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

Abstract

The chief value of the recorded heart sound is the possibility of accurately timing its occurrence in relation to the events of the cardiac cycle." This is even truer to-day than when Lewis first stated it (1915), because methods for recording events in the cardiac cycle have become more precise. In spite of this, there is still lack of agreement on the origin of the heart sounds. The causes of the second heart sound have been clearly defined, but there is a divergence of opinion on the origin of the major components of the first heart sound. On auscultation in young people the first sound may be heard to be narrowly split at the lower left sternal border. This splitting is widest and therefore most easily heard at the end of expiration (Potain, 1866). The origin of these two components has been attributed by some to closure of the auriculo-ventricular valves followed by opening of the semilunar valves (Orias and Braun-Menendez, 1939; Rappaport and Sprague, 1942; Luisada et al., 1949; Nazzi et al., 1954). Others have suggested that it is due to asynchronism in contraction of the ventricles (Wolferth and Margolies, 1945), and so to closure of the mitral valve followed by closure of the tricuspid valve (Leatham, 1954). Much of the evidence for these views has been obtained by relating the components of the heart sounds to dynamic events in the cardiac cycle by the use of reference tracings. Phlebograms and indirect carotid artery tracings have been mainly used for this purpose but these have the disadvantage that an arbitrary time interval has to be allowed for pulse wave transmission when assessing results.