Extracorporeal carbon dioxide removal requirements for ultraprotective mechanical ventilation: Mathematical model...

Extracorporeal carbon dioxide (CO₂ ) removal (ECCO₂ R) facilitates the use of low tidal volumes during protective or ultraprotective mechanical ventilation when managing patients with acute respiratory distress syndrome (ARDS); however, the rate of ECCO₂ R required to avoid hypercapnia remains unclear. We calculated ECCO₂ R rate requirements to maintain arterial partial pressure of CO₂ (PaCO₂ ) at clinically desirable levels in mechanically ventilated ARDS patients using a six-compartment mathematical model of CO₂ and oxygen (O₂ ) biochemistry and whole-body transport with the inclusion of an ECCO₂ R device for extracorporeal veno-venous removal of CO₂ . The model assumes steady state conditions. Model compartments were lung capillary blood, arterial blood, venous blood, post-ECCO₂ R venous blood, interstitial fluid and tissue cells, with CO₂ and O₂ distribution within each compartment; biochemistry included equilibrium among bicarbonate and non-bicarbonate buffers and CO₂ and O₂ binding to hemoglobin to elucidate Bohr and Haldane effects. O₂ consumption and CO₂ production rates were assumed proportional to predicted body weight (PBW) and adjusted to achieve reported arterial partial pressure of O₂ and a PaCO₂ level of 46 mmHg at a tidal volume of 7.6 mL/kg PBW in the absence of an ECCO₂ R device based on average data from LUNG SAFE. Model calculations showed that ECCO₂ R rates required to achieve mild permissive hypercapnia (PaCO₂ of 46 mmHg) at a ventilation frequency or respiratory rate of 20.8/min during mechanical ventilation increased when tidal volumes decreased from 7.6 to 3 mL/kg PBW. Higher ECCO2R rates were required to achieve normocapnia (PaCO2 of 40 mmHg). Model calculations also showed that required ECCO2R rates were lower when ventilation frequencies were increased from 20.8/min to 26/min. The current mathematical model predicts that ECCO2R rates resulting in clinically desirable PaCO2 levels at tidal volumes of 5-6 mL/kg PBW can likely be achieved in mechanically ventilated ARDS patients with current technologies; use of ultraprotective tidal volumes (3-4 mL/kg PBW) may be challenging unless high mechanical ventilation frequencies are used.