Additionally, we determined the plasma disappearance rate of indocyanine green [15]. The Simplified Acute Physiology Score II [16] and the Therapeutic Intervention Scoring System score [17] were determined in order to characterize the severity of the underlying disease. Exclusion criteria comprised severe alkalosis (pH >7.55) or acidosis (pH <7.1) and deficiency of ionized calcium (Caion <0.9 mmol/l). Two patients had to be excluded because of severe hypocalcemia, and two other patients because of severe acidosis.This prospective observational study was approved by the institutional review board of the Technical University of Munich, Germany. Written informed consent was obtained from the patients or their legal representatives.Continuous venovenous hemodialysis treatmentCommercially available hemodialysis devices were used for CVVHD treatment: the HF 440 (Duomedica GmBH, Maintal, Germany) for eight runs, or the Multifiltrate (Fresenius Medical Care, Bad Homburg, Germany) for 35 runs. For all CVVHD treatments, a blood flow of 100 ml/minute and a dialysate flow of 2,000 ml/hour were applied.Custom-made 4% sodium citrate (136 mmol/l; 39.8 g/l sodium citrate in 1,000 ml aqua destillata; Fresenius Medical Care) and calcium-free dialysate solution (sodium 133.0 mmol/l, potassium 2.0 mmol/l, calcium 0 mmol/l, magnesium 0.75 mmol/l, chloride 116.5 mmol/l, bicarbonate 20 mmol/l, glucose 1 g/l) were used. Calcium chloride solution was produced in the local hospital pharmacy (500 mmol/l; 73.5 g calcium chloride dihydrate in 1,000 ml aqua destillata). Sodium citrate and calcium chloride flow was started with 4 mmol/l and 1.7 mmol/l treated blood, respectively, and adapted according to the required limits of calcium between 0.25 and 0.35 mmol/l post filter and 1.12 and 1.20 mmol/l in the patient’s circulation. According to the study protocol, citrate and total calcium in serum were measured at baseline just before the beginning of CVVHD treatment, after 30 minutes, as well as after 1, 4, 12, 24, and 72 hours. Blood gas analyses of the patient’s circulation and Caion post filter were measured at baseline, after 30 minutes, as well as after 1, 2, 4, 8, 12, 16, 20, and 24 hours and then every 8 hours up to 72 hours of the expected CVVHD treatment time.Citrate levels were measured enzymatically by the citrate-lyase method [18] (MVZ Labor Limbach, Heidelberg, Germany). Applying this method, citrate is metabolized to oxalacetate and acetate catalyzed by the enzyme citrate lyase.