State space modeling of bioimpedance for haematocrit measurement

Abstract

Haematocrit are recorded during artificial dialysis resulting in remarkable changes in the blood conductivity that can reach up to 20%. Traditional methods for HCT-measurement like centrifugation or photometry are well known and precise. The drawback of the first method is the need for extracting a blood sample out of the closed loop system, which takes time and increases the cost. The latter method requires an optical window to access the blood and therefore it does require an additional precision fabricated disposable component. In order to eliminate the disadvantages of the mentioned HCT- measurement methods a new approach for blood bioimpedance modeling is presented.

The principles of functional identification of electrical impedance of biological tissues are considered. To determine the frequency characteristic of the electrical impedance the method of transient functions is proposed. Transient function of electrical impedance recorded as a response on unit step-current. The frequency characteristic is defined as the Carson—Heaviside transform of the electrical impedance transient function. Functional identification realized by Levy method.

For modeling the component analysis of a biological tissue the electrical impedance frequency characteristic is considered in the state space of parallel RC circuits, where the state variable has the meaning of electrical charge. The modeling of electrical impedance in blood samples with different haematocrit level is carried out. The dependences of the equivalent circuit parameters upon the haematocrit level are obtained. The obtained dependences of the impedance modulus characteristic frequency and the equivalent scheme parameters upon the haematocrit level may be used for the haematocrit level determination in a blood sample.