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Mixed predilution and postdilution online hemodiafiltration compared with the traditional infusion modes.

BACKGROUND
On postdilution hemodiafiltration (post-HDF), convective removal of medium-high molecular weight solutes is, at the highest ultrafiltration rates, limited by high blood viscosity and protein concentration. Prefilter reinfusion (pre-HDF) may overcome this problem, but plasma dilution may affect the overall efficiency of the technique. In this study, an experimental system of online HDF with combined prefilter and postfilter infusion (mixed HDF) was evaluated and compared with the traditional predilution and postdilution modes.


METHODS
Removal of urea (U), creatinine (Cr), phosphate (Phos), and beta(2)-microglobulin (beta(2)m), ultrafiltration coefficients of the dialyzer (K(UF)), and rheologic conditions of the blood circuit were evaluated during the three infusion modes (a total of 36 runs lasting 180 min), performed with a polysulfone hemofilter 1.8 m(2), blood flow (Q(b)) 400 mL/min, dialysate flow (Q(d)) 700 mL/min, and infusion rate 120 mL/min (pre-HDF and post-HDF), or 60 + 60 mL/min (mixed HDF).


RESULTS
The mean effective U and Cr clearances and urea index of dialysis dose (eKt/V) were significantly higher on post-HDF than on pre-HDF (K(WB) (U) 210 vs. 193 mL/min, K(DQ) (Cr) 152 vs. 142 mL/min, eKt/V 1.41 vs. 1.30), while mixed HDF did not show significant differences versus post-HDF (K(WB) (U) 201 mL/min, K(DQ) (Cr) 149 mL/min). K(DQ) for Phos and beta(2)m were higher on post-HDF in only absolute values. Similar differences were found for instantaneous dialyzer clearances (K(I)) at 60, 120, and 180 minutes of the sessions, with a common trend to decrease with time. K(UF) and the apparent beta(2)m sieving coefficient showed their lowest values toward the end of post-HDF sessions. Increasing filtration fractions (FFs) were associated with increasing transmembrane pressure (TMP) and solute clearances up to FF values of 0.45. These were values achieved in only post-HDF, at which point the curve of the relationship between TMP and FF assumed its steepest exponential trend.


CONCLUSIONS
Mixed HDF, by better preserving the characteristics of water and solute transport of the membrane, ensured safer operating conditions than post-HDF, while achieving similar removal of small- and large-size solutes. Optimizing the ratio of prefilter/postfilter infusion and the total infusion according to the relationships found in our study between solute clearances, FF, and TMP, convective flux and transport may avoid excessive hemoconcentration and dangerous pressure gradients.