Transthoracic Electrical Impedance Test Signal Generation and Potential Use of Optocoupler

Transthoracic electrical impedance plethysmography also referred to as impedance cardiography (ICG), is a non-invasive, easy in application, and inexpensive technology for hemodynamic parameters assessment and heart diseases diagnosis. Existing ICG simulators reproduce the ICG signal with a constant shape and frequency with no permission to set the heart rate. It complicates the testing procedure of new electrical impedance measurement systems and ICG signal processing methods. In this work, the problem of generating ICG signals reproducing different heart rates is considered. The experimental data analysis showed that the dependences of three ICG wave periods duration (before, during, and after the left ventricular ejection) on heart rate are very well approximated by exponential functions with coefficients of determination of 0.96, 0.97, and 0.97 respectively. The duration of each period was estimated using averaged ICG waves preprocessed by R-waves detection and Akima spline interpolation. As a way to obtain a smooth ICG signal, similar to a physiological one, it is proposed to use an optocoupler, consisting of a photodiode and a photoresistor in an opaque case. Since the dependence of the output resistance on the photodiode current is known, this will allow to generate a smooth ICG signal and easily realign it to different heart rate values.