On the Function of the Aortic Valve and the Mechanism of the First and Second Sounds

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Abstract
The exact timing of the aortic component of the 2nd sound in relationship to aortic pressure and flow velocity, and to aortic valve closure was reported. These studies were performed in dogs by means of catheter-tip probes and an electric contact introduced into the aortic valve. They were supplemented by ECG studies of the aortic valve, and impedance cardiograms in man. Opening of the aortic valve and its relationship to the 1st heart sound were studied. The 2nd component of the 1st heart sound grossly coincides with the opening of the aortic valve. The aortic component of the 2nd sound starts a few milliseconds after aortic valve closure. It starts after the incisura of the aortic pressure tracing and the drop of the velocity curve to the zero line; it then increases attaining its maximum at the peak of the rapid rebound of the aortic pressure, which coincides with the trough of the velocity tracing. The final interpretation on the mechanism of the second sound agrees in principle with studies attributing the sound vibrations to release of the energy stored in the aortic wall, which accelerates the flow and causes deceleration of numerous structures (aortic and valvar) as well as of the blood. Studies of the rate of change of acceleration of pressure confirm this interpretation.

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