ECMO (Extracorporeal Membrane Oxygenation)

ECMO (Extracorporeal Membrane Oxygenation) is a technique that can be used in a variety of ways to offer cardiopulmonary support to persons with severe cardiac and/or pulmonary failure. In cases of heart transplants, ECMO can give a “bridge to heart transplant (BTT)” to individuals who are not fit enough to be able to wait for the donated heart. Extracorporeal membrane oxygenation can thus be used as a “bridge to heart transplantation” to offer temporary cardiopulmonary support till a suitable donor heart is found. In this paper, we will focus on what ECMO is, why it is used as a BTT, the procedure, benefits, and risks associated with its use.

What is ECMO?

In general, Extracorporeal Membrane Oxygenation (ECMO) refers to a machine that takes over the role of the heart and/or lungs in critically ill persons. In general, there are two main types of ECMO:

VA-ECMO (Veno-Arterial Extracorporeal Membrane Oxygenation)

This type of ECMO is used to offer support to patients who need both cardiac and pulmonary assistance. Generally, the machine withdraws blood from the venous system, oxygenates it, and then re-introduces it into the arterial system. The cannulation sites include the femoral vein and artery or the carotid artery and femoral vein. VA-ECMO is primarily used for cardiogenic shock, heart failure, and as a bridge to heart transplant.

VV-ECMO (Veno-Venous Extracorporeal Membrane Oxygenation)

This type of ECMO is primarily used to offer support to patients with severe pulmonary failure. Similar to VA-ECMO, VV-ECMO withdraws blood from the venous system, oxygenates it, and then re-introduces it into the venous system. It is primarily indicated for acute respiratory failure.

For use as a bridge to heart transplant, VA-ECMO is primarily used as both cardiac and pulmonary functions need to be supported while awaiting the donor heart.

ECMO Indications as a Bridge to Heart Transplant

Extracorporeal Membrane Oxygenation is primarily used for patients with severe cardiac failure who are not fit enough to wait for a donor heart. The conditions that warrant the use of ECMO as a bridge to heart transplant include:

Cardiogenic shock

This happens when the heart is unable to pump enough blood to meet the body’s needs. This leads to malfunction of other organs as they are not getting enough oxygen in the blood. The most common cause of cardiogenic shock is a heart attack or myocardial infarction.

Acute Myocardial Infarction (MI) or Heart Attack

This happens when the blood flow to the heart is impeded due to blockage in the coronary artery. This leads to damage to part of the heart muscle due to lack of oxygen supply. This may lead to heart failure.

End-Stage Heart Failure

This refers to heart failure that is resistant to treatment. Patients with end-stage heart failure may need heart transplantation.

Post-cardiac Surgery

This includes patients who may present with cardiac arrest, cardiac dysrhythmias, or cardiomyopathy following heart surgery.

Post-cardiopulmonary bypass

This refers to patients who are unable to “awaken” from the cardiopulmonary bypass following heart surgery. Such patients may need to be placed on ECMO as a bridge to heart transplantation.

Severe cardiac arrhythmias

This refers to abnormal heart rhythms that may require the use of ECMO as a bridge to heart transplant.

ECMO: The Procedure and Set Up

The procedure of placing patients on an ECMO machine is very invasive and is mostly performed invasively in the intensive care unit (ICU). The main steps undertaken when placing patients on an ECMO machine include:

Cannulation

This refers to the process of inserting tubes into large veins and arteries. The cannulas are inserted into the femoral vein, internal jugular vein, or subclavian vein and the femoral artery or carotid artery for VA-ECMO; for VV-ECMO, the cannulas are inserted into the femoral vein and artery. With the tubes in place, blood is withdrawn from the venous system into the machine, oxygenated, and then re-introduced into the arterial system or venous system depending on the type of ECMO. The general procedure is depicted below.

ECMO Circuit

The circuit consists of a pump, an oxygenator, and a heater. The oxygenator is a device that oxygenates the blood and removes some of the waste gases such as carbon dioxide in a manner similar to the lungs. The pump helps to circulate the blood in a manner similar to the heart. The heater helps to maintain the temperature of the blood. The machine is connected to the cannulas and then used to provide temporary cardiopulmonary support to the patient.

Monitoring and Management

The patient’s blood pressure, saturation, heart sounds, and the functioning of the ECMO machine are constantly monitored. The ECMO machine also requires frequent maintenance and monitoring to ensure it is working properly and supporting the patient’s cardiopulmonary functions. The patient may also need blood thinners or anti-clotting drugs to prevent the formation of clots in the machine.

Patient Management

The patient is placed on a ventilator and sedated most of the time. Some patients who only need partial assistance can be allowed to wake up if they are able to breathe on their own. After being placed on the machine, the patient will require to stay in the ICU for monitoring and management.

Benefits of Using an ECMO as a Bridge to Heart Transplant

Ability to Stabilize Heart Function

ECMO can be used to stabilize the heart function and circulation in persons presenting with cardiogenic shock or acute heart failure.

Ability to Bridge to a Transplant

ECMO is used as a “bridge to transplant” to offer temporary cardiopulmonary support to persons with severe heart failure.

Ability to Protect Other Organs

ECMO can be used to provide oxygenation and circulation support to protect other organs such as the kidney, liver, and brain from damage. This would prevent the need for multiple transplants.

Ability to Reverse End-stage Heart Failure

ECMO can reverse end-stage heart failure by helping the heart to rest while the machine supports oxygenation and circulation needs.

Ability to Offer Temporary Support

ECMO offers temporary support to persons with severe heart failure while waiting for a donor heart. As a rule, patients are only placed on this machine for a short time, ranging from a few days to a few weeks.

Risks and/or Complications of an ECMO

Although ECMO is a highly effective procedure, it comes with a variety of risks and complications, including:

Infections

ECMO requires cannulation of large veins and arteries which can lead to infections in the body.

Bleeding

ECMO can lead to bleeding complications due to coagulation problems that can arise during the procedure or the use of antiplatelet drugs to prevent clotting in the machine.

Blood Clots

ECMO can lead to the formation of blood clots in the machine thus interfering with its functioning.

Organ Damage

ECMO can cause damage to other organs such as the liver, kidneys, and brain due to poor oxygenation or circulation.

Failure of ECMO Machine

The ECMO machine can fail to function which may result in damage to other organs such as the kidneys, lungs, and liver leading to death.

Cannonball Effect

This is a condition that occurs due to the ECMO machine causing right heart failure. It is characterized by an increase in right ventricular pressure that eventually leads to pulmonary hypertension.

Mechanical Complications and/or Malfunction

ECMO can cause mechanical complications such as displacement of cannulas or malfunction of the machine or clotting of the circuit.

Following ECMO and Heart Transplant

Following ECMO, the patient will be evaluated for a heart transplant. Once a donor heart has been identified, the heart transplant procedure will be performed. Once the donor heart has been transplanted, the ECMO machine can be removed. Following transplant, the patient will also be monitored for other post-transplant complications such as organ rejection and infection. The patient will also need to be on immunosuppressive drugs to prevent organ rejection. The prognosis following heart transplant is generally very good with the survival rate being very high, although there are multiple factors that can affect this.

Prognosis and Survival Rates

Prognosis and survival rates of persons going on ECMO as a bridge to heart transplant vary depending on the general condition of the patient and how long they were on the machine. Generally, the survival rate of persons who go on ECMO is 60-70% at one year. At one year following a heart transplant, the survival rate increases to about 85-90%, while the five-year survival rate is 70-75%.

In general, ECMO is a very powerful life-saving technology that can be used as a bridge to a heart transplant for persons with severe cardiac failure. While it comes with several risks and complications, it can help to provide temporary cardiopulmonary support till a donor heart is found. Overall, it is a very invasive procedure that should only be reserved for patients who need it.