Microprocessor-based system for measurement of electrical impedances during haemodialysis and in postoperative care

Abstract

The paper describes an instrument designed for accurate measurement of the mean specific electrical impedance of tissue. Two basic measurements are possible. One implementation called tetrapolar impedance plethysmography is employed for investigation with respect to transcellular fluid shifts during haemodialysis. The other measuring configuration called differential tetrapolar impedance plethysmography, deals with measurement of intrathoracic fluid content, which is important in monitoring treatment and prevention of lung oedema. An alternating current of 1 mA is passed through the tissue at different frequencies. Two measuring channels are available for differential tetrapolar measurement. Only one channel is required for straightforward tetrapolar measurement. Recovery of the impedance signals is achieved by synchronous demodulation. The impedance signal is passed through an analogue-to-digital convertor for digital processing to estimate the specific impedance in case of a differential tetrapolar measurement. Output display is possible via an xy recorder. The design of the instrument is discussed and some results shown. © 1988 IFMBE.