The goal of this observational study is to learn about amino acid loss during continuous renal replacement therapy and plasma amino acid levels in intensive care patients. The main questions it aims to answer are:

What amount of amino acids is lost over the duration of continuous renal replacement therapy? How do amino acid plasma concentrations change over time in patients with and without continuous renal replacement therapy?

Amino acid concentrations will be measured in the effluent and in the plasma of patients receiving continuous renal replacement therapy as part of their regular medical care. In addition, plasma concentrations of amino acids will be studied in patients without renal replacement therapy.

In order to quantify the amount of amino acids lost via the continuous renal replacement (CRRT) machine, it is warranted to measure amino acid concentrations in the blood and effluent over the entire treatment period.

To this end, 20 intensive care patients will be examined throughout the entire duration of CRRT as part of this exploratory non-interventional study. Additionally, 10 intensive care patients without renal replacement therapy will be enrolled. In the CRRT-group amino acid loss through the effluent will be measured at different time points. Amino acid plasma concentrations will be determined in both groups. Amino acid urine concentration, nitrogen balance, as well as blood urea nitrogen, creatinine and albumin in the plasma will be measured as secondary outcome parameters. Additionally, observations regarding nutritional status and mobility (indirect calorimetry, bioimpedance analysis, ultrasound muscle and adipose tissue thickness, strength and mobility scoring) will be analyzed as exploratory secondary outcomes.

Duration of CRRT, length of intensive care and hospital stay as well as mortality will be monitored.

High-performance liquid chromatography fluorescence as well as liquid chromatography-mass spectrometry will be performed to measure amino acid concentrations in plasma, effluent and urine.

To answer the primary research question, change in amino acid plasma concentrations and effluent loss between treatment start and day 4 of continuous renal replacement therapy will be analysed using a two-sided Wilcoxon signed rank test.

Inclusion criteria:

• Postoperative ICU patients
• Age ≥ 18 years
• Patients on enteral or/and parenteral nutrition given according to internal standard operating procedure

A) CRRT group:

- CRRT treatment planned; all types of CRRT (continuous veno-venous hemodialysis (CVVHD), continuous veno-venous hemodiafiltration (CVVHDF), continuous veno-venous hemofiltration (CVVH))

B)Non-CRRT group:

• no current or previous (<30 days) continuous or intermittent RRT
• no chronic kidney disease stage G3-G5

Exclusion criteria:

• Preoperative intensive care patients
• Patients with liver cirrhosis stage 1-3 according to the Child-Pugh classification
• Patients with acute liver failure

This study will enroll 30 intensive care patients: 20 patients receiving continuous renal replacement therapy and 10 patients without renal replacement therapy. The study will be carried out at the Medical University of Vienna at a 16-bed intensive care unit admitting patients after cardiothoracic and major vascular surgery.