Explain what is Third Heart Sound S₃ ?

S, is generated during the rapid filling pliase. It is a low frequency event. Mechanism of S, production: It is due to a sudden limitation of the rapidly expanding ventricle at the end of early rapid filling pliase. The continued rapid increase in blood volume in ventricle acts as a sudden distending force that causes the sound.

Physiological S₂ is rarely heart in normal subjects after 30 years age. Exaggerated flow across the mitral valve as in left to right shunts (VSD, PDA) or MR can cause physiological S₂. However, exaggerated flow across the tricuspid valve doesn't usually cause a RV S₂. This is because RV is more compliant than LV and hence expands easily to accommodate increased blood flow, Pathological S, is associated with high LA pressure and reduced LV compliance, most commonly in cardiomyopathies. RV S₃ can be heard in patients with high RA pressures and RV dilatation secondary to severe TR due to pulmonary hypertension or sudden RV out flow obstruction as in massive pulmonary embolism. S₃, is best heard at apex unless RVS, which is best heard in left parasternal area or early loud.S₃ due to constrictive pericarditis (Pericardial Knock), which is best, heard in left parasternal area.