ANALYTIC SOLUTION OF THE VARIABLE-VOLUME DOUBLE-POOL UREA KINETICS MODEL APPLIED TO PARAMETER-ESTIMATION IN HEMODIALYSIS

An analytic solution of the Variable-Volume Double-Pool urea kinetics model and its application to the estimation of clinically relevant par ameters of the patient-machine system, are presented. These include th e urea distribution volume and generation rate and the mean dialyzer c learance. The estimation of these parameters is based on the assumptio n of constant values for the diffusion coefficient between the two poo ls and the intra-extracellular volume ratio. Results obtained by compu ter simulations show that the effect of a +/-50% variation of these pa rameters influences the estimates less than standard measurement error s. Starting from these results, four methods to in vivo estimate the u rea distribution volume and generation rate from blood samples are com pared. Two methods are based on the analytic solution of the double-po ol model using seven samples (reference method) or three samples (new clinical method). The remaining methods are based on urea mass-balance and are largely used in the clinical practice. These last techniques differ from each other for the blood sample taken at the end of the tr eatment or 30 min later. The results obtained from hemofiltration sess ions show that the urea generation rate is accurately estimated by all methods. The total distribution volume is still accurately estimated by the new clinical method while it is systematically underestimated b y the urea mass-balance when the blood sample at the end of dialysis i s used. Instead, a high overcompensation results using the blood sampl e taken 30 min after the end of dialysis. Finally, the new clinical me thod also provides reliable estimates for the dialyzer clearance start ing from only three blood samples all taken during dialysis.