The third heart sound.

A unitary concept is proposed to explain the genesis of the third heart sound and associated "rapid filling wave" of the apexcardiogram in physiological and pathological states including constrictive pericarditis. This theory not only clarifies the hitherto unexplained phenomenon such as presence of S3 in significant mitral stenosis, but also places in proper perspective the hemodynamic significance of S3 in physiological and various pathological states.