Dose of dialysis predicted by continuous measurement of dialysate urea concentration

On-line estimation of urea kinetic parameters from dialysate urea concentration (DUN) is proposed and applied to dialysis dose prediction during therapy. Continuous measurement of DUN was performed by a post-filter enzymatic sensor (Hospal Dasco Spa) on six patients undergoing standard haemodialysis. In order to allow for early and reliable prediction of the end of dialysis values, a variable-volume double-pool model is used since the classical first order model cannot account for the fast-to-slow dynamics occurring during the first hour. Three parameter combinations are estimated at each time on the basis of the past DUN history: K_d/V₀, K_dC₀ and VR, where K_d, V_o, C_o, and VR are the dialyzer clearance, the total urea distribution volume, the initial plasma concentration, and the intra-extracellular volume ratio, respectively. Residuals between model-predicted and measured data have an average root mean square error of about 3% of the DUN excursion and model parameters show realistic values. The final dose of dialysis is predicted by multiplying K_d/V_o for the scheduled duration of the dialysis session. Prediction after 70 min from the beginning of the session differs on the average by less than 5% from the final value thus demonstrating the applicability of the proposed method. Furthermore, knowledge of model parameters allows for early and accurate prediction of the total urea removed