The primary objectives of the BiPhox-Trial are to demonstrate, that the use of Biphozyl® as a replacement fluid in adult critically ill acute kidney injury (AKI) patients, results in a lower rate of pH excursions and of bicarbonate (HCO3-) excursions compared to the use of Phoxilium® during the studied continuous veno-venous hemofiltration (CVVH) interval with regional citrate anticoagulation (RCA).

The secondary objectives of the BiPhox-Trial are to evaluate the time to pH level normalization and the HCO3- substitution rates after initiation of CVVH treatment. Further, to demonstrate that the use of Biphozyl® as a replacement fluid in adult critically ill AKI patients, results in a more stable acid-base-status as well as improved respiratory situation due to lower intracorporeal HCO3- and carbon dioxide levels compared to the use of Phoxilium® during the studied CVVH interval with RCA.

After being fully eligible by meeting all inclusion and none of the exclusion criteria, participants will be randomly assigned to one of two groups, either the Phoxilium® - Group or Biphozyl® - Group. After randomization, patients receive either Phoxilium® or Biphozyl® for CVVH initiation and maintenance as a replacement fluid during the first 48 hours (h) of treatment. After the first 48h of CVVH with either Phoxilium® or Biphozyl® a cross-over follows, with another 48h of CVVH with the opposite replacement fluid (Phoxilium® switched to Biphozyl® or Biphozyl® switched to Phoxilium®). In comparison, all patients should receive one session of CVVH with 96h. Resulting from 48h of CVVH with Phoxilium® and 48h of CVVH with Biphozyl® as a replacement fluid. The order is determined by randomization.

Anticoagulation is always delivered as pre-filter RCA with Regiocit® (Gambro Lundia AB, Sweden). For antagonisation of Regiocit®, a calcium solution (calcium chloride, with or without magnesium chloride) will be used post-filter.

Inclusion Criteria:

1. Age ≥ 18 years
2. Admission to Intensive Care Unit
3. Indication for CVVH as determined by the attending physician
4. Planned CVVH treatment time ≥ 48 hours
5. Written informed consent or deferred consent or legally acceptable representative consent

Exclusion Criteria:

1. Lack of commitment to provide CVVH as part of limitation of ongoing life support
2. Presence of a drug overdose that may result in acid-base-disorders and/or a shift of electrolytes
3. Receipt of CVVH within the previous 72 hours
4. Dialysis dependent end-stage renal disease
5. Pregnancy, must be ruled out by anamnesis and/or blood or urine pregnancy test
6. Combination of severely impaired liver function and shock with muscle hypoperfusion
7. Co-enrollment in another trial, which could have a plausible interaction with the acid-base-status and/or any electrolytes
8. Subjects, who are legally exempted from participation in clinical trials (e.g. persons held in an institution by legal or official order)