Gas exchange calculation may estimate changes in pulmonary blood flow during veno-arterial extracorporeal membrane oxygenation in a porcine model.

Abstract

Veno-arterial extracorporeal membrane oxygenation (V-A ECMO) is used as rescue therapy for severe cardiopulmonary failure. We tested whether the ratio of CO2 elimination at the lung and the V-A ECMO (V̇co2ECMO/V̇co2Lung) would reflect the ratio of respective blood flows and could be used to estimate changes in pulmonary blood flow (Q̇Lung), i.e., native cardiac output. Four healthy pigs were centrally cannulated for V-A ECMO. We measured blood flows with an ultrasonic flow probe. V̇co2ECMO and V̇co2Lung were calculated from sidestream capnographs under constant pulmonary ventilation during V-A ECMO weaning with changing sweep gas and/or V-A ECMO blood flow. If ventilation-to-perfusion ratio (V̇/Q̇) of V-A ECMO was not 1, the V̇co2ECMO was normalized to V̇/Q̇ = 1 (V̇co2ECMONorm). Changes in pulmonary blood flow were calculated using the relationship between changes in CO2 elimination and V-A ECMO blood flow (Q̇ECMO). Q̇ECMO correlated strongly with V̇co2ECMONorm (r2 0.95–0.99). Q̇Lung correlated well with V̇co2Lung (r2 0.65–0.89, P < = 0.002). Absolute Q̇Lung could not be calculated in a nonsteady state. Calculated pulmonary blood flow changes had a bias of 76 (−266 to 418) mL/min and correlated with measured Q̇Lung (r2 0.974–1.000, P = 0.1 to 0.006) for cumulative ECMO flow reductions. In conclusion, V̇co2 of the lung correlated strongly with pulmonary blood flow. Our model could predict pulmonary blood flow changes within clinically acceptable margins of error. The prediction is made possible with normalization to a V̇/Q̇ of 1 for ECMO. This approach depends on measurements readily available and may allow immediate assessment of the cardiac output response.