Overview of Extracorporeal Membrane Oxygenation Anticoagulation
Extracorporeal Membrane Oxygenation (ECMO) is a medical treatment that’s becoming more widespread globally. Essentially, it is a technique used to support the heart and lungs in critically ill patients. Its main aim is to ensure that the patient’s body is receiving enough oxygen, and it gives time for the heart and lungs to heal. The ECMO treatment can also be used as a waiting phase for patients who will be undergoing a more definite treatment like organ transplantation.
There are two methods of ECMO: veno-arterial and veno-venous. Both these types help support the lungs, but veno-arterial ECMO also supplies extra support to the heart, making sure blood circulates properly around the body.
Like any other medical procedure, ECMO has its risks. The two most common complications are hemorrhage (bleeding) and thromboembolism (the formation of a clot in a blood vessel which can block blood flow). However, the danger of bleeding outweighs thromboembolism (where blood clots occur), which comes in as the second major complication reported in patients on ECMO.
Anatomy and Physiology of Extracorporeal Membrane Oxygenation Anticoagulation
Extracorporeal Membrane Oxygenation (ECMO) is a complicated life-support method often used for critically sick patients. Its setup, also known as an ECMO circuit, varies based on the patient’s specific needs.
There are two main ECMO configurations: Veno-venous (VV) and Veno-arterial.
Veno-venous (VV) ECMO is commonly started by inserting a large tube into the femoral vein – a major vein in the leg. The size of the tube depends on the patient’s age and size. Another tube is then placed in the internal jugular vein, located in the neck and it ends where the heart and the major vein meet. Doctors often use an ultrasound scan to ensure the correct placement of these tubes. Another method uses a special double-tube, which has 3 ports and is designed to process and return deoxygenated blood back to the heart. This method is advantageous as it requires making a single incision, reducing the chances of bleeding and allowing the patient to move with relative ease which can help them potentially actively rehabilitate and avoid muscular weakness typically associated with long stays in the Intensive Care Unit (ICU).
Veno-arterial ECMO, unlike VV ECMO which supports the lungs, provides support for both the heart and the lungs. Using this type, the ECMO device takes over the heart and lungs’ functions completely. To implement this, a tube is placed in either the internal jugular vein or the femoral vein to drain deoxygenated blood. Oxygenated blood is returned via a tube placed in the femoral, axillary (located near the armpit), or carotid artery (located in the neck). The most commonly used site for this is the femoral artery. However, using the axillary artery is often more beneficial as it provides better blood flow, reduces the chance of limb blood supply issues, promotes easier patient mobility, and increases oxygen supply to the brain. The challenge with the axillary method is that it requires a surgical procedure to create access. The carotid artery method is generally used for children weighing less than 15 kg.
Another, albeit less used type of ECMO, is Central cannulation. This more invasive method poses higher risks of bleeding and infection as it requires a cut through the breastbone (sternotomy). The tubes are placed directly in the right atrium of the heart and the aorta, securely fixed to the patient to prevent accidental removal. Its advantage is improved venous drainage and better arterial blood flow, thereby reducing pressure in the left ventricle of the heart.
The ECMO circuit setup includes an oxygenator (a device that adds oxygen to the blood), a heat exchanger, a pump, and various cannulas – parts used to connect a patient’s blood vessels with the ECMO device. The oxygenator is prone to blood clot formation and therefore needs to be monitored frequently. Severe clots are often dealt with by changing the entire ECMO circuit. Blood clot formation is primarily due to the blood’s exposure to the artificial surfaces of the ECMO machine, leading to a release of specific proteins that trigger the formation of clots. Also, the pump used in the ECMO machine can sometimes damage the red blood cells leading to their breakdown. However, newer pump designs are better and are now used more frequently.
Why do People Need Extracorporeal Membrane Oxygenation Anticoagulation
If someone’s lungs are severely affected and not responding to respiratory support, they may need a procedure known as veno-venous (VV) ECMO. This type of ECMO is used when only the lungs are failing and the heart is functioning normally. Conditions that might require VV ECMO include extremely low oxygen levels in the blood, despite providing extra oxygen (PaO2/FiO2 less than 100mmHg or a Murray score of 3-4). For children, the oxygenation index (OI) is used instead of the PaO2/FiO2 ratio, but there isn’t an agreed-upon threshold to start ECMO. Healthcare providers generally agree to watch for severe PARDS, a condition that affects the lungs, signaled by an OI greater than 16.
VV ECMO might also be used if there’s a buildup of carbon dioxide in the blood, despite mechanical ventilation, or in immediate respiratory collapse due to suffocation or severe asthma. It’s also used if a patient is awaiting a lung transplant and their condition worsens.
A different type of ECMO, called veno-arterial (VA) ECMO, is used when both the heart and lungs are failing. It can help in conditions like heart attack, serious heart failure, heart muscle disease, or difficulty recovering after heart surgery. It can also be used as a transition to using a ventricular assist device (a mechanical pump that supports heart function and blood flow) or before a heart transplant.
When a Person Should Avoid Extracorporeal Membrane Oxygenation Anticoagulation
There aren’t any firm restrictions against using ECMO (Extracorporeal Membrane Oxygenation, a treatment which provides heart and lung support), and doctors look at each person’s circumstances individually. However, certain aspects may affect the decision to start ECMO. These can include having a weaker immune system, suffering an irreversible brain injury, dealing with an end-stage cancer, or being advanced in age. All of these factors will be considered thoroughly before deciding an ECMO treatment.
