Repetition Training - Basic Heart Sounds Fourth 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 Fourth Heart Sound Gallop A#401 (lesson# 2) within course number 2. The fourth heart sound occurs in late diastole just prior to the first heart sound.The first heart sound is decreased in intensity and the second heart sound is increased in intensity.The fourth heart sound is produced by an increase in stiffness of the left ventricle due to scar tissue formation. This may be a manifestation of coronary heart disease. A fourth heart sound can also be caused by a greatly thickened left ventricular wall such as with essential hypertension or aortic stenosis. This is shown in the anatomy tab.A fourth heart sound is never heard with atrial fibrillation because the contraction of the atria is ineffective in this condition.The fourth heart sound is a low frequency sound best heard with the bell of the stethoscope pressed lightly on the skin of the chest.