A MATHEMATICAL-ANALYSIS OF A 2-COMPARTMENT MODEL OF UREA KINETICS

A model of urea distribution comprising well-mixed intracellular and e xtracellular compartments, with diffusive transfer of urea between com partments, is used to study blood urea variation during haemodialysis. Assuming that a typical adult patient (weight 70 kg and urea generati on rate 5 mg min(-1)) is dialysed using a high-efficiency dialyser (ur ea clearance 0.20.251 min(-1)) at an ultrafiltration rate of 10 ml min (-1), it is shown that a perturbation analysis can account for the eff ects of ultrafiltration and urea generation. This permits an analytica l solution of the equations which describe the variation in solute con centration in each of the compartments, which may be compared with the solution obtained when urea generation and ultrafiltration are neglec ted. For a typical adult patient with urea distribution volume in the range 25-401 undergoing high-efficiency haemodialysis, the analysis su ggests that the contribution of ultrafiltration to the variation in ur ea concentration during dialysis is similar in magnitude to the experi mental errors in measuring the blood concentration of urea and that a constant volume model will be sufficiently accurate to describe urea c learance in many patients.