The Microcirculation, Dialysis Modality and Sequestered Salt

The aim of this clinical trial is to investigate the effect of 5 different dialysis treatments (combinations of dialysis mode and dialysis fluid sodium content) on the microcirculation (MC) and sequestered sodium content (SSC) in adult prevalent end-stage kidney disease (ESKD) patients treated with hemodialysis (HD) or hemodiafiltration (HDF). The main questions it aims to answer are: 1. What are the effects on the sequestered sodium content and microcirculation after 4 weeks of treatment with the following dialysis modes? * HDF with an expected zero diffusive sodium balance (Dialysate sodium concentration (DNa)= Plasma sodium concentration (PNa)) compared to * HDF with an expected diffusive sodium efflux (DNa \< PNa, difference 3 mmol/L) compared to * HD with an expected zero diffusive sodium balance (DNa = PNa) compared to * HD with an expected diffusive sodium efflux (DNa \< PNa, difference 3 mmol/L) compared to * Isolated ultrafiltration for 30 minutes followed by HD with an expected zero diffusive sodium balance (DNa = PNa) 2. Are the SSC and MC interrelated in this patient group? This study is a randomized cross-over trial. Participants will be subjected to the abovementioned dialysis treatment modes in random order.