A mathematical model of CO 2 , O 2 and N 2 exchange during venovenous extracorporeal membrane oxygenation

Background Venovenous extracorporeal membrane oxygenation (vv-ECMO) is an effective treatment for severe respiratory failure. The interaction between the cardiorespiratory system and the oxygenator can be explored with mathematical models. Understanding the physiology will help the clinician optimise therapy. As others have examined O 2 exchange, the main focus of this study was on CO 2 exchange. Methods A model of the cardiorespiratory system during vv-ECMO was developed, incorporating O 2 , CO 2 and N 2 exchange in both the lung and the oxygenator. We modelled lungs with shunt fractions varying from 0 to 1, covering the plausible range from normal lung to severe acute respiratory distress syndrome. The effects on P a CO 2 of varying the input parameters for the cardiorespiratory system and for the oxygenator were examined. Results P a CO 2 increased as the shunt fraction in the lung and metabolic CO 2 production rose. Changes in haemoglobin and F I O 2 had minimal effect on P a CO 2 . The effect of cardiac output on P a CO 2 was variable, depending on the shunt fraction in the lung. P a CO 2 decreased as extracorporeal circuit blood flow was increased, but the changes were relatively small in the range used clinically for vv-ECMO of > 2 l/min. P a CO 2 decreased as gas flow to the oxygenator rose and increased with recirculation. The oxygen fraction of gas flow to the oxygenator had minimal effect on P a CO 2 . Conclusions This mathematical model of gas exchange during vv-ECMO found that the main determinants of P a CO 2 during vv-ECMO were pulmonary shunt fraction, metabolic CO 2 production, gas flow to the oxygenator and extracorporeal circuit recirculation.