The fourth heart sound is caused by
A fourth sound can sometimes be heard immediately before the first sound when atrial pressure is high or the ventricle is stiff in conditions such as ventricular hypertrophy. It is due to ventricular filling and is rarely heard in normal adults.
Closure of the aortic and pulmonary valves.
Vibrations in the ventricular wall during systole.
Ventricular filling.
Closure of the mitral and tricuspid valves.