systolic Murmur) 4Aortic Regurgitation - Moderate 5Mitral Stenosis (Diastolic Murmur) 6Mitral Regurgitation 7Innocent Murmur Practice Quiz This video is an animation of Third Heart Sound Gallop A#401 (lesson# 1) within course number 1. A third heart sound occurs early in diastole. In young people and athletes it is a normal phenomenon. In older individuals it indicates the presence of congestive heart failure.The third heart sound is caused by a sudden deceleration of blood flow into the left ventricle from the left atrium. In the anatomy tab you will see a thin-walled, dilated left ventricle with generalized decreased vigor of contraction.In the presence of a third heart sound (S3) the first heart sound is decreased in intensity while the second heart sound is increased in intensity.The third heart sound is a low frequency sound best heard with the bell of the stethoscope pressed lightly on the skin of the chest.