CO(2) and O(2) removal during continuous veno-venous hemofiltration: a pilot study

BACKGROUND: Carbon dioxide (CO(2)) accumulation is a challenging issue in critically ill patients. CO(2) can be eliminated by renal replacement therapy but studies are scarce and clinical relevance is unknown. We prospectively studied CO(2) and O(2) behavior at different sample points of continuous veno-venous hemofiltration (CVVH) and build a model to calculate CO(2) removal bedside. METHODS: In 10 patients receiving standard CVVH under citrate anticoagulation, blood gas analysis was performed at different sample points within the CVVH circuit. Citrate was then replaced by NaCl 0.9% and sampling was repeated. Total CO(2) (tCO(2)), CO(2) flow (V̇CO(2)) and O(2) flow (V̇O(2)) were compared between different sample points. The effect of citrate on transmembrane tCO(2) was evaluated. Wilcoxon matched-pairs signed rank test was performed to evaluate significance of difference between 2 data sets. Friedman test was used when more data sets were compared. RESULTS: V̇CO(2) in the effluent (26.0 ml/min) correlated significantly with transmembrane V̇CO(2) (24.2 ml/min). This represents 14% of the average expired V̇CO(2) in ventilated patients. Only 1.3 ml/min CO(2) was removed in the de-aeration chamber, suggesting that CO(2) was almost entirely cleared across the membrane filter. tCO(2) values in effluent, before, and after the filter were not statistically different. Transmembrane tCO(2) under citrate or NaCl 0.9% predilution also did not differ significantly. No changes in V̇O(2) were observed throughout the CVVH circuit. Based on recorded data, formulas were constructed that allow bedside evaluation of CVVH-attributable CO(2) removal. CONCLUSION: A relevant amount of CO(2) is removed by CVVH and can be quantified by one simple blood gas analysis within the circuit. Future studies should assess the clinical impact of this observation. TRIAL REGISTRATION: The trial was registered at https://clinicaltrials.gov with trial registration number NCT03314363 on October 192,017. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (10.1186/s12882-019-1378-y) contains supplementary material, which is available to authorized users.