Application of cardiac impedance signal in the reservoir-wave model of circulatory system in humans

Reservoir-wave model assumes that aortic blood pressure curve (P_ao (t)) is a sum of two pressure components: first related to change in the blood volume within aorta(P_wk(t)), second related to axial velocity of blood along the aorta (P_ex(t)). If impedance cardiography (ICG) signal tracks changes of blood volume within segment of thorax, it can be used to estimate P_wk(t) noninvasively in humans. During late diastole, according to reservoir-wave model P_ao(t)= P_wk(t). Moreover, P_wk(t) in aorta and pressure in peripheral arteries should be similar. If these assumptions hold true, during late diastole, shapes of P_ao(t), of pressure in peripheral arteries, and of impedance cardiography signal should be exponential decay curves with the same time constant. Parameters of exponential decay curves of radial pressure obtained with Finapres and of cardiac impedance were compared in 4 subjects. Different locations of ICG sensing electrodes were tested We found that degree of similarity between ICG and Finapres signals depends on the location of ICG sensing electrodes. Motion artifacts and O wave distorted ICG curves and diminished the similarity between the two curves. Least effected and most similar to decaying portion of radial pressure curve was the impedance signal obtained with sensing electrodes localized upon the abdominal aorta, but the shape of two signals still considerably varied. Thus, we conclude that impedance signal can be used to determine the parameters of P_wk(t) curve in humans only if essential improvement in gaining and processing of this signal will be applied.