Relations between components of impedance cardiogram analyzed by means of finite element model and sensitivity theorem.

ABSTRACT

The main aim of the study is to establish a relation between different sources of Impedance cardiogram (ICG) as function of spatial distribution of conductivity. A three-dimensional model of a human thorax using the finite element method has been constructed. The model includes 35 horizontal layers consisting of up to 519 pentahedral elements that are automatically divided into tetrahedral ones before calculating the potential distribution. Electrode array configuration proposed by Kubicek et al. (Aerosp. Med. 37 1208-1212, 1996) has been studied. A relationship proposed by Geselowitz (IEEE Trans. Biomed. Eng. 18 38-41, 1971) has been used to calculate the sensitivity of the examined electrode array to conductivity changes inside the thorax. This relationship has allowed for the calculation of the contributions to ICG from spatially separated sources when modeling all changes in conductivity simultaneously. It has been confirmed that the main contributions to ICG signal come from ventricles, atria, aorta, and lungs. The relations between these components have been found to be dependent nonlinearly on spatial conductivity distribution. As a result, reliable and reproducible measurements of stroke volume (SV) using ICG are impossible. Nevertheless, ICG can be used to monitor relative changes of SV in all cases where the spatial distribution of conductivity and geometry of the subject, during the examination, is preserved. However, it does not mean that the accuracy of SV measurement will be the same in all these cases.