A modeling solution to the origin of the electrical impedance cardiogram

Impedance cardiography has been practiced as a noninvasive measure for stroke volume and cardiac output since first proposed in 1966. One reason that the method has not achieved universal acceptance is because the origin of the impedance waveform is not well understood. To investigate the impedance signal, the authors developed a Simulink(R) circulatory system model, with a first-derivative of a weighted sum of instantaneous volumes, from a lumped-parameter model. It was hypothesized that this variable is analogous to the dz/dt of the impedance cardiogram and will resemble the latter if the relative contribution of each lumped volume is correctly adjusted. The contribution of each volume was controlled through variable-gain amplifiers manipulated through a Matlab(R) Optimization Toolbox routine. A reference waveform, synthesized by averaging the impedance signals of five healthy subjects, was used as the standard against which the simulated waveforms were compared. The optimization program adjusted the amplifier gains to minimize differences between the simulated and standardized waveforms. The results indicate that the dz/dt waveform is not strongly related to volume changes in the ventricles nor the aorta, but results primarily from the volume changes representing the pulmonary venous circulation