[Evolutionary biological aspects of the physiology of extracorporeal CO2 removal]

Extracorporeal CO2 elimination (ECCO2-R) is a new approach to the treatment of severe respiratory failure. Gas exchange is separated into oxygen uptake by apneic oxygenation through the natural lungs while CO2 is removed extracorporeally with an artificial organ. The physiological conditions of both processes can thus be optimized. In the course of evolution, a similar bimodal gas exchange has developed during the respiratory transition from aquatic gas exchange to pulmonary gas exchange: In air-breathing fish or amphibia oxygenation is accomplished predominantly via the lungs while CO2 is eliminated via gill or skin. Today's air-breathing vertebrates maintain a considerable respiratory acidosis which has to be compensated for by an appropriate bicarbonate level. This is dependent upon gill reduction and skin armor to prevent evaporation leading to a rise in pCO2 from 3-4 to 40 mmHg and a tenfold increase of serum bicarbonate levels. We believe that the developmental history of respiration justifies the use of a bimodal gas exchange system. It is clinically applied as extracorporeal CO2 removal with membrane lungs (ECCO2-R) or, still under investigation, in a hemodialysis-related procedure (extracorporeal bicarbonate/CO2 removal: ECBicCO2).