A multifrequency multichannel electrical impedance data acquisition system for body fluid shift monitoring.

This paper discusses some important issues for the design of electrical impedance measurement systems intended for body fluid shift monitoring, in particular during dialysis treatments. We have studied two common signal generation systems: digital synthesis and carrier recovery. We have found that in prolonged measurement applications, digital synthesis yields the best performance. On the demodulation side, we balance the demodulator errors between the real and imaginary parts by rotating the demodulation axes. We use segmental multifrequency impedance measurements to estimate the values of intracellular and extracellular impedance by adjusting the parameters of a Cole-Cole model for each segment measured. We stress the need to perform segmental measurements in order to accurately measure the segments of interest, in particular the trunk during dialysis treatments. Our results show that there is a sharp disequilibrium between the intracellular and extracellular compartments in the very first dialysis period. This fact generates the need to continuously measure segmental impedance instead of comparing initial and final values.