ECMO (Extracorporeal Membrane Oxygenation) is a life-support technique used to provide cardiopulmonary support to patients with severe heart and/or lung failure. In the context of heart transplantation, ECMO is often used as a "bridge to heart transplant" (BTT) for patients who are in acute heart failure or cardiogenic shock and cannot survive the wait for a donor heart without support. ECMO temporarily takes over the function of the heart and lungs, allowing the organs to rest and providing time for stabilization or until a suitable donor heart becomes available.

What is ECMO?

ECMO is a machine that temporarily takes over the function of the heart and/or lungs in patients who are critically ill. There are two main types of ECMO:

1. VA-ECMO (Veno-Arterial ECMO):
• This type of ECMO is used when both cardiac and pulmonary support are needed.
• It draws blood from a large vein (usually the femoral vein), oxygenates it outside the body, and then returns it to the body through an artery (usually the femoral artery or the carotid artery). This provides support for both heart and lung function.
• VA-ECMO is commonly used in patients with severe cardiogenic shock, heart failure, or those awaiting heart transplantation.
2. VV-ECMO (Veno-Venous ECMO):
• This type of ECMO is used when only pulmonary support is needed.
• It draws blood from a large vein, oxygenates it, and returns it to the body through another vein. This helps with oxygenation when the heart is still functioning, but the lungs are not able to provide adequate gas exchange.

For heart transplant patients, VA-ECMO is typically used because it supports both the heart and lungs while the patient is awaiting a donor heart.

Indications for ECMO as a Bridge to Heart Transplant

ECMO is typically used for patients who are in severe heart failure and are not stable enough to wait for a heart transplant without mechanical support. Common indications for using ECMO as a bridge to heart transplant include:

1. Cardiogenic Shock:
• A life-threatening condition where the heart is unable to pump sufficient blood to meet the body’s needs, leading to multi-organ failure.
2. Acute Myocardial Infarction (MI) or Heart Attack:
• Severe heart attack leading to massive damage to the heart muscle and resulting in acute heart failure.
3. End-Stage Heart Failure:
• Patients with end-stage heart failure who have failed medical therapy and cannot wait without mechanical circulatory support.
4. Post-Cardiac Surgery Complications:
• Patients who suffer complications after heart surgery, such as cardiac arrest, severe arrhythmias, or low cardiac output syndrome.
5. Post-Cardiopulmonary Bypass:
• Some patients after major heart surgery, such as a valve replacement or coronary artery bypass, may need ECMO if their heart fails to resume normal function.
6. Severe Cardiac Arrhythmias:
• Life-threatening arrhythmias that cannot be controlled with medications or electrical cardioversion may require ECMO to stabilize the patient.
7. Refractory Ventricular Arrhythmias:
• Patients with ventricular fibrillation or sustained ventricular tachycardia who fail conventional treatments and need time to stabilize before a transplant.

ECMO Procedure and Setup

The ECMO procedure is highly invasive and is typically performed in an intensive care setting (ICU) or during surgery. The steps for ECMO initiation include:

1. Vascular Access:
• Cannulas (tubes) are inserted into the patient’s veins and/or arteries to allow blood to be drawn out of the body and returned after being oxygenated. The cannulas are placed into large veins, usually the femoral vein, internal jugular vein, or subclavian vein, and large arteries such as the femoral artery or carotid artery.
2. ECMO Circuit:
• Blood is drawn through the cannulas, pumped through an oxygenator (which adds oxygen and removes carbon dioxide), and then returned to the patient’s body.
• The ECMO machine consists of a pump, oxygenator, heat exchanger, and cannulas. The oxygenator mimics the function of the lungs, while the pump supports the circulatory system, essentially functioning as a heart.
3. Monitoring and Management:
• The patient is continuously monitored with various parameters, including blood pressure, oxygen levels, heart function, and the performance of the ECMO circuit. The ECMO machine needs to be closely regulated to ensure the blood is oxygenated properly and that blood flow is sufficient.
• Anticoagulation (blood-thinning medication) is often required to prevent blood clots from forming in the ECMO circuit.
4. Patient Management:
• ECMO patients are usually sedated and intubated (on a ventilator) to manage the mechanical support. In some cases, patients may be awake and alert if only limited support is needed.

