Experimental and Numerical Study on Optimal Spot-electrodes Arrays in Transthoracic Electrical Impedance Cardiography

Transthoracic electrical impedance (or admittance) cardiography is a simple technique for the non-invasive and continuous monitoring of stroke volume or cardiac output by detecting the electrical impedance of a thorax which is roughly assumed to be a two-compartment coaxial cylindrical model composed of the aorta and its surrounding thoracic tissues. A tetra-polar band-electrodes method by Kubicek et al. has been widely used for the detection of the electrical impedance. However, this band-electrodes attachment makes a subject troublesome. Replacement of the band- to a spot-electrodes array is therefore highly required for practical use. In our previous reports, we have confirmed that a thorax is nearly assumed as an electrical cylinder model when a pair of current injection spot-electrodes are placed far from the thorax like a placement of the forehead - left medial knee or the mastoid process - lower right abdomen, and that the changes in current distributions associated with cardiac blood ejection are roughly homogeneous around the medial line of the thorax. The present study concerns with determination of an optimal spot-electrodes array for voltage pick-up through the detailed measurements of pulsatile components of the thoracic impedance along the medial line of the thorax using an 11times2 channels impedance mapping system. Additionally, we have investigated the influence of blood volume change in the heart itself by a finite element method. From these experimental and numerical investigations, it was suggested that the impedance change due to the blood volume change in the aorta without that in the heart can mainly be detected between the level of the clavicle and that of the xiphisternum. Consequently, an optimal electrodes-location for voltage pick-up was to be determined as the medial portion at the level of the clavicle and the portion above the xiphisternum.