Fig. 1
From: Physiological adaptations during weaning from veno-venous extracorporeal membrane oxygenation
The anatomy and physiology of the extracorporeal circuit, depicted in a femoral–jugular configuration. Blood is drained from the central venous system (CV) via a cannula and centrifugal pump which generates extracorporeal blood flow (ECBF). Pre-oxygenator blood is a mixture of central venous (Cv) and recirculating (QR) blood. It is pumped across hollow fibres within the membrane oxygenator across which there is sweep gas flow (SGF). Post-oxygenator blood passes through the return lumen where it becomes mixed with the CV blood in the right ventricle and pulmonary arteries to form the mixed venous blood (CVmixed) before being distributed through the native pulmonary circulation. Mixed central venous bloods oxygen content (CVmixedO2) will be determined by: the central venous oxygen content (CvO2), the post-oxygenator blood’s oxygen content (Cpost-oxy O2), the extracorporeal blood flow (ECBF), recirculation flow (QR) and overall cardiac output (Qt) according to the formula: CVmixedO2·Qt = [CvO2·(Qt-ECBF + Qr)] + [Cpost-oxyO2·(ECBF-QR)]. Although the ECBF contributes to the calculation of the overall CO2 clearance of the membrane lung [V’CO2ML = (Cpost-oxyCO2 − Cpost-oxyCO2)·ECBF·25)], during the usual V–V ECMO ECBF levels (> 2.5 L) the primary determinant of V’CO2ML is the SGF rate which generates the gradient for CO2 diffusion and resulting difference in pre- and post-oxygenator CO2 content