RELATION OF THE FIRST AND SECOND HEART SOUNDS TO EVENTS IN THE CARDIAC CYCLE

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Abstract
In order to study the nature of the first and second heart sounds and to relate their components to events in the cardiac cycle, medium frequency phonocardiograms were taken synchronously with border electrokymograms, lead II ecgs and apexcardiograms. The two major components of the first heart sound, which give the impression of splitting on auscultation, are separated by an average interval of 0.03 seconds and are of greater amplitude and contain more high frequency vibrations than other components. Since the onset of the first major component was correlated with the onset of left ventricular isometric contraction and the time of mitral valve closure and the second major component was synchronous with the onset of isometric right ventricular systole and tricuspid valve closure, it was concluded that each of these components are due to closure of the mitral and tricuspid valves respectively. The relationship of auricular contraction and P-R interval to the amplitude of the sound components support this conclusion. Comparison of the aortic and pulmonary dicrotic notches on the electrokymogram with the two components of the split second heart sound lead to the conclusion that the first is due to aortic valve closure and the second to pulmonary valve closure. They are separated by an average interval of 0.05 seconds in mid-respiration and this increases during inspiration.