Modeling of the Transfer Function of the Heart-Thorax Acoustic System in Dogs

A computer model capable of reflecting the dynamic bevior of the heart-thorax acoustic system is used to study the transmission of the first and second heart sounds originating within the left ventricle through the heart muscle and the thoracic tissues of dogs. The input signal of the model is the phonocardiogram recorded within the left ventricle, while the output signal is the phonocardiogram recorded on the chest wall over the apex of the heart. Changes in the transmission path characteritics are modeled by varying the frequency response of an equivalent acoustic system throughout the cardiac cycle. Experimental measurements in dogs were used to determine the general characteristics of left ventricular and apical first and second heart sound spectra, and from these, the transfer and coherence functions of the heart-thorax acoustic system. Results show that the transfer function of the heart-thorax acoustic system changes during the cardiac cycle. For the seven animals studied, it would appear that the contribution of left ventricular first and second sounds to the apical phonocardiogram is significant for frequencies below 70 Hz and negligible for frequencies above 250 Hz. In addition, it is shown that 80 percent of the power of sounds recorded on the chest wall arises from linear transmission of sounds recorded within the left ventricle through the heart-thorax acoustic system. The other 20 percent is due to thoracic noise, noncoherent cardiac contributions, and nonlinearity of the acoustic system.