Fluid volumes determination by impedance spectroscopy and hematocrit monitoring: application to pediatric hemodialysis.

A method for extracting fluid volumes from multifrequency bioimpedance, which takes into account the body geometry and the presence of nonconducting elements, was tested on 12 young dialyzed patients against correlations for total body water volumes (TBW) from Watson et al. and Humes et al. Our calculations of TBW from impedance were found to overestimate Humes' values by 0.25 L (0.8%) postdialysis and by 2.08 L (6.5%) predialysis. Extracellular water (ECW) was found to contribute an average of 93% of ultrafiltered volume. Intracellular water volume (ICW) determination from impedance was found to be too imprecise to predict its variation during dialysis; therefore, ICW variations were calculated as the difference between ultrafiltration and ECW changes. The continuous recording of hematocrit by an optical device monitored changes in plasma and interstitial volumes. In most cases, ultrafiltration was compensated mainly by a contribution from interstitial fluid, and the drop in plasma volume was generally moderate.

Continuous measurements by impedance of haematocrit and plasma volume variations during dialysis.

A technique for continuous measurements of haematocrit and plasma volume in the arterial line of dialysed patients has been tested in vitro and in vivo. This method uses impedance measurements at 5 kHz and requires a single haematocrit measurement. It relies on two assumptions: that plasma resistivity does not change during dialysis and that blood resistivity obeys Hanai's model. Both assumptions are verified during in vitro tests. Haematocrits measured in vivo by this method are found to be in good agreement with direct measurements from blood samples. The haematocrit variation is then used to monitor changes in plasma volume, assuming conservation of erythrocyte volume. In addition, it is possible to obtain the variation in interstitial volume by combining these data with body impedance measurements.

Extra- and intracellular volume monitoring by impedance during haemodialysis using Cole-Cole extrapolation.

A method is presented for monitoring the relative variation of extracellular and intracellular fluid volumes using a multifrequency impedance meter and the Cole-Cole extrapolation technique. It is found that this extrapolation is necessary to obtain reliable data for the resistance of the intracellular fluid. The extracellular and intracellular resistances can be approached using frequencies of, respectively, 5 kHz and 1000 kHz, but the use of 100 kHz leads to unacceptable errors. In the conventional treatment the overall relative variation of intracellular resistance is found to be relatively small.

Extracellular and intracellular fluid volume monitoring during dialysis by multifrequency impedancemetry.

Impedance techniques can, in principle, permit non-invasive monitoring of extracellular (Ve) and intracellular (Vi) fluid volumes during dialysis. The authors present a method that determines the resistances Re and Ri of extracellular and intracellular compartments, respectively, by extrapolating impedance measurement toward zero, as well as infinite frequencies, according to the Cole-Cole model of biologic issues. These measurements were made using a XITRON 4000 B impedance meter (Xitron Technologies Inc., San Diego, CA) at frequencies ranging from 5 kHz to 1 MHz. The fact that the body is not a cylindric, homogeneous conductor is taken into account by introducing shape factors ke and ki and different resistivities pe and pi for the extracellular and intracellular fluid compartments. Assuming that these four unknown parameters can be regarded as constant during dialysis, the authors obtain: [formula: see text] where subscript o denotes initial values at start of dialysis. Impedances were measured at 30 min intervals on 11 pediatric patients and two adults, whereas total body water volume was determined by measuring urea in collected dialysate. Without ultrafiltration, Ve and Vi do not change significantly in percentage whereas, in the presence of ultrafiltration, Ve decreased by 15% to 25%. In cases when Vi does not change, it is possible to determine Ve and its variation during dialysis.

Urea, creatinine and phosphate kinetic modeling during dialysis: application to pediatric hemodialysis.

The kinetics of urea, creatinine and phosphate removal during dialysis were investigated in pediatric patients using a two-pool model taking into account fluid shifts and mass transfer between the two compartments. It is found that even urea must be described by a two-pool model since it presents a post dialysis rebound due to equilibration between the two compartments. Phosphate plasma concentration drops very sharply during the first hour of dialysis and rises rapidly during the rebound period. This pattern cannot be accounted for by the classical two-pool model with constant generation rate and mass transfer coefficients, but corresponds to a large time-dependent phosphate influx from the intracellular compartment in which phosphate is generated by biochemical reactions or liberated from the bones. This influx was calculated for four patients representing 8 dialysis sessions and was found to reach a plateau after 90 minutes of dialysis, dropping rapidly during the rebound period.