Extra-corporeal membrane oxygenation (ECMO) is primarily used to provide cardiopulmonary support.
Extracorporeal membrane oxygenation (ECMO) is a lifesaving technique used in critically ill patients presenting acute cardiac and/or pulmonary dysfunctions, who are at high risk of developing acute kidney injury (AKI) and fluid overload (FO). ECMO is commonly used in the following situations:
- Heart and/or lung failure despite usual medical therapy to allow heart and/or lung to recover.
- As a bridge option to further treatment such as heart assist device or left ventricular assist device (LVAD)
- As a bridge to heart or lung transplantation
Venous-venous ECMO (VV ECMO) provides pure respiratory support through blood oxygenation and carbon dioxide (CO2) removal. It does not provide circulatory support.
Veno-arterial ECMO (VA ECMO) is similar to the heart-lung machine and provides cardiac and pulmonary support for patients in cardiogenic shock.
- Mechanical cardiopulmonary support is most often applied intraoperatively to facilitate cardiac surgery (ie, cardiopulmonary bypass). However, cardiopulmonary support can also be delivered in a more prolonged fashion in an intensive care unit, although it is less common.
- Prolonged cardiopulmonary support is called extracorporeal membrane oxygenation (ECMO), extracorporeal life support, or extracorporeal lung assist. There are two types of ECMO – venoarterial (VA) and venovenous (VV). Both provide respiratory support, but only VA ECMO provides hemodynamic support.
The survival of patients undergoing ECMO can be categorized according to the indication for the ECMO: severe acute respiratory failure or cardiac failure.
Acute respiratory failure — Multiple studies have evaluated the effect of ECMO on mortality in patients with severe acute respiratory failure. The potential benefit from ECMO should always be weighed against the risk of transfer. In experienced ECMO centers, approximately 25 percent of patients will improve and recover without ECMO, while 75 percent of patients will require ECMO. Among those who require ECMO, 60 to 70 percent will survive.
Cardiac failure — Venoarterial (VA) ECMO can provide acute support in cardiogenic shock or cardiac arrest in adults. Assuming that the brain function is normal or only minimally impaired, ECMO is provided until the patient recovers or receives a long-term ventricular assist device as a bridge to cardiac transplantation.
Observational studies and case series have reported survival rates of 20 to 50 percent among patients who received ECMO for cardiac arrest, severe cardiogenic shock, or failure to wean from cardiopulmonary bypass following cardiac surgery and including older adults.
ECMO can be venovenous (VV) or venoarterial (VA):
- During VV ECMO, blood is extracted from the vena cava or right atrium and returned to the right atrium. VV ECMO provides respiratory support, but the patient is dependent upon his or her own hemodynamics.
- During VA ECMO, blood is extracted from the right atrium and returned to the arterial system, bypassing the heart and lungs. VA ECMO provides both respiratory and hemodynamic support.
Initiation — Once it has been decided that ECMO will be initiated, the patient is anticoagulated (usually with intravenous heparin) and then the cannulae are inserted. ECMO support is initiated once the cannulae are connected to the appropriate limbs of the ECMO circuit.
Cannulation — Cannulae are usually placed percutaneously by Seldinger technique. The largest cannulae that can be placed in the vessels are used.
For VV ECMO, venous cannulae are usually placed in the right or left common femoral vein (for drainage) and right internal jugular vein (for infusion). The tip of the femoral cannula should be maintained near the junction of the inferior vena cava and right atrium, while the tip of the internal jugular cannula should be maintained near the junction of the superior vena cava and right atrium. Alternatively, a double lumen cannula is available that is large enough to accommodate 4 to 5 L/min of blood flow. It is available in a variety of sizes, with 31 French being the largest and most appropriate for adult males. The drainage and infusion ports have been engineered to minimize recirculation.
For VA ECMO, a venous cannula is placed in the inferior vena cava or right atrium (for drainage) and an arterial cannula is placed into the right femoral artery (for infusion).
Femoral access is preferred for VA ECMO because insertion is relatively easy. The main drawback of femoral access is ischemia of the ipsilateral lower extremity. The likelihood of this complication can be decreased by inserting an additional arterial cannula distal to the femoral artery cannula and redirecting a portion of the infused blood to the additional cannula for “reperfusion” of the extremity. Alternatively, a cannula can be inserted into the posterior tibial artery for retrograde flow to the extremity.
Occasionally, the femoral vessels are unsuitable for cannulation for VA ECMO (eg, patients with severe occlusive peripheral artery disease or prior femoral arterial reconstruction). In such circumstances, the right common carotid artery or subclavian artery can be used. In our experience, there is a 5 to 10 percent risk of a large watershed cerebral infarction when the right common carotid artery is used. Use of the subclavian artery offers the advantage of allowing patients on ECMO to ambulate.
For postcardiotomy ECMO, the cannulae employed for cardiopulmonary bypass can be transferred from the heart-lung machine to the ECMO circuit, with blood drained from the right atrium and reinfused into the ascending aorta.
Titration — Following cannulation, the patient is connected to the ECMO circuit and the blood flow is increased until respiratory and hemodynamic parameters are satisfactory. Reasonable targets include:
- An arterial oxyhemoglobin saturation of >90 percent for VA ECMO, or >75 percent for VV ECMO
- A venous oxyhemoglobin saturation 20 to 25 percent lower than the arterial saturation, measured on the venous line
- Adequate tissue perfusion, as determined by the arterial blood pressure, venous oxygen saturation, and blood lactate level
Maintenance — Once the initial respiratory and hemodynamic goals have been achieved, the blood flow is maintained at that rate. Frequent assessment and adjustments are facilitated by continuous venous oximetry, which directly measures the oxyhemoglobin saturation of the blood in the venous limb of the ECMO circuit. When the venous oxyhemoglobin saturation is below target, interventions that may be helpful include increasing one or more of the following: blood flow, intravascular volume, or hemoglobin concentration. Decreasing the systemic oxygen uptake by reducing the temperature may also be helpful.
Anticoagulation is sustained during ECMO with a continuous infusion of unfractionated heparin or direct thrombin inhibitor titrated to an activated clotting time (ACT) of 180 to 210 seconds. The ACT target is decreased if bleeding develops. ACT is easily determined at the point of care, but plasma PTT (1.5 times normal) can also be used. Thromboelastography is a useful adjunct. When heparin is used, the anticoagulant effect is dependent on the amount of endogenous antithrombin (AT3). If AT3 deficiency is suspected, the level can be measured. If less than 50 percent normal, AT3 is replaced by fresh frozen plasma. Less commonly, some specialized centers follow factor Xa levels. One review of 16 studies suggested that optimal targets vary among centers resulting in variable rates of bleeding and thromboembolism .
Platelets are continuously consumed during ECMO because they are activated by exposure to the foreign surface area. Platelