ECMO stands for Extra Corporeal Membrane Oxygenation. Extra corporeal means ‘outside the body’ and membrane oxygenation is the process of performing the function of the lungs. The ECMO circuit performs the function of the heart and/or lungs thereby allowing those organs to rest and heal.
When a patient is placed on ECMO, blood is drained from the patient, sent through an oxygenator where oxygen is added, and carbon dioxide is removed and is then pumped back into the patient via an artery or a vein.
ECMO is not a treatment modality. It is used as a BRIDGE for patients. Bridge to transplant in patients who deteriorate while waiting for suitable organs, Bridge to next decision in emergency scenarios when patients suffer a cardiovascular collapse and finally, Bridge to recovery.
VA (Veno arterial) and VV (Veno venous), are the two main configurations of ECMO with VAV and VVA being the newer, hybrid cannulation strategies. Depending on cannulation sites, ECMO can be Central or Peripheral.
VA ECMO supports both the heart and lung. Blood is drained, oxygenated, and pumped back into an artery, hence providing complete cardiopulmonary support and bypassing both the heart and lungs.
VV ECMO, on the other hand, provides only respiratory support. Blood is drained, oxygenated, carbon dioxide removed and is then sent back into the patient via a central vein. This modality is only for patients in respiratory failure and does not provide any cardiovascular support whatsoever.
VA ECMO is indicated for any patient with severe cardiac failure due to almost any cause:
VV ECMO is indicated for respiratory failure of any cause:
▪ primary graft failure after lung transplantation ▪ bridge to lung transplant ▪ intraoperative ECMO
The ECMO circuit consists of a pump, membrane oxygenator, and a heat exchanger connected with circuit tubing between the outflow cannula and either the arterial (VA) or venous (VV) inflow cannula. Circuit tubing used is made from a polyvinylchloride (PVC) – based plastic compound. In addition to this, they can include pressure monitors, oxyhemoglobin saturation monitors, circuit access sites and a bridge connecting the venous access and arterial infusion limbs of the circuit. Blood is exposed to a large surface area as it moves through the ECMO circuit, activating a prothrombotic state, thus necessitating the use of anticoagulation to prevent clot formation.
Hemorrhage: The most frequent complication during ECMO is hemorrhage, ranging between 10-30%. Bleeding can occur at the surgical site, at the cannula site, intracranial, intrathoracic, abdominal, or retroperitoneal hemorrhage may also occur. Treatment is by decreasing or stopping heparin and infusion of platelets and clotting factors.
Hemolysis: This usually does not occur during ECMO unless there is a circulatory or patient problem. Plasma-free hemoglobin should be checked frequently. Values above 10 mg% should be investigated further to determine and treat the cause.
Systemic thromboembolism due to thrombus formation within the extracorporeal circuit, is a rare complication. In most patients, thromboembolism can be prevented by infusing Heparin to achieve targets for activated clotting time (ACT) and closely monitoring the circulation for signs of clot formation.
Neurological complications: these include seizures, infarction, and intracranial hemorrhage. Adults have the lowest incidence of major neurological disorders, with 2% suffering from stroke, 4% from infarction and 2% from hemorrhage. These could be due to systemic heparinization, thrombocytopenia, coagulopathies, or systolic hypertension.
Sepsis: Septic complications can happen because the ECMO circuit is a large foreign body within the blood vessel and frequent manipulation increases the risk of infection. All invasive lines and cannulae must be handled with extremely sterile precautions to avoid sepsis.
VA-ECMO, especially peripheral VA ECMO, is associated with specific complications like Harlequin syndrome and LV distension.
During peripheral VA-ECMO, fully oxygenated blood is infused into the femoral artery in a retrograde manner towards the aorta and will preferentially perfuse the lower extremities and the abdominal viscera. If the patient has good or recovering LV function with poor lung function, deoxygenated blood ejected from the heart will perfuse heart, brain, and upper extremities. As a result of this, the oxyhemoglobin saturation of the blood perfusing the upper extremities and brain will be lower than the blood perfusing the lower extremities and abdominal viscera. This Differential hypoxia is also known as north-south syndrome, proximal-distal syndrome, and Harlequin syndrome. This is detected by monitoring the saturation simultaneously in the right upper extremity and lower extremities. This syndrome is managed by improving the pulmonary shunt through ventilator settings, reduction of native cardiac output, improvement in ECMO blood flow by increasing pump rate or by infusing oxygenated blood into the right atrium through an additional arterial inflow cannula, which is called VAV ECMO.
A particular problem associated with VA-ECMO is LV distention, due to the increased afterload on the LV. This is usually detected by evidence of pulmonary edema on the chest radiograph and a TEE (transesophageal echocardiography) can confirm the diagnosis identifying a severely dilated LV. Hence, if LV contractility is profoundly reduced, it can lead to an increase in left heart pressures resulting in LV distention. Mechanical LV venting may be necessary if LV unloading is not successful with inotropic support, pharmacologic afterload reduction, or intra-aortic balloon pump afterload reduction.
Cannulation for ECMO is a crucial step with numerous pitfalls and complications. The type, size and site of cannulation must be carefully chosen based on each patient’s clinical condition. Cannulation can be percutaneous or surgical. Several types of cannulas are currently available in a variety of sizes, with distinct features that can be used for cannulation. Single-lumen cannulas are most used to provide venous and arterial access for patients receiving VA ECMO and VV ECMO. Cannulas sizes vary from 6F (2 mm diameter) to 51F (17 mm diameter). Most cannulas are manufactured with wire-reinforced bodies that are designed to prevent luminal occlusion and they are designed for percutaneous vascular access using cannula-specific guide wire introducer sets.
The AVALON Dual-lumen cannula is a recent advancement that allows us to insert a single cannula in the Right IJV. This cannula drains venous blood from the IVC and SVC through one lumen and oxygenated blood is returned to the right atrium through a separate lumen. It is inserted under echo and fluoroscopy guidance since positioning is crucial. It must be positioned such that the outflow port is adjacent to the tricuspid valve and the drainage ports are in the SVC and IVC. The Avalon cannula is available in sizes ranging from 13F to 31F and the main advantage of this cannula is that it allows for early mobilization of patients.
Dr Arjun Ashok Cardiac Anesthetist Department of Heart and Lung Transplant
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