Dialysis
Figure 1: Online Haemodiafiltration can be of the Pre-dilution, Post-dilution, Mixed Dilution or Mid-dilution Type
A Substitution fluid
Dialysate effluent + Ultrafiltrate
Dialysate effluent substitution fluid
Water UF UF
Infusion pump(s)
B
Dialysate effluent + Ultrafiltrate
Dialysate effluent substitution fluid
Infusion pump(s)
UF UF Online UF
haemodiafiltration machine
Dialysate effluent
Online
haemodiafiltration machine
UF ultrapure
Dialysate effluent
• pre-dilution, where the substitution fluid is infused into the blood stream at the pre-filter site. The absence of haemoconcentration in the filter in this HDF mode presents the advantage of decreasing the clotting risk and permitting the use of higher ultrafiltration to blood flow rate ratios;
• post-dilution, in which the substitution fluid is administered after the filter. This modality presents the advantage of providing a higher convective clearance at a given ultrafiltration rate since solutes are not diluted by the substitution fluid before entering the filter;
• Water ultrapure •
mixed dilution, which is a more recent modality, comprises the infusion of substitution fluid both before and after the filter. This technique has the added value of maintaining a stable ultrafiltration rate and preserving the transmembrane pressure4 and could increase middle-sized molecules and reduce albumin loss; and5,6
C Substitution fluid
Dialysate effluent + Ultrafiltrate
Dialysate effluent substitution fluid
UF UF
Infusion pump(s)
Substitution fluid
D
Dialysate effluent + Ultrafiltrate
Dialysate effluent substitution fluid
Infusion pump(s)
Substitution fluid
UF UF Online
haemodiafiltration machine
UF
Dialysate effluent
Online
haemodiafiltration machine
UF
Dialysate effluent
mid-dilution, which relies on a particular haemodiafilter constituted of two separate concentric fibre bundles, one providing blood flow away from and the other ensuring blood flow towards the filter head where both arterial and venous lines are attached. The two bundles are in continuity at the distal head, where substitution fluid is infused, therefore providing both pre- and post-dilution HDF at the same time. A variant has recently been developed (reverse mid-dilution) that could optimise fluid exchange.7,8
Water ultrapure
or that separate the convective and diffusive solute fluxes (such as paired HF, HDF with endogenous reinfusion)13,14
Other techniques that rely on the direct backfiltration of the dialysate through the haemodiafilter (such as internal HDF, HDF with two filters, push/pull HDF)9–12
have
Notwithstanding the treatment schedule prescribed, adequate anticoagulation and the use of high blood flow rates are of utmost importance to achieve high solute clearances.
Comparative Solute Removal Water ultrapure
When looking at the existing data on the removal capacities of HDF, one has to consider three broad groups of uraemic substances:
• •
Pre-dilution (A), post-dilution (B), mixed dilution (C) or mid-dilution (D) types. UF = ultrafiltration.
• •
large surface area for exchange (1.80–2.10 m2); and low resistance (arteriovenous pressure drop less than 300 mmHg).
Moreover, the patient’s vascular access must be able to permit blood flow rates in the 400 ml/minute realm and the HDF machine must be able to generate 600–800 ml/minute of dialysate fluid. Online HDF machines have the capacity to divert a fraction of the proportioned
144
Multiple studies have shown a greater solute reduction ratio (a surrogate of mass removal) with HDF than with low- or high-flux haemodialysis.17–21
higher-molecular-weight solutes.22
This difference is even greater when looking at Below, we describe the most
relevant results looking at the comparative removal of solutes belonging to each of the three categories already described.
EUROPEAN NEPHROLOGY •
water-soluble, low-molecular-weight solutes (less than 5 kDa), such as urea;
medium-molecular-weight solutes (over 5 kDa), such as B2-microglobulin; and
highly protein-bound solutes, irrespective of their molecular weight, such as para-cresyl sulphate.16
also been described and adopted to varying degrees in different parts of the globe. Finally, the treatment schedule of conventional HDF usually consists of thrice-weekly four-hour sessions. Daily HDF has been described and its impact on solute removal and patient outcomes has been evaluated.15
dialysate (varying from 50 to 200 ml/minute) to be used as the substitution fluid. Depending on where the substitution volume is infused in the extracorporeal blood circuit, four common types of HDF can be identified (see Figure 1):
Patient
Patient
Patient
Patient
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