Inclusion Criteria:  Regular dialysis (HD or HDF) therapy for >3 months Age >18 years Stable and functional arteriovenous (AV)-fistula Able to achieve a dialysis blood flow > 250 mL Able to stand up for a minimum of 10 min Able to undergo a 4 hour dialysis session without eating, drinking or sleeping Proven cabable of cumulative ultrafiltration of 2% of end-dialytic weight (EDW) Able to give informed consent to participation in the study Hematocrit >30% BMI >18 and <35  Exclusion Criteria:  Central venous catheter for HD or HDF Recirculation in AV-fistula Acute myocardial infarction within 3 months Atrial fibrillation Active malignant or infectious diseases Cerebrovascular incident within 3 months Pregnancy Alcohol or drug abuse History of interruptions during HD or untimely termination of HD treatment