The fourth heart sound occurs in late diastole just before the first heart sound. When the fourth heart sound is present, the first heart sound is decreased and the second heart sound is increased in intensity. The fourth heart sound is created by an increased stiffness of the left ventricle, often caused by 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. Use the 'Visual' tab to see our animation of this type of fourth heart sound. 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. A fourth heart sound is never heard with atrial fibrillation because the contraction of the atria is ineffective in this condition.