Effect of increasing dialysate flow rate on diffusive mass transfer of urea, phosphate and beta2-microglobulin during clinical haemodialysis

Abstract

Background: Diffusive clearance depends on blood and dialysate flow rates and the overall mass transfer area coefficient (K(o)A) of the dialyzer. Although K(o)A should be constant for a given dialyzer, urea K(o)A has been reported to vary with dialysate flow rate possibly because of improvements in flow distribution. This study examined the dependence of K(o)A for urea, phosphate and β(2)-microglobulin on dialysate flow rate in dialyzers containing undulating fibers to promote flow distribution and two different fiber packing densities.

Methods: Twelve stable haemodialysis patients underwent dialysis with four different dialyzers, each used with a blood flow rate of 400 mL/min and dialysate flow rates of 350, 500 and 800 mL/min. Clearances of urea, phosphate and β(2)-microglobulin were measured and K(o)A values calculated.

Results: Clearances of urea and phosphate, but not β(2)-microglobulin, increased significantly with increasing dialysate flow rate. However, increasing dialysate flow rate had no significant effect on K(o)A or K(o) for any of the three solutes examined, although K(o) for urea and phosphate increased significantly as the average flow velocity in the dialysate compartment increased.

Conclusions: For dialyzers with features that promote good dialysate flow distribution, increasing dialysate flow rate beyond 600 mL/min at a blood flow rate of 400 mL/min is likely to have only a modest impact on dialyzer performance, limited to the theoretical increase predicted for a constant K(o)A.

Figures

Fig. 1

Urea (panel A) and phosphate (panel B) Ko as a function of average dialysate velocity. Average dialysate velocity was calculated by dividing the dialysate flow rate by the free cross-sectional area of the dialysate compartment (cross-sectional area of the dialyzer housing − cross-sectional area of fibers).

Fig. 2

Predicted urea clearance versus measured urea clearance. Urea clearance predicted using the Michaels equation and a constant KoA (derived from experimental data for QD = 500 mL/min) did not differ from the measured clearances at dialysate flow rates of 350 and 800 mL/min. Data are presented as mean ± SEM for n = 12. The solid line is the line of identity.