Extracorporeal Therapy Modes Used in ESRD Patients Figure

In a typical haemodialysis procedure, although transmembrane mass transfer occurs predominantly by diffusion, a modest degree of convective mass transfer is also achieved in association with the ultrafiltered plasma water. However, the recent recognition of ft2M and other low molecular mass proteins as important uraemic toxins has prompted interest in using dialytic therapies with increased convective removal capabilities for these poorly diffusible solutes. In haemodialysis, the total ultrafiltration volume and net ultrafiltration rate are determined by the degree to which a patient's plasma volume needs to be reduced and the duration of the treatment. (The total ultrafiltration requirement is dictated by the amount of fluid ingested by the patient in the period between dialysis treatments.) The total volume of plasma water ultrafil-tered is approximately 3-4 L, resulting in a typical net ultrafiltration rate of 15-20 mL min"1.

As a means to augment convective solute removal, haemofiltration (HF) was developed by Henderson,

Haemofiltration Haemodiafiltration

Figure 1 Extracorporeal therapy modes used in end-stage renal disease.

Haemofiltration Haemodiafiltration

Figure 1 Extracorporeal therapy modes used in end-stage renal disease.

Lysaght and colleagues in the early 1970s. This is a purely convective therapy in which no dialysate is used but an ultrafiltration rate that far exceeds the net ultrafiltration requirements of the patient is employed. As plasma water is typically ultrafiltered at an absolute rate of at least 100mLmin~1 (6Lh_1) in HF, the much lower net ultrafiltration rate required for fluid removal from the patient is achieved by 'replacing' most of the ultrafiltrate with a bicarbonate-based solution. For the large volume of intravenous-quality 'replacement fluid' that is required, the filtrate produced by sequential ultrafiltration of dialysate is used. This 'on-line' mechanism, in which the dialysate precursor of the replacement fluid is produced by the same HD machine that performs the HF treatment, allows very high volumes of ultrafiltrate to be produced. In HF, only dialysers with very high hydraulic permeability (see below) are used.

Although HF is a significant improvement over HD with respect to relatively large sized uraemic toxin removal, the absence of diffusion renders it only a marginal therapy with respect to small solute removal. To overcome this deficiency of HF, Canaud and colleagues approximately 15 years ago first employed online haemodiafiltration (HDF). As its name implies, this therapy is essentially a HD/HF hybrid in which both dialysate flow and high ultrafiltration rates are used. At present, HDF offers the broadest solute removal spectrum of all dialytic therapies.

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