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

Background Dialysis efficacy is mostly influenced by dialyzer clearance. Ur ea clearance may be estimated in vitro by total ion clearance, which can be obtained by conductivity measurements. We have previously used this approa ch to assess in vitro clearances in a system mimicking predilutional and po stdilutional online hemodiafiltration with a wide range of Q(D), Q(S), and ultrafiltration rates. Our current study elaborates on a formula that allow s the prediction of the influence of ultrafiltration on small molecule clea rances, and validates the mathematical approach both experimentally in vitr o 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 o smosis 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 predilution al hemodiafiltration. Urea clearances were determined in the in vivo studie s, which included 0, 50, 100, and 150 mL/min ultrafiltration rates. Results. The ultrafiltration rate and clearance were significantly correlat ed (R > 0.9, P < 0.001) and fitted a linear model (P < 0.001) in all of the experimental conditions. The following formula fitted the experimental poi nts with an error <2% for both postdilutional and predilutional online diaf iltration in vitro, respectively. K = K-0 + [(Q(B) - K-0)/(Q(B))] x ultrafiltration rate K = K-0 + [((Q(D) x Q(B))/(Q(B) + Q(D)) - K-0)/Q(D)] x ultrafiltration rate where K is the clearance; K-0 is the clearance with nil ultrafiltration rat e; Q(D) is the total dialysate produced tin commercial HDF, Q(D) = Q(Di) Q(inf)) Since weight loss was maintained at 0, ultrafiltration rate = infus ion flow. Q(B) is the "blood" line flow. The formula was also verified in v ivo in clinical postdilutional hemodiafiltration with a Q(B) taking into ac count the cellular and water compartments. Discussion. In vitro, by simply determining the clearance in conventional d ialysis, 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 hemodiafiltrat ion when the formula takes into account the cellular and water composition of blood.