Electrical admittance method for estimating fluid removal during artificial dialysis

Because the conductivity of blood changes remarkably during artificial dialysis, sometimes by more than 20%, changes in tissue admittance at low frequency are caused by changes not only in the extracellular fluid volume but also in blood conductivity. Therefore the changes in blood conductance due to artificial dialysis must be considered for the estimation of water removal by the admittance method. An accurate bio-impedance measurement system was developed. Measurement error was less than 1% at low frequency and 10% at high frequency. A new electrical bio-admittance method (NAM) was evaluated for the continuous measurement of removed fluid volume, using a three parallel-compartment tissue model, consisting of intracellular, interstitial and blood compartments, which takes into account the blood conductivity change. NAM used the equivalent conductivity calculated from the leg admittances of patients, measured at 1 min intervals during various artificial dialysis procedures. The actual amount of excess water removed by ultra-filtration agreed with the NAM-estimated amount within an error of 20%. NAM was also applied to estimate the intra- and extracellular fluid changes. The results were consistent with the physiological changes known to occur during the various forms of dialysis.