Impact of sweep gas flow on extracorporeal CO(2) removal (ECCO(2)R)

BACKGROUND: Veno-venous extracorporeal carbon dioxide (CO(2)) removal (vv-ECCO(2)R) is increasingly being used in the setting of acute respiratory failure. Blood flow rates range in clinical practice from 200 mL/min to more than 1500 mL/min, and sweep gas flow rates range from less than 1 to more than 10 L/min. The present porcine model study was aimed at determining the impact of varying sweep gas flow rates on CO(2) removal under different blood flow conditions and membrane lung surface areas. METHODS: Two different membrane lungs, with surface areas of 0.4 and 0.8m(2), were used in nine pigs with experimentally-induced hypercapnia. During each experiment, the blood flow was increased stepwise from 300 to 900 mL/min, with further increases up to 1800 mL/min with the larger membrane lung in steps of 300 mL/min. Sweep gas was titrated under each condition from 2 to 8 L/min in steps of 2 L/min. Extracorporeal CO(2) elimination was normalized to a PaCO(2) of 45 mmHg before the membrane lung. RESULTS: Reversal of hypercapnia was only feasible when blood flow rates above 900 mL/min were used with a membrane lung surface area of at least 0.8m(2). The membrane lung with a surface of 0.4m(2) allowed a maximum normalized CO(2) elimination rate of 41 ± 6 mL/min with 8 L/min sweep gas flow and 900 mL blood flow/min. The increase in sweep gas flow from 2 to 8 L/min increased normalized CO(2) elimination from 35 ± 5 to 41 ± 6 with 900 mL blood flow/min, whereas with lower blood flow rates, any increase was less effective, levelling out at 4 L sweep gas flow/min. The membrane lung with a surface area of 0.8m(2) allowed a maximum normalized CO(2) elimination rate of 101 ± 12 mL/min with increasing influence of sweep gas flow. The delta of normalized CO(2) elimination increased from 4 ± 2 to 26 ± 7 mL/min with blood flow rates being increased from 300 to 1800 mL/min, respectively. CONCLUSIONS: The influence of sweep gas flow on the CO(2) removal capacity of ECCO(2)R systems depends predominantly on blood flow rate and membrane lung surface area. In this model, considerable CO(2) removal occurred only with the larger membrane lung surface of 0.8m(2) and when blood flow rates of ≥ 900 mL/min were used.