Extracorporeal Membrane Oxygenation (ECMO) is a form of life support that temporarily takes over the function of the heart and/or lungs. It is used when a patient’s organs are too sick to function, providing time for them to rest and heal. The system works by circulating blood outside the body through an external pathway of tubing and machinery known as the ECMO circuit. This circuit adds oxygen to the blood and removes carbon dioxide, mimicking the natural processes of the respiratory and circulatory systems.
Core Components of the Circuit
Cannulae are large, flexible tubes surgically placed into major blood vessels in the neck, groin, or chest. These tubes act as access points, allowing blood to be drained from the body and later returned. The number and placement of cannulae depend on the patient’s needs and the type of support required.
The pump propels the blood through the circuit. Modern systems use centrifugal pumps, which use a rapidly spinning cone to create a vortex. This gently pulls blood from the drainage cannula and pushes it forward through the rest of the circuit.
The oxygenator functions as an artificial lung. Inside, blood flows across a membrane of tiny hollow fibers while a controlled gas mixture is swept across the outside. This interface allows carbon dioxide to diffuse out of the blood and oxygen to diffuse in, turning the dark, deoxygenated blood bright red.
Before the blood is returned to the body, it passes through a heat exchanger. This device, often integrated with the oxygenator, warms the blood to a normal body temperature. Maintaining this temperature is necessary for normal metabolic function and to prevent hypothermia.
The Path of Blood Flow
Deoxygenated blood is drained from the patient through a venous cannula and pulled into the circuit by the pump. The pump then actively propels the blood forward through the oxygenator for gas exchange. After being oxygenated, the blood moves through the heat exchanger to be warmed to a normal body temperature.
This step prevents a dangerous drop in the patient’s core body temperature. Finally, the warm, oxygenated blood is returned to the patient through a separate cannula. This completes the circuit and allows the heart and lungs to rest.
Types of ECMO Configurations
ECMO circuits have different configurations depending on whether the patient requires support for their lungs, heart, or both. The two primary types are Veno-Venous (VV) and Veno-Arterial (VA) ECMO. The names refer to the blood vessels used for draining and returning blood.
Veno-Venous ECMO is used for patients with severe respiratory failure but adequate heart function. Blood is drained from a large vein and, after being oxygenated, is returned to another large vein. The patient’s own heart is still responsible for pumping the newly oxygenated blood throughout the body, so VV ECMO acts only as an artificial lung.
Veno-Arterial ECMO provides support for both the heart and lungs and is used for patients in cardiogenic shock or with combined cardiopulmonary failure. Blood is drained from a large vein and returned to a major artery. By returning oxygenated blood to the arterial system, the circuit bypasses both the heart and lungs, taking over the work of both organs.
Monitoring the Circuit and Patient
The operation of an ECMO circuit requires continuous monitoring by a specialized medical team to ensure safety and effectiveness. Management involves several checks of both the machine and the patient:
- Pressures at various points in the tubing are monitored to detect obstructions or kinks that could impede blood flow.
- Blood samples are taken from the circuit for analysis to confirm the oxygenator is working effectively and to allow for adjustments to oxygen and carbon dioxide levels.
- The blood’s clotting ability is carefully monitored because patients receive anticoagulants, like heparin, to prevent clots from forming within the circuit.
- The patient’s heart rate, blood pressure, and other vital signs are tracked to assess how they are responding to ECMO support and to guide ongoing care.