Optimization of mid-dilution haemodiafiltration: technique and performance.

Background: Mid-dilution haemodiafiltration (MD-HDF), reported as a highly efficient convective-mixed technique, has demonstrated serious drawbacks in relation to the high pressure originating inside the blood compartment of the filter during clinical application. This randomized crossover design study was planned to optimize the efficiency of the MD-HDF technique while reducing its inherent risks.

Methods: Fifteen patients on RRT were submitted in random sequence to standard and reverse MD-HDF under similar operating conditions. Efficiency in solute removal was evaluated by measuring urea (U), phosphate (P) and beta2-microglobulin (beta2-m), mean dialysate clearances (K(DQ)) and eKt/V. Blood and dialysate compartment pressures were monitored on-line during the sessions, and instantaneous hydraulic and membrane permeability indexes were calculated.

Results: During standard MD-HDF sessions, unlike with reverse MD-HDF, excessive blood inlet and transmembrane pressure prevented the planned infusion from being maintained. Resistance index and membrane permeability to water and middle molecules substantially improved with reverse MD-HDF. This resulted in higher beta2-m removal (221.3 +/- 81.3 versus 185.1 +/- 65.5 mg/session, P = 0.007). Phosphate removal was comparable, while U removal was greater with standard MD-HDF (K(DQ) 272 +/- 35 versus 252 +/- 29 ml/min, P = 0.002; eKt/V 1.63 +/- 0.23 versus 1.49 +/- 0.17, P = 0.005).

Conclusions: This study demonstrated the ability of MD-HDF to remove significant amounts of medium-sized uraemic compounds and phosphate, but safe rheologic and hydraulic conditions were only maintained by carrying out treatments with the dialyser used in reverse configuration. For this purpose, the larger MD-220 dialyser ensured better tolerance together with higher middle molecules clearance, even though small molecule removal was slightly worsened. The results of this study may provide some insight into the complex interactions between pressures and flux within the original structure of MD-dialysers and help optimize the clinical application of the technique and reduce its risks.