Extracorporeal Membrane Oxygenation (ECMO)

Cardiovascular Devices

Chiara Tarenghi

When a patient suffers from critical conditions which prevent to lungs or heart to work properly and to grant adequate blood flow and oxygen supply, either because of congenital disease, surgery or other reasons (severe pneumonia, sepsis, heart malformations, congenital diaphragmatic hernia, Respiratory Distress Syndrome ), a mechanical circulatory support is needed to take over temporarily the cardiopulmonary functions. These devices are known as extracorporeal life support (ECLS).

ECMO is an invasive, non-therapeutic technique, consists essentially of an artificial external lung, a membrane oxygenator, which reproduces the physiological function of the gas exchange in the lungs (the blood releases CO2 and gains O2 according to the partial pressure gradient). It has successfully been employed as a bridge to recovery, cardiac transplantation, or implantation of a ventricular assist device, and it has met great application in paediatric surgery field (i.e. premature infants). It grants a steady amount of oxygen to the body while resting  and recover the lungs and heart.

The device pumps blood from the body through the oxygenator with a roller pump (or a centrifugal pump, which causes major haemolysis though). There are three types of oxygenator commercially available: bubble oxygenator ( the gas is bubbled into blood), membrane oxygenator (allows exchanges of diffusion through a semipermeable membrane of silicon rubber that completely separates gas and blood), and hollow - fibres oxygenator (fibres of PVC or PTFE hollow inside and semipermeable to gases).

The blood is then warmed up through a heat exchanger (a coiled tubing containing warm water flowing counter current) and is injected back into the body. Each section of the machine is connected through cannulae. The blood flow is monitored continuamente to avoid the presence of microscopic air bubbles, thrombi and other emboli. Hazardous occlusions of tubing or thrombosis are detected by pressure monitors.

Before letting blood flow through the tubes, the latter are primed by flushing with CO2 and then with crystalloid prime, albumin (to limit platelets adherence) and finally blood. The flow and amount of oxygenation is adjusted according to the needs of the patient. As the heart function improves, the flow is lowered. Blood volume can be assessed by urine output and central venous and mean arterial pressure. After a few days of mechanical assistance, the device is usually removed.

ECMO is a valid alternative to pulmonary ventilation, since the latter pushes oxygen into patients' lungs, regulating its amount and pressure, but high gas pressure could further damage lungs. Risks associated to ECMO include blood clot formation (which is prevented through anticoagulant like heparin) and, related to this, haemorrhage, infection, transfusions problems (thromboembolism and related stroke or ischaemia), mechanical failures.

There are different ways of perfusion :

ECMO VENO ARTERIAL (VA) : It requires three tubes to be inserted, venous blood is removed from the right atrium or vena cava, passing through the  jugular or femoral vein, and returned to ascending aorta (central ECMO) or femoral artery (peripheral artery).

It is a hemodynamic and respiratory support for patients suffering from cardiogenic shock, sepsis, severe pneumonia, persistent pulmonary hypertension.

Advantages: Low cost, can be applied to children and adults, support double heart and lungs. Disadvantages: It is more invasive than VV ECMO is a complex circuit, can cause embolism, neurological complications and require a sternotomy.

ECMO VENO VENOUS  (VV):  In this case two tubes are inserted into the largest veins as jugular or femoral, but most often a single Y shaped cannula is inserted in the both veins. Blood is drained from the vena cava , blood is returned through the right atrium into the tricuspid valve.

It is only a respiratory support for patients suffering from severe respiratory failure, asthmas, persistent pulmonary hypertension. Disadvantages: No cardiac support.

ARTERIO VENUS  (AV):uses patient's arterial pressure to pump the blood from artery to vein, it can be used only as a short-term support.