How is Extracorporeal Membrane Oxygenation Anticoagulation performed
ECMO, or extracorporeal membrane oxygenation, is a medical technique where blood is pumped outside of the body to a heart-lung machine that removes carbon dioxide and sends oxygen-filled blood back to tissues in the body. The process of placing these tubes, or ‘cannulas’, in your body can be done in three ways, depending on your specific situation. It can be done through a non-surgical method called Seldinger’s technique, a fusion of non-surgical and surgical method called open cut-down Seldinger, or a fully surgical method that connects a graft known as end-to-side anastomosis. When the procedure is performed centrally, it means that it is done directly into the large blood vessels of the heart and requires a surgical incision in the chest, known as a sternotomy.
When blood comes into contact with the ECMO machine, it can cause an inflammatory response. This means that parts of your immune system, like platelets, neutrophils, and different pathways in your blood that help clotting, get activated.
Neutrophils, a type of white blood cells, produce substances called cytokines in response to inflammation.
While on ECMO, a medication called unfractionated heparin (UFH) is commonly used to prevent blood clots. UFH works by binding to a substance in your blood called antithrombin and enhancing its ability to stop clot formation a thousand or two thousand times stronger.
The dose of UFH used varies and is mostly guided by a test called ACT (activated clotting time), which measures how long your blood takes to clot, although this can vary from hospital to hospital. It’s good to note that apart from UFH, your doctor will also monitor the levels of antithrombin in your blood, and might adjust your dose of heparin or give you antithrombin supplements based on these levels.
Another group of medicines to prevent blood clots that might be used are called direct thrombin inhibitors (DTIs), which work by directly blocking the action of a protein involved in clot formation called thrombin. Two examples are Bivalirudin and Argatroban. However, unlike heparin, these medications do not have a reversal agent, meaning that if severe bleeding occurs, there is no specific antidote to stop their effects.
During ECMO, various tests monitor the effectiveness of the blood-thinning medications and the overall blood clotting profile of your body. These include the activated clotting time (ACT), the activated partial thromboplastin time (aPPT), which measures how long it takes for your blood to clot, the anti-factor Xa, which specifically measures how well heparin is working, and the TEG or ROTEM test, which provide a more detailed picture about your blood clotting process.
Your blood parameters, including levels of UFH, antithrombin, hemoglobin, platelets, and fibrinogen will be monitored daily while on ECMO, and adjustments to your treatment may be made based on these results.
Possible Complications of Extracorporeal Membrane Oxygenation Anticoagulation
For patients on Extracorporeal Membrane Oxygenation (ECMO), a type of life support machine which adds oxygen to the blood and pumps it through the body, several serious complications can occur. For patients on venovenous ECMO, where the blood is drawn from and returned to the veins, the main issue could be inadequate flow. This situation can occur frequently in patients with severe infections, as their bodies can go into overdrive, making it difficult for the ECMO machine to oxygenate all the blood.
Veno-arterial (VA) ECMO, where the blood is drawn from a vein and returned to an artery, can also cause complications. When the femoral artery in the leg is used for VA ECMO, backward flow can occur in the circulatory system, blocking the artery and preventing sufficient blood flow to the leg. To correct this problem, various methods have been developed to redirect the bloodflow to the lower leg.
In VA ECMO, this backward flow can cause serious heart problems too, including left ventricular distension (or enlargement) and “watershed phenomena,” which can increase the risk of damage to the heart. For patients with severely decreased left ventricle (LV) or lower heart chamber function, a blood clot can form in the LV. In this case, a type of heart pump or a special ventilation device can be used to help with heart function. In some cases, a medical procedure to create an opening between the upper chambers of the heart can also be implemented.
A common problem in both the venovenous and venoarterial types of ECMO is the formation of clots, or thrombosis. This can be due to the stress of the ECMO machine on the blood, the machine parts, and factors specific to the patient themselves such as severe infections, liver failure or other conditions that mess with the body’s normal clotting process. To prevent clotting, unfractionated heparin, a medicine that interferes with the blood’s ability to clot, is used.
One serious side effect can be Heparin-induced thrombocytopenia (HIT). In this case, the body creates antibodies to a factor in the platelets when it combines with heparin. This leads to a huge increase in clot formation and can cause life-threatening clots. A 50% drop in platelet count from the start should be a warning sign to the doctors, and heparin should be stopped right away, with a direct clot inhibitor being started instead.
What Else Should I Know About Extracorporeal Membrane Oxygenation Anticoagulation?
Extracorporeal membrane oxygenation (ECMO) is a treatment that uses a machine to take over the work of your lungs and sometimes your heart. Its use as extended support for heart and lung function has been growing worldwide. However, like all treatments, it can have complications. Two common ones with ECMO use for a long time are bleeding and clot formation (thrombosis).
Thrombosis often happens due to the interaction between the ECMO system parts and the patient’s blood. To prevent thrombosis, it’s crucial to closely follow guidelines on using drugs to thin the blood (anticoagulation).
Understanding how thrombosis forms, how common blood-thinning drugs work, and how to monitor the effects of these drugs can lower the chances of thrombosis happening.
