Genesis of the third heart sound. Phonoechocardiographic studies.

To clarify the genesis of the third heart sound (IIIS), phonoechocardiographic analysis was made in 31 cases with apparent IIIS. The mitral valve, either anterior or posterior, showed no definite movement coincided with the IIIS. At the time of IIIS, mitral valve is in the closing process, but it is still widely open, and there was no evidence of the stretch or tension of the valve apparatus. On the other hand, the endocardial echoes, either of interventricular septum or of posterior wall of the left ventricle, almost always showed definite check point (point of inflexion) during (or exactly at the end of) rapid ventricular filling as manifested by the sudden change in the diastolic dimension of the left ventricle near the apex. The IIIS seems likely to originate from the abruptly arrested momentum of the moving mass of blood at this check point and then the vibration of the relaxed cardiohemic system.