Assessment of chest impedance in relation to phonocardiography

Weak sounds originating from the heart, coronary arteries or the lungs can be used to perform a noninvasive diagnosis of a certain diseases. However, the sounds of interest can be difficult or even impossible to pick up due to loss of signal when the sound is transmitted through the tissue and from the surface of the skin to the transducer. If the impedance of the skin is known, it may be possible to optimize the transducer to achieve an improved signal for a certain frequency range, while attenuating disturbing noise. Further, from a classical stethoscope it is known, that the sound picked up by the stethoscope can be influenced by changing the pressure on the chest piece of the stethoscope. A high pressure will stretch the skin similar to a drum skin, and attenuate lower frequencies, while lighter pressure will broaden the frequency range.

By using an impedance tube (also known as standing wave tube), it is possible to measure the impedance of the surface of the skin and at the same time investigate the influence of different pressures and diameters of a transducer. The impedance tube is made specifically with the purpose of measuring chest impedances in the frequency range from 50 Hz to 5 kHz. An MLS sequence is used as the excitation signal, and based on the measured impulse responses, the impedance of the surface is calculated. The diameters used for the setup are in the same range as diameters of normal stethoscopes, and the force applied to the tube ranges from close to zero to pressures in the range normally used for auscultation. The study involves the measurement of the chest impedance on several people at locations on the chest, where auscultation of the heart is normally carried out.

Knowledge of the chest impedance is aimed at use in optimal selection of transducer (e.g. microphone, force transducer or accelerometer), sensitivity and dynamic range, frequency range, coupling area, coupler geometry etc. for a system to pick up chest sounds. Especially the weak sounds of the heart, e.g. the murmur sounds originating from coronary arteries or the fetal heart sounds, are of interest.