Gas exchange calculation may estimate changes in pulmonary blood flow during veno-arterial extracorporeal membrane oxygenation in a porcine model
Kaspar F. Bachmann, Matthias Hænggi, Stephan M. Jakob, Jukka Takala, Luciano Gattinoni, David Berger
Abstract
Veno-arterial extracorporeal membrane oxygenation (V-A ECMO) is used as rescue therapy for severe cardiopulmonary failure. We tested whether the ratio of CO 2 elimination at the lung and the V-A ECMO (V˙co 2ECMO /V˙co 2Lung ) would reflect the ratio of respective blood flows and could be used to estimate changes in pulmonary blood flow (Q˙ Lung ), i.e., native cardiac output. Four healthy pigs were centrally cannulated for V-A ECMO. We measured blood flows with an ultrasonic flow probe. V˙co 2ECMO and V˙co 2Lung 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 (V˙/Q˙) of V-A ECMO was not 1, the V˙co 2ECMO was normalized to V˙/Q˙ = 1 (V˙co 2ECMONorm ). Changes in pulmonary blood flow were calculated using the relationship between changes in CO 2 elimination and V-A ECMO blood flow (Q˙ ECMO ). Q˙ ECMO correlated strongly with V˙co 2ECMONorm ( r 2 0.95–0.99). Q˙ Lung correlated well with V˙co 2Lung ( r 2 0.65–0.89, P < = 0.002). Absolute Q˙ Lung 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 Q˙ Lung ( r 2 0.974–1.000, P = 0.1 to 0.006) for cumulative ECMO flow reductions. In conclusion, V˙co 2 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 V˙/Q˙ of 1 for ECMO. This approach depends on measurements readily available and may allow immediate assessment of the cardiac output response.