Benefits of ECMO as a Bridge to Heart Transplant

1. Stabilization of Heart Function:
• ECMO provides immediate circulatory support for patients in cardiogenic shock or acute heart failure, stabilizing their condition and improving organ perfusion until a donor heart is available.
2. Allows Time for Transplantation:
• By stabilizing the patient, ECMO allows time for a heart transplant to be arranged, offering a bridge for the patient to survive while waiting for a donor heart.
3. Organ Protection:
• ECMO helps maintain blood flow and oxygenation to vital organs, including the kidneys, liver, and brain, preventing further organ failure while waiting for a transplant.
4. Reversing End-Stage Heart Failure:
• For some patients, ECMO allows the heart to rest and recover temporarily, improving heart function and potentially making them more suitable candidates for heart transplantation.
5. Temporary Support:
• ECMO is a short-term solution, allowing the patient to wait for a donor heart. It is not a long-term therapy and typically is used for a few days to a few weeks.

Risks and Complications of ECMO

While ECMO can be life-saving, it carries a range of risks and complications:

1. Infection:
• The presence of invasive cannulas and the need for continuous monitoring can increase the risk of infection, particularly bloodstream infections.
2. Bleeding:
• Patients on ECMO are typically given anticoagulants to prevent blood clots in the ECMO circuit, but this increases the risk of bleeding, particularly from the cannulation sites or other parts of the body.
3. Blood Clots:
• The ECMO machine can also create a risk of forming blood clots within the circuit, which can block the flow of blood and cause complications such as stroke or organ failure.
4. Organ Damage:
• Prolonged ECMO use can lead to kidney damage, liver dysfunction, or neurological complications due to reduced perfusion, especially if the ECMO machine is not adequately managed.
5. Device Failure:
• Although rare, ECMO machines can fail, which could result in severe complications, including organ damage or death if not quickly addressed.
6. Cannonball Effect:
• This is a term used to describe a condition in which patients on ECMO develop right heart failure due to the sudden increase in pressure in the circulatory system caused by ECMO.
7. Mechanical Complications:
• Cannula displacement, pump failure, and circuit clotting can all cause significant issues, and ECMO requires constant monitoring and frequent adjustments.

Post-ECMO and Heart Transplant

After ECMO has stabilized the patient, the goal is to proceed with a heart transplant once a suitable donor heart becomes available. The steps following ECMO include:

1. Heart Transplant Surgery:
• Once a donor heart is found, the patient will undergo heart transplant surgery. The surgeon will remove the patient’s heart and implant the donor heart.
2. ECMO Removal:
• ECMO is removed during the heart transplant procedure. After the new heart is connected, ECMO support is discontinued.
3. Post-Transplant Care:
• After transplant, the patient will be monitored in the ICU for rejection, infection, or other complications. Immunosuppressive medications will be started to prevent rejection of the new heart.
4. Recovery and Rehabilitation:
• The recovery process after heart transplant involves careful management, including cardiac rehabilitation, to regain strength and function.

Prognosis and Survival Rates

The survival rates for patients who undergo ECMO as a bridge to heart transplant vary based on the patient's underlying health, the length of ECMO support, and any complications that arise. Generally:

• One-year survival rate for patients on ECMO is around 60-70%, depending on the complexity of the heart failure and ECMO duration.
• After a successful heart transplant, the survival rate for the first year is approximately 85-90%.
• Long-term survival after heart transplant (5 years or more) is around 70-75%.

Conclusion

ECMO is a powerful life-saving technology that can be used as a bridge to heart transplant for patients in severe heart failure. It provides temporary support for the heart and lungs, allowing patients to survive while awaiting a donor heart. However, ECMO carries risks, including infection, bleeding, and mechanical complications, and requires careful monitoring and management. When used appropriately, ECMO can significantly improve outcomes for patients awaiting heart transplantation.