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.