Spectral analysis and acoustic transmission of mitral and aortic valve closure sounds in dogs. Part 1. Modelling the heart/thorax acoustic system.

A system model based on the simultaneous recording and analysis of the intracardiac and thoracic phonocardiograms to estimate the time-varying properties of the heart/thorax acoustic system of the dog is described. The presence of instrumental noise in the recording of intracardiac phonocardiograms is characterised, and it is demonstrated that its effect on the estimate of the transfer and coherence functions of the system can be quantified and corrected. Application of the model to study the spectral characteristics and the acoustic transmission properties of the mitral component M1 of the first heart sound and of the aortic component A2 of the second heart sound in the dog shows that the heart/thorax acoustic system acts like a bandpass filter having a higher attenuation for A2 than for M1. Between 20 and 100 Hz, the mean attenuation of M1 is 30 dB while that of A2 is 46 dB. Above 100 Hz, the attenuation slope is -12 dB per octave for M1 and -6 dB per octave for A2.