Extra-cellular volume estimation by electrical impedance - phase measurement or curve fitting: a comparative study

In order to determine body fluid shifts between the intra- and extra-cellul ar spaces, multifrequency impedance measurement is performed. According to the Cole-Cole extrapolation, lumped values of intra- and extra-cellular con duction can be estimated which are commonly expressed in resistances R-i an d R-e respectively. For this purpose the magnitude and phase of the impedan ce under study are determined at a number of frequencies in the range betwe en 5 kHz and 1 MHz. An approach to determine intra- and extra-cellular conduction on the basis of Bode analysis is presented in this article. On this basis, estimation of the ratio between intra- and extracellular conduction could be performed b y phase measurement only, midrange in the bandwidth of interest. An importa nt feature is that the relation between intra- and extra-cellular conductio n can be continuously monitored by phase measurement and no curve fitting w hatsoever is required. Based on a two frequency measurement determining R-e at 4 kHZ and phi(max) at 64 kHz it proved possible to estimate extra-cellu lar volume (ECV) more accurately compared with the estimation based on extr apolation according to the Cole-Cole model in 26 patients. Reference values of ECV were determined by sodium bromide. The results show a correlation o f 0.90 with the reference method. The average error of ECV estimation was - 3.6% (SD 8.4), whereas the Cole-Cole extrapolation showed an error of 13.2% (SD 9.5). An important feature of the proposed approach is that the relati on between intra- and extra-cellular conduction can be continuously monitor ed by phase measurement and no curve fitting whatsoever is required.