PCO 2 Gradient Between Inlet and Outlet Blood of Extracorporeal Respiratory Support Is a Reliable Marker of CO 2 Elimination.

Our objective was to assess the relationship between the pre-/post-oxygenator gradient of the partial pressure of carbon dioxide (∆ EC PCO 2 ; dissolved form) and CO 2 elimination under extracorporeal respiratory support. All patients who were treated with veno-venous extracorporeal membrane oxygenation and high-flow extracorporeal CO 2 removal in our intensive care unit over 18 months were included. Pre-/post-oxygenator blood gases were collected every 12 h and CO 2 elimination was calculated for each pair of samples (pre-/post-oxygenator total carbon dioxide content in blood [ ct CO 2 ] × pump flow [extracorporeal pump flow {Q EC }]). The relationship between ∆ EC PCO 2 and CO 2 elimination, as well as the origin of CO 2 removed. Eighteen patients were analyzed (24 oxygenators and 293 datasets). Each additional unit of ∆ EC PCO 2 × Q EC was associated with an increase in CO 2 elimination of 5.2 ml (95% confidence interval [CI], 4.7-5.6 ml; p < 0.001). Each reduction of 1 ml STPD/dl of CO 2 across the oxygenator was associated with a reduction of 0.63 ml STPD/dl (95% CI, 0.60-0.66) of CO 2 combined with water, 0.08 ml STPD/dl (95% CI, 0.07-0.09) of dissolved CO 2 , and 0.29 ml STPD/dl (95% CI, 0.27-0.31) of CO 2 in erythrocytes. The pre-/post-oxygenator PCO 2 gradient under extracorporeal respiratory support is thus linearly associated with CO 2 elimination; however, most of the CO 2 removed comes from combined CO 2 in plasma, generating bicarbonate.