Gas Exchange Calculation may Estimate Changes in Pulmonary Blood Flow during Veno-Arterial Extracorporeal Membrane Oxygenation in a Porcine model.

Background

Veno-arterial extracorporeal membrane oxygenation (VA-ECMO) is used as rescue for severe cardiopulmonary failure. We tested whether the ratio of CO2 elimination at the lung and the ECMO (VCO2_ECMO/VCO2_Lung) 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.

Methods

Four healthy pigs were centrally cannulated for VA-ECMO. We measured blood flows with an ultrasonic flow probes. VCO2_ECMO and VCO2_Lung were calculated from sidestream capnographs under constant pulmonary ventilation during ECMO weaning with changing sweep gas and/or ECMO blood flow. If ventilation/perfusion (V/Q) ratio of ECMO was not one, the VCO2_ECMO was normalized to V/Q=1 (VCO2_ECMONORM). Changes in pulmonary blood flow were calculated using the relationship between changes in CO2 elimination and ECMO blood flow.

Results

Q_ECMO correlated strongly with VCO2_ECMONORM (r² 0.95 – 0.99). Q_LUNG correlated well with VCO2_LUNG (r² 0.65 – 0.89, p<=0.002). Absolute Q_Lung could not be calculated in a non-steady state. Calculated pulmonary blood flow changes had a bias of 76 (-266 to 418) ml/min and correlated with measured Q_LUNG (r² 0.974 – 1.000, p = 0.1 to 0.006) for cumulative ECMO flow reductions.

Conclusions

VCO2 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 a 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.