Urea, sodium, and water changes in profiling dialysis

Abstract

Control of osmolarity, as well as sodium, urea, and water balance in anuric patients under intermittent haemodialysis therapy is fundamentally different from subjects with normal kidney function. Whereas in subjects with normal kidney function excretion of solutes and water is performed continuously, during dialysis, within a short period of time most sodium and only a small amount of urea are removed convectively with the ultrafiltration of water (1 litre of ultrafiltrate = 8.5 g sodium chloride and about 1.5 g urea). On the other hand most urea and a small amount of sodium is removed by diffusion, depending on the dialyser clearance and the difference in concentration between plasma and dialysis fluid. The relatively short period of treatment induces acute changes in fluid volume, urea, and sodium concentration as well as in plasma osmolarity, which in healthy subjects never happen. The consequence are side-effects of dialysis therapy such as osmotic dysequilibrium syndrome, muscle cramps, symptomatic hypotension, and thirst. Having no mechanism to excrete sodium during the long period between two dialyses, the anuric patient only has the possibility to dilute a high sodium concentration by drinking or to minimize sodium ingestion. Consequently the amount of fluid ingestion depends on the amount of sodium chloride ingestion by eating and probably on an increase in sodium concentration due to dialysis treatment itself. Among other methods profiling of ultrafiltration and sodium concentration in dialysis fluid has been proposed to optimize dialysis therapy. To evaluate the benefit of profiling to the patient the basic conditions of water and solute distribution and exchange in dialysis therapy have to be considered.