Transmittion of Heart Sounds by Means of Auscultation

Abstract

The article presents the results of the study of the amplitude-frequency characteristics of the electron stethoscope / phonendoscope with a sound-conducting tube in the frequency range (50 - 400) Hz, which according to different estimates belongs to the spectrum of II tones of the human heart. Modeling is carried out taking into account mechanical impedance of the human chest area. In order to generalize the results, in contrast to the zone of registration of II tone, the area located in the fourth intercostal margin along the right middle clavicle line with known parameters was chosen. Mechanical impedance of the site, according to the measurement data, was interpreted by the system of sequentially connected elements with concentrated parameters: elasticity, friction resistance and vibrational mass. The friction resistance was determined by the minimum of the impedance modulus, the elasticity of the latter at low frequencies (50 Hz), and the vibrational mass at high frequencies (1000 Hz). The stethoscope/phonendoscope were modeled by elements with concentrated and distributed parameters. The stiffness of the phonendoscope membrane was determined by experimental measurements and the resonance frequency by calculations. The sound conducting tube was simulated by the system with distributed parameters. It was accepted that the mechanical impedance of the load of the sound-conducting tube in the frequency range under study had an elastic character, taking into account the frequency range of the microphone. At low frequencies, the input impedance of the phonendoscope prevailed over that of the stethoscope. For the rigidity of the microphone membrane of more than 103 N/m in the studied frequency range there are bands with a significant predominance of impedance of the human body or means of auscultation of heart sounds. In the first case, the heart's sound system operates in the preset pressure mode, while in the second case, it operates at an oscillatory speed. In the case of a preset pressure, the transmission functions of the stethoscope and the phonendoscope are different, while the preset oscillatory speed is similar. In the frequency bands in which the ausculators functioned at a given oscillatory speed, there was a decrease in sounds and a slight increase in the given pressure.