Factors responsible for normal splitting of first heart sound. High-speed echophonocardiographic study of valve movement.

Echocardiographic studies have confirmed that normal splitting of the first heart sound is a result of mitral valve closure preceding tricuspid valve closure. This fits in with contraction of the left ventricle before the right, but the width of splitting is usually greater than would be expected from the known ventricular asynchrony. The importance of atrial systole, which initiates closure of the atrioventricular valves, has not been considered. In this study of 67 normal subjects, high-speed echophonocardiograms were recorded and the timing of mitral and tricuspid valve closure was related to the PR interval and the beginning of left and right ventricular ejection. The timing of mitral valve closure was strongly influenced by the PR interval; in patients with a long PR interval the valve leaflets were nearly apposed by atrial systole and closure was completed early after the Q wave on the electrocardiogram, while with a short PR interval the leaflets were still widely separated at the time of ventricular activation and closure was completed later. The timing of tricuspid closure was affected similarly but less strongly, and even when the PR interval was so long that mitral closure was complete before the onset of left ventricular systole, tricuspid closure occurred up to 48 ms later. The mean interval between mitral and tricuspid closure was 34 ms. Thus mitral closure was effected nearer the onset of left ventricular systole than was tricuspid closure in relation to the onset of right ventricular systole, and this accounts for the surprising width of splitting of the first sound. Variations in splitting of the first sound were mainly the result of variations in the timing of tricuspid rather than of mitral closure. When tricuspid closure was delayed, so was pulmonary valve opening, suggesting that the cause was delay in the onset of right ventricular systole or a slower rate of pressure development in the right ventricle.