The first heart sound during the isovolumetric contraction

We make the first attempt to construct a qualitative theory covering the whole process of the major part of the first heart sound from an electrical activation to the phonocardiographic observations at the thorax. We calculate the amplitudes and frequencies of the radiated pressures during the isovolumetric contraction period generated by the muscular wall of the left ventricle and by the valves considered as a spherical shell and two-dimensional membranes, respectively. The analysis shows that both the hemodynamic and the valvular theory are able to explain most of the characteristic features of the first heart sound (linear relation between the amplitudes of the radiated pressure and the slope of the left ventricular pressure-time curve; directional polarity of the amplitudes; equidistant frequency peaks with a decline in amplitudes). However, existing magnitudes of the set of physiological parameters involved seems to favour the hemodynamic theory of the first heart sound. The aortic valve can be neglected as a source of sound. The initial conditions (like valve closure velocity), according to our theory, cannot be important. The predicted time-plot and frequency spectrum of the radiated pressure show a general resemblance with the recorded ones. It is essential to have considerably more quantitative acoustic data both for normal and diseased hearts for subsequent theoretical development.