PCO2 Gradient Between Inlet and Outlet Blood of Extracorporeal Respiratory Support is a Reliable Marker of CO2 Elimination

Abstract

Our objective was to assess the relationship between the pre-/post-oxygenator gradient of the partial pressure of carbon dioxide (∆EC PCO2; dissolved form) and CO2 elimination under extracorporeal respiratory support. All patients who were treated with veno-venous extracorporeal membrane oxygenation and high-flow extracorporeal CO2 removal in our intensive care unit over 18 months were included. Pre-/post-oxygenator blood gases were collected every 12 h and CO2 elimination was calculated for each pair of samples (pre-/post-oxygenator total carbon dioxide content in blood [ctCO2] × pump flow [extracorporeal pump flow {QEC}]). The relationship between ∆EC PCO2 and CO2 elimination, as well as the origin of CO2 removed. Eighteen patients were analyzed (24 oxygenators and 293 datasets). Each additional unit of ∆EC PCO2 × QEC was associated with an increase in CO2 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 CO2 across the oxygenator was associated with a reduction of 0.63 ml STPD/dl (95% CI, 0.60−0.66) of CO2 combined with water, 0.08 ml STPD/dl (95% CI, 0.07−0.09) of dissolved CO2, and 0.29 ml STPD/dl (95% CI, 0.27−0.31) of CO2 in erythrocytes. The pre-/post-oxygenator PCO2 gradient under extracorporeal respiratory support is thus linearly associated with CO2 elimination; however, most of the CO2 removed comes from combined CO2 in plasma, generating bicarbonate.