Influence of convection on small molecule clearances in online hemodiafiltration Technical Note

Abstract

Influence of convection on small molecule clearances in online hemodiafiltration. Background Dialysis efficacy is mostly influenced by dialyzer clearance. Urea clearance may be estimated in vitro by total ion clearance, which can be obtained by conductivity measurements. We have previously used this approach to assess in vitro clearances in a system mimicking predilutional and postdilutional online hemodiafiltration with a wide range of Q D , Q B , andultrafiltration rates. Our current study elaborates on a formula that allows the prediction of the influence of ultrafiltration on small molecule clearances, and validates the mathematical approach both experimentally in vitro and clinically in vivo data. Methods Two conductivimeters in the dialysate side of an E-2008 Fresenius machine were used. HF80 and HF40 polysulfone dialyzers were used; reverse osmosis water and dialysate were used for blood and dialysate compartments, respectively. Study conditions included Q B of 300 and 400 mL/min and Q D of 500 and 590 mL/min, with a range of ultrafiltration rate from 0 to 400 mL/min in postdilutional hemodiafiltration and to 590 mL/min in predilutional hemodiafiltration. Urea clearances were determined in the in vivo studies, which included 0, 50, 100, and 150 mL/min ultrafiltration rates. Results The ultrafiltration rate and clearance were significantly correlated ( R > 0.9, P P where K is the clearance; K 0 is the clearance with nil ultrafiltration rate; Q D is the total dialysate produced (in commercial HDF, Q D = Q Di + Q inf ). Since weight loss was maintained at 0, ultrafiltration rate = infusion flow. Q B is the “blood” lineflow. The formula was also verified in vivo in clinical postdilutional hemodiafiltration with a Q B taking into account the cellu-lar and water compartments. Discussion In vitro, by simply determining the clearance in conventional dialysis, the total clearance for any ultrafiltration rate may be estimated in both predilutional and postdilutional online diafiltration with an error of less than 2%. The same applies to in vivo postdilutional hemodiafiltration when the formula takes into account the cellular and water composition of blood.