The purpose of this study is to compare the effect of low flux hemodialysis with online hemodiafiltration on all cause mortality and a combination of cardiovascular morbidity and mortality in chronic hemodialysis patients.

Today, an increasing number of patients with chronic renal failure (CRF) is treated with (on-line) hemodiafiltration (HDF). This practice is based on the assumption that the high incidence of cardiovascular (CV) disease, as observed in patients with CRF, is at least partially related to the retention of uremic toxins in the middle and large-middle molecular (MM) range. As HDF lowers these molecules more effectively than HD, it has been suggested that this treatment improves CV outcome, if compared to standard HD.

Thus far, no definite data on the effects of HDF on CV parameters and/or clinical end-points are available. Promising data include a reduction of left ventricular mass index (LVMi) after one year of treatment with acetate free bio-filtration (AFB). Furthermore, relatively high survival rates were reported in a single center non-experimental study on patients who were treated with HDF, if compared to the EDTA registry data on HD-treated patients. Yet, these data are of observational nature, with the possibility of being biased by confounding by indication.

As the accumulation of MMW substances has been implicated in increased oxidative stress and endothelial dysfunction, a reduction of these compounds might improve these derangements. In addition, cardiac dysfunction, atherosclerosis (as measured by left ventricular mass index [LVMi], carotid intima media thickness [CIMT]) and vascular stiffness (as measured by pulse wave velocity [PWV]) might be reduced during HDF, as compared to low-flux HD.

Therefore, we propose a prospective, randomized multicenter trial, comparing (on-line) HDF with HD. After a stabilization period, an expected number of 700 chronic HD patients will be randomized to either HDF or low-flux HD and followed during 1-6 years. Primary end points are all cause mortality and combined CV events and mortality. In addition, LVMi, PWV, CIMT and various parameters of oxidative stress, acute phase reaction (APR) and endothelial function will be assessed and compared between treatment groups.

This study will provide strong evidence on the efficacy of HDF compared to low flux HD on CV morbidity and mortality, which is currently lacking but urgently needed. It is highly likely that the outcome of this study will affect current clinical practice considerably, in the Netherlands as well as internationally. Moreover, the study will point towards the mechanisms underlying the effects of HDF.

The following hypotheses will be tested:

1. All-cause mortality and combined CV morbidity and mortality in patients treated with (on-line) HDF is lower than in patients treated with standard low-flux HD.
2. A reduction in MMW uremic toxins by HDF leads to an improvement of the 'uremic profile' (as measured by AGE-levels, homocysteine levels, oxidative stress, and endothelial dysfunction), if compared to standard low-flux HD.
3. The improvement of the 'uremic profile' in HDF-treated patients results in an improvement of endothelial function with a reduction in the progression of vascular injury (as measured by CIMT and PWV) and a reduction in LVMi, if compared to standard low-flux HD.

Inclusion Criteria:

• Patients treated by HD 2 or 3 times a week, for at least 2 months
• Patients able to understand the study procedures
• Patients willing to provide written informed consent

Exclusion Criteria:

• Current age < 18 years
• Treatment by HDF or high flux HD in the preceding 6 months
• Severe incompliance (severe non-adherence to the dialysis procedure and accompanying prescriptions, especially frequency and duration of dialysis treatment and fluid restriction)
• Life expectancy < 3 months due to non renal disease
• Participation in other clinical intervention trials evaluating cardiovascular outcome