Dialysis
Updating Online Haemodiafiltration in the Renal Replacement Therapy of Chronic Kidney Disease Patients
Alexandre Granger Vallée1 and Bernard Canaud2
1. Nephrologist; 2. Professor and Head, Department of Nephrology, Dialysis and Intensive Care and Institute for Research and Training in Dialysis, Montpellier Regional University Hospital, Hôpital Lapeyronie, Montpellier
Abstract
While a growing number of patients worldwide depend on renal replacement therapy for survival, outcomes still remain dismal for patients on haemodialysis. This poor survival on haemodialysis may emanate, in part, from the insufficient removal of medium- and high-molecular-weight uraemic toxins and the inflammation inevitably created by this technique. Haemodiafiltration has the capacity to tackle both of these problems at once: by combining convective- to diffuse-base clearance, it has the ability to remove higher-molecular-weight solutes, while its use of ultrapure water for dialysate and substitution fluid generation and its better biocompatibility profile render it a much less inflammatory technique. In this review, we present some key technical aspects relevant to haemodiafiltration before summarising the most relevant studies evaluating the removal capacity of haemodiafiltration for a wide range of solutes. We end with an analysis of the evidence on the impact of the technique on the most pertinent patient outcomes.
Keywords
Convection, end-stage renal disease, haemodialysis, haemodiafiltration, patient outcomes, renal replacement therapy, ultrapure water, uraemic toxins
Disclosure: The authors have no conflicts of interest to declare. Received: 26 May 2011 Accepted: 29 July 2011 Citation: European Nephrology, 2011;5(2):143–7 Correspondence: Bernard Canaud, Lapeyronie Hospital, 371 av Doyen Gaston Giraud, 34295, Montpellier CEDEX 5, France. E:
b-canaud@chu-montpellier.fr
According to the latest US Renal Data System data, the adjusted rate of prevalent cases of end-stage renal disease (ESRD) is nearly 1,700/million population.1
On a larger scale, recent estimates evaluate that nearly two million patients depend on renal replacement therapy (RRT) for survival worldwide. While for most patients, kidney transplantation constitutes the best therapeutic option, logistical reasons (the principal of which being organ shortage) limit the number of patients who can beneficiate from such therapy. Consequently, the great majority of ESRD patients rely on dialysis (more often blood-borne than peritoneal) to compensate for the deteriorating function of their failing kidneys.
While conventional haemodialysis relies solely on the diffusion of molecules across a semi-permeable membrane according to their concentration gradient, convective modalities more closely mimic the glomerular filtration process by forcing fluid across a semi-permeable membrane (and the solutes it contains according to their sieving coefficient) using a hydraulic pressure gradient. The fluid removed during the convective process is replaced extemporaneously with a substitution fluid aimed at restoring the internal milieu. Modalities that rely solely on convective clearance (haemofiltration HF]) or, more frequently, that combine both diffusive and convective clearances (haemodiafiltration [HDF]) have been developed and are currently used in clinics worldwide.
In the following sections, we present the technical aspects inherent to the realisation of HDF on a large scale, the comparative removal of solutes with convective and diffusive therapies and clinical data looking at the impact of these modalities on patient outcomes.
© TOUCH BRIEFINGS 2011
While the exhaustive description of these systems is beyond the scope of this article, their main characteristics are two reverse osmosis modules in series, a distribution loop free of stagnant water and the frequent disinfection of the whole system to prevent biofilm formation. Moreover, HDF machines generate the substitution fluid by proportioning the ultrapure water thus produced with acid and base concentrates and by using a cold sterilising process (based on ultrafilters, usually two in series) on the fluid before its use.
This prerogative has led to the generation of water-treatment systems capable of delivering virtually sterile and non-pyrogenic ‘ultrapure’ water.3
Apart from the water used, the HDF technique necessitates a specific filter showing:
• •
high hydraulic permeability (coefficient of ultrafiltration [hydraulic permeability] [KUF] ≥50 ml/hour/mmHg);
high small- (coefficient of mass transfer for a solute [urea, creatinine, etc] KoA urea over 600 ml/minute) and large-solute (B2-microglobulin clearance over 60 ml/minute) permeability;
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Technical Aspects of Online Haemodiafiltration One distinctive component of convective techniques resides in their use of substitution fluid. Water used for the generation of this fluid has to comply with very high microbial criteria since it is directly infused into the patient’s bloodstream (contrary to dialysate, which is separated from the blood by the dialysis membrane). Indeed, the microbial quality of the substitution fluid has to comply with more stringent criteria than those applied to the production of intravenous fluids.2
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