Estimation of parameters in a two-pool urea kinetic model for hemodialysis.

A two-pool, variable volume urea kinetic model for estimation of solute removal in hemodialysis is solved analytically, and closed form expressions are presented for urea concentration in both compartments, both during dialysis and between dialyses. This approach also includes an estimation of the extent of the post dialysis rebound phenomenon of urea concentration. A method is presented to estimate values for the urea generation rate G, the distribution volume V and its partition in two compartments with volumes alpha 1V and alpha 2V (alpha 1-alpha 2-1), the total clearance K, and intercompartmental transfer coefficient X. To apply this analysis, several measurements are needed as input; the urea concentration at the end of a dialysis, the evolution of this concentration during the next dialysis, with at least four measurements including the initial and the final concentration, the volume of the dialysate, and its urea concentration. The main results are: the magnitude of the rebound is approximately proportional to alpha 2(2) K/X; the accuracy of the parameter estimation does not improve much further by taking more than six measurements during dialysis.