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Quantitative analysis of convective dose in hemofiltration and hemodiafiltration: “Predilution” vs. “postdilution” reinfusion

In hemofiltration (HF) and hemodiafiltration (HDF), removal of medium and high‐molecular‐weight solutes is greatly enhanced by convective mechanisms as compared with simple diffusion; increasing convective flows may allow greater removal rates of these solutes. Use of “predilution” (pre‐H[D]F) may allow higher ultrafiltration rates than the “postdilution” mode (post‐H[D]F); yet, the dilution of plasma water may have unpredictable effects on “endogenous” water convection. We have applied a mathematical analysis to evaluate and compare endogenous water convective flow rates in pre‐H(D)F vs. post‐H(D)F. Endogenous plasma water recovered in ultrafiltrate was calculated according to patient (hematocrit, total protein level) and session parameters (blood flow, ultrafiltration rate, programmed weight loss), in absolute terms and as a fraction of endogenous plasma water delivery to the filter. Maximally efficient post‐H(D)F was modelled according to a preset postfilter hematocrit or filtration fraction. Nomograms were constructed expressing endogenous water convective fluxes in relation to parameters of interest (ultrafiltration rate, blood flow, hematocrit) with both post‐H(D)F and pre‐H(D)F, and “efficiency” of pre‐H(D)F vs. post‐H(D)F (as the ratio of endogenous water convective flow rate with the 2 techniques) as a function of the ultrafiltration/reinfusion rate. In post‐H(D)F, the model predicts maximal ultrafiltration rates within the limits of a preset hemoconcentration at the filter outlet; additionally, the model allows to calculate ultrafiltration/reinfusion quantities to be set in pre‐H(D)F to equal and overcome maximal convective efficiency of post‐H(D)F. This “equivalence” ultrafiltration rate may greatly vary according to patient's hematocrit and blood flow, so that the ultrafiltrate‐reinfusate volume available in the system dictates, in any patient, which mode of reinfusion may attain higher “endogenous” convective flow rates. Pre‐H(D)F may allow higher fractional and absolute “endogenous” convective flow rates as compared with post‐H(D)F, provided that adequate amounts of reinfusate are available. For lower reinfusate volumes than “equivalence” values, post‐H(D)F remains a better option.