Paradigm depicting the mechanism of differential oxygen return. (A) Differential venous oxygen return between the IVC and the SVC exists in IVC-FA. Oxygen-rich blood is drained back to the ECMO circuit by the drainage cannula at the IVC, and the oxygen-poor blood from the SVC enters the heart and perfuses the upper body, which leads to differential hypoxia. (B) In SVC-FA, oxygen-poor blood in the SVC is drained to the ECMO circuit, whereas the oxygen-rich blood from the IVC enters the RA. (C) In IVC-CA, the oxygenated blood from the ECMO circuit is directly supplied to the whole body. (D) In FA-IJV, a certain amount of oxygenated blood is shunted into the SVC to improve upper body oxygenation. Differential venous oxygen return is attenuated in B, C, D. ECMO: extracorporeal membrane oxygenation; FA-IJV: an additional return cannula was added into the internal jugular vein on the basis of femoral veno-arterial extracorporeal membrane oxygenation; IVC-CA: a drainage cannula was inserted into the inferior vena cava and a return cannula was inserted into the carotid artery; IVC-FA: a drainage cannula was placed into the inferior vena cava through the femoral vein and a return cannula was inserted into the femoral artery; RA: right atrium; SO2: oxygen saturation; SVC-FA: a drainage cannula was placed into the superior vena cava through the femoral vein and a return cannula was placed into the femoral artery.