What Determines the Arterial Partial Pressure of Carbon Dioxide on Venovenous Extracorporeal Membrane Oxygenation?

Rapid reductions in PaCO2 during extracorporeal membrane oxygenation (ECMO) are associated with poor neurologic outcomes. Understanding what factors determine PaCO2 may allow a gradual reduction, potentially improving neurologic outcome. A simple and intuitive arithmetic expression was developed, to describe the interactions between the major factors determining PaCO2 during venovenous ECMO. This expression was tested using a wide range of input parameters from clinically feasible scenarios. The difference between PaCO2 predicted by the arithmetic equation and PaCO2 predicted by a more robust and complex in-silico mathematical model, was <10 mm Hg for more than 95% of the scenarios tested. With no CO2 in the sweep gas, PaCO2 is proportional to metabolic CO2 production and inversely proportional to the "total effective expired ventilation" (sum of alveolar ventilation and oxygenator ventilation). Extracorporeal blood flow has a small effect on PaCO2, which becomes more important at low blood flows and high recirculation fractions. With CO2 in the sweep gas, the increase in PaCO2 is proportional to the concentration of CO2 administered. PaCO2 also depends on the fraction of the total effective expired ventilation provided via the oxygenator. This relationship offers a simple intervention to control PaCO2 using titration of CO2 in the sweep gas.