SOLUTE CLEARANCE DURING CRRT: ANALYSIS WITH A FINITE ELEMENT MODEL

Abstract

We developed a finitite element model (FEM) of a hollow fiber to simulate CVVHD, including blood and dialysate flow, ultrafiltration, internal filtration/backfiltration (IF/BF), convection-diffusion interaction, and concentration polarization. We simulated conditions of postdilution CVVHD for urea and vitamin B12 flux in a AN69 hollow fiber dialyzer. We conducted the following analyses: 1) Determination of the total solute removal when substitution/dialysate (S/D) fluid was varied from pure hemofiltration to pure hemodialysis, 2) comparison of CVVH and CVVD to evaluate efficiency of convective vs. diffusive flux, and 3) evaluate the contribution of IF/BF. The efficiency in urea removal was inversely related to the ratio of dialysate to substitution fluid. Clearance during CVVD was less than CVVH with increasing total S/D infusion, thus CVVH is more efficient than CVVD at the same flow rates. Hemofiltration with 6 L/min substitution fluid in CVVH resulted in 51% more total clearance than 6 L/min dialysate in CVVD. At 1 L/min, however, CVVH only afforded a 4% increase in total clearance. Under zero net ultrafiltration, IF/BF accounted for only 0.45% and 0.47% of total clearance for urea and B12 despite differences in diffusivity and sieving coefficients. FEM allows evaluation of CRRT performance, including quantitation of convective and diffusive fluxes, and flux during IF/BF. Our findings suggest that CVVH is considerably more efficient than CVVHD at high but not traditional volumes. Clearance from IF/BF contributes less than 0.5% of total clearance under traditional dialyzer design.