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 video.
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.