On-Line Hemodiafiltration with Pre- and Postdilution: A Comparison of Efficacy

Since the introduction of on-line substituate preparation, high substituate rates (Q_s) in pre- and postdilution for hemodiafiltration (HDF) procedures can be realized. During postdilution HDF (POD-HDF) and additional convective removal is possible, but in vivo Q_s is limited to approx. 1/3Q_b (bloodflow). With predilution HDF (PRD-HDF) higher Q_s and therefore high convective transport rates by ultrafiltration can be reached. On the other hand the blood concentration is diminished by predilution. Further decrease of the diffusive transport is caused by reduced dialysate flow Q_d due to separation of the substituate from the dialysate (Fresenius 4008 On-Line HDF, Gambro AK100 Ultra). The theoretical description of the combined diffusive-convective transport is limited to 1-dimensional models and small UF-rates. Therefore for practical and theoretical purposes the assessment of the efficacy of on-line PRD-HDF and POD-HDF in different molecular weight ranges is desirable. By means of in vitro experiments the effective clearances K_eff of hemodialysis (HD, dialyzer: Fresenius F60) for urea, creatinine, vitamin B12 and inulin were compared with measured and theoretical K_eff of POD- and PRD-HDF. The theoretical expectation is confirmed that K_eff for small molecular weight substances decreases slightly with PRD-HDF and increases for larger molecules. In the case of POD-HDF K_eff for small molecular weight substances increases slightly and strongly for larger molecules. In vivo experiments were performed to measure the real substance removal from patient's blood and to figure out the impact of dialysate flow (collection of the used dialysate during the 1. treatment hour and concentration measurements for urea, creatinine, phosphate, ß₂-MG). The results show that the substraction of Q_s from Q_d reduces K_eff for urea, creatinine and phosphate but not for ß₂-MG. PRD-HDF with Q_d = 500 ml/min is significantly less effective for small molecules than HD. There is no significant difference of K_eff for urea, creatinine, phosphate during HD and PRD-HDF with Q_d = 800 ml/min, but a significant increase of 10-15% for POD-HDF K_eff for ß₂-MG increases by 75% for PRD-HDF and 95% for POD-HDF compared with HD (Q_d = 500 ml/min).