Effective Diffusion Volume Flow Rates (Q_e) for Urea, Creatinine, and Inorganic Phosphorous (Q_eu, Q_ecr, Q_eiP) during Hemodialysis

In vivo solute clearances can be estimated from dialyzer blood (Q _b) and dialysate (Q_d) flow rates and a solute- and dialyzer-specific overall permeability membrane area product (KoA). However, these calculations require knowledge of the flow rate of the effective solute distribution volume in the flowing bloodstream (Q_e) in order to calculate in vivo clearances and KoAs. We have determined Q_e for urea, creatinine, and inorganic phosphorus from changes in concentrations across the blood compartment and mass balance between the blood and dialysate streams. We made four serial measurements over one dialysis in 23 patients and found that Q_eu equals the total blood water flow rate, Q _ecr equals the plasma water flow rate plus 61% of red cell Water flow rate, and Q_eiP is limited to the plasma water flow rate. Equations are derived to calculate Q_e for each of these solutes from Q_b and hematocrit and in vivo KoAs for each solute were calculated.