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

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 ((V) over dotCO(2ECMO)/(V) over dotCO(2Lung)) would reflect the ratio of respective blood flows and could be used to estimate changes in pulmonary blood flow ((Q) over dot(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) over dotCO(2ECMO) and (V) over dotCO(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) over dot/(Q) over dot) of V-A ECMO was not 1, the (V) over dotCO(2ECMO) was normalized to (V) over dot/(Q) over dot = 1 ((V) over dotCO(2ECMONorm)). Changes in pulmonary blood flow were calculated using the relationship between changes in CO2 elimination and V-A ECMO blood flow ((Q) over dot(ECMO)). (Q) over dot(ECMO) correlated strongly with (V) over dotCO(2ECMONorm) (r(2) 0.95-0.99). (Q) over dot(Lung) correlated well with (V) over dotCO(2Lung) (r(2) 0.65-0.89, P < = 0.002). Absolute (Q) over dot(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) over dot(Lung) (r(2) 0.974-1.000, P = 0.1 to 0.006) for cumulative ECMO flow reductions. In conclusion, (V) over dotCO(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) over dot/(Q) over dot of 1 for ECMO. This approach depends on measurements readily available and may allow immediate assessment of the cardiac output response.