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

Abstract

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.

Influence of Hemofiltration on Intradialytic Plasma Volume Decrease

“…We could not achieve optimal effectiveness of convective transport in post-ol-HDF [6]; the blood-flow rates generally used in our country are lower than those recommended by the relevant European and American societies [1, 2]. In contrast, we could achieve optimal effectiveness even with low blood-flow rates in pre-ol-HDF; the estimated effective convective rates were > 30% of blood-flow even with low blood-flow rates.…”

“…Relative post-dialysis Ht decrease leads to IPVD underestimation. Hemoconcentration in dialyzer hollow fibers is a cause of intradialytic erythrocyte loss and is severer in post-ol-HDF compared with HD and pre-ol-HDF, so it follows that post-ol-HDF would have larger intradialytic erythrocyte loss compared with the other modes [6, 7]. Hence, if intradialytic erythrocyte loss had occurred, the findings that IPVD in post-ol-HDF was not smaller than IPVD in HD would have indicated facilitation rather than prevention of IPVD by post-ol-HDF compared with HD.…”

“…Body fluid removal causes intradialytic plasma volume decrease (IPVD), which is a leading cause of intradialytic hypotension (IDH) [3-5]. Compared with the primary mode of hemodialysis (HD), where only diffusive transport is performed, hemodiafiltration (HDF), a mode of dialysis where convective transport is used in combination with diffusive transport [6, 7], reduced the episodes of IDH [8, 9]. Thus, HDF is expected to prevent IPVD.…”

Hemodiafiltration: the addition of convective flow to hemodialysis

“…This enables enhanced ultrafiltration flow in the dialyzer, over the plasma water flow. To improve efficiency in the predilution mode, the convective flow should be high enough to ensure increased solute clearance [5,20,21] despite blood dilution [5,8,9]. In practice ( Table 2), this is superior to 50% of the blood flow, Substitution fluid is obtained from ultrapure dialysate by ultrafiltration through a filter.…”

Evaluation of a New Online Hemodiafiltration Mode with Automated Pressure Control of Convection

“…Moreover, the ULTRAc system automatically optimizes the individual convective volumes in online therapies, making the treatments easy to manage in a postdilution configuration, particularly when a Q B of ≤300 ml/min is limited by vascular access status. This limitation is often overcome by using the predilution mode with the constraint of a two-fold exchange convective volume [31] or the use of a mix of the two solutions, but adding complexity to the prescription and treatment surveillance [14,18,27]. …”