Theoretical aspects of various ultrafiltration methods in artificial kidney therapy

A mathematical model including urea, creatinine and other osmotically important solutes (such as sodium, potassium and chloride) is applied to calculate volume shifts, caused by ultrafiltration, between the fluid compartments of the body. The volume shifts between the intracellular (ICV) and the extracellular (ECV) compartments are mainly caused by alteration of extracellular sodium concentration. Various methods of achieving ultrafiltration, including conventional dialysis, initial ultrafiltration using Cuprophan (without dialysis) or hemofiltration, produce different responses. In choosing a method, one must consider that both a rapid decrease of ECV and a fast shift of water from ICV to ECV should be avoided. In pure hemofiltration, ultrafiltrate is isotonic and water is removed from ECV only. Hemofiltration with dilution produces a very slow shift of water between ICV and ECV dependent on sodium concentration of plasma and diluting fluid. In initial ultrafiltration through Cuprophan, water is shifted from ICV to ECV. With ultrafiltration throughout the entire dialysis, there are pronounced shifts between ICV and ECV dependent on the difference of the sodium concentration between plasma and dialysate.