The third heart sound is a low frequency heard after the second heart sound. It occurs during early diastole during the time of rapid ventricular filling. This sound occurs about 140 to 150 milliseconds after the second sound.
The S3 is often heard in normal children or young adults. But when heard in individuals over the age of 40, it usually reflects cardiac disease characterized by ventricular dilatation, decreased systolic function and elevated ventricular diastolic filling pressure.
The abnormal or pathological 3rd heart sound may be heard in individuals with coronary artery disease viz. cardiomyopathy, incompetent valves, left to right shunts, ventricle septal defect (VSD) or patent ductus arteriosis (PDA).
The pathological S3 may be the first clinical sign of congestive heart failure. The S3 may have its origin in either left or right heart. The S3 of left ventricular origin is best heard at the apex. The S3 of right ventricular origin is best heard at the left lateral sternal border.
Both S3 and S4 increase with exertion and become fainter with the patient sitting or standing. 3rd heart sounds are often associated with sharp outward precordial movements which frequently can be seen or felt.