ECMO for Respiratory Failure
What is ECMO?
Extracorporeal Membrane Oxygenation (ECMO) is a mechanical system that temporarily takes over the function of the lungs in patients with acute respiratory failure. It can also provide cardiac support until the patient recovers or is able to receive further treatment.
In most approaches to ECMO for respiratory failure, a catheter is placed in a central vein. A mechanical pump draws blood from the vein into the ECMO circuit, where the blood passes into a chamber (referred to as an "oxygenator"). The oxygenator allows oxygen to diffuse into the blood and carbon dioxide to diffuse out of the blood. The blood may be warmed or cooled as needed, and returned either to a central vein ("venovenous ECMO") for respiratory support or to an artery ("venoarterial ECMO") for combined respiratory and cardiac support.
How ECMO Works
The ECMO pump delivers venous blood to the oxygenator. This device is divided into two separate chambers by a semipermeable membrane. The venous blood enters the oxygenator and travels along one side of the membrane (the blood side), while fresh gas, known as sweep gas, is delivered to the other side (the gas side). Gas exchange (oxygen uptake and CO2 elimination) take place across the membrane. The oxygenated blood is then reinfused into the patient’s venous or arterial systems. The composition of the gas on the gas side of the oxygenator membrane is determined by adjustment of a blender that mixes room air with oxygen for delivery into the oxygenator.
Single-site approach to venovenous ECMO cannulation
A dual-lumen cannula is inserted in the internal jugular vein (extending through the right atrium and into the inferior vena cava). Venous blood is withdrawn through one “drainage” lumen with ports in both the superior and the inferior vena cava. Reinfusion of oxygenated blood occurs through the second lumen, with a port situated in the right atrium. Inset: The two ports of the “drainage” lumen are situated in the superior and inferior vena cavae, distant from the reinfusion port. The reinfusion port is positioned so that oxygenated blood is directed across the tricuspid valve and directly into the right ventricle. This arrangement significantly reduces recirculation of blood when the cannula is properly positioned.
Two-site approach to venovenous ECMO cannulation:
Cannulae are inserted in the internal jugular vein (extending to the right atrium) and the femoral vein (extending into the inferior vena cava). When the ECMO circuit is connected, venous blood is withdrawn via the femoral venous “drainage” cannula into the pump, passes through the oxygenator where gas exchange takes place, then is reinfused into the venous system via the internal jugular venous cannula. Inset: With the two-site approach, a portion of the oxygenated blood returning via the internal jugular venous cannula can be drawn directly into the femoral venous cannula without passing through the systemic circulation. Blood that is recirculated in this fashion does not contribute to systemic oxygenation.