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 O2 exchange, the main focus of this study was on CO2 exchange.

METHODS: A model of the cardiorespiratory system during vv-ECMO was developed, incorporating O2 , CO2 and N2 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 Pa CO2 of varying the input parameters for the cardiorespiratory system and for the oxygenator were examined.

RESULTS: Pa CO2 increased as the shunt fraction in the lung and metabolic CO2 production rose. Changes in haemoglobin and FI O2 had minimal effect on Pa CO2 . The effect of cardiac output on Pa CO2 was variable, depending on the shunt fraction in the lung. Pa CO2 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. Pa CO2 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 Pa CO2 .

CONCLUSIONS: This mathematical model of gas exchange during vv-ECMO found that the main determinants of Pa CO2 during vv-ECMO were pulmonary shunt fraction, metabolic CO2 production, gas flow to the oxygenator and extracorporeal circuit recirculation.