**Principles of Haemodiafiltration: Rationale for Improved Patients' Survival**

Goran Imamović, Bernard Canaud, Nusret Mehmedović and Cäcilia Scholz

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/63067

#### **Abstract**

Haemodiafiltration (HDF) is a renal replacement modality that combines diffusion and enhanced convection in order to remove small- and middle-molecular-weight com‐ pounds, respectively. They are removed along solvent drag effect of ultrafiltration through increased transmembrane pressure (TMP), whereas the replacement solution is infused intravenously at equal amount minus the desired fluid volume removal for achieving dry weight. Limiting factors for high-volume on-line haemodiafiltration (HV oHDF) are blood flow and viscosity (haematocrit, protocrit), filter performance and technical features of HDF monitor. Most recent advanced technology of dynamic analysis of pressure pulses along the blood flow pathway in the dialyser has enabled optimal ultrafiltration flow performances. HV oHDF offers today the best compromise of cardioprotective option by reducing cardiovascular risk factors in end-stage kidney disease patients. Recent randomised controlled trials (RCTs), individual participant data meta-analyses and a number of observational studies have shown the evidence of survival advantage of HDF over conventional haemodialysis (HD). The convective volume has become the key quantifier for HV oHDF as the measure of dialysis dose. Its cut-off values for better survival have been recognised, but the research is still needed in the years to come to set the required optimal volumes tailored to individual patients' needs.

**Keywords:** haemodialysis, haemodiafiltration, convection, ultrafiltration, mortality

### **1. Introduction**

The uremic syndrome is characterised by an accumulation of uremic toxins due to inadequate kidney function. There have been more than 90 compounds considered to be uremic toxins listed

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by the European Uremic Toxin Work Group. Sixty-eight have a molecular weight less than 500Da, 10have amolecularweightbetween500 and12,000Da and12 exceed12,000Da.Twentyfive solutes (28%) are protein bound [1]. These figures are further increasing with the adop‐ tion of new knowledge and technology of uremic toxins detection.

The clearance of solutes during conventional haemodialysis (HD) depends on their size and the concentration gradient across the dialysis membrane. Solutes weighing less than 500 Da are considered low-molecular-weight solutes and they are removed by passive diffusion down a favourable concentration gradient. Urea is considered a marker of such toxins. Its clearance, as measured by Kt/Vurea, correlates with patient morbidity [2] showing the evidence that such toxins contribute to the uremic syndrome.

However, despite improvements in technology and patient care, the mortality rate of patients on maintenance dialysis remains high, at approximately 15–20% per year [3]. As specified in the United States Renal Data System report, the expected remaining life span after the initiation of renal replacement therapy was eight years for dialysis patients aged 40–44 and 4.5 years for those 60–64 years of age. In general population the ranges of the expected remaining life span at specified ages are 30–40 years and 17–22 years, respectively. The values in older dialysis patients are only slightly better than those in patients with lung cancer [4]. Therefore, there is an obvious need to change the practices and to attempt to approach the problem differently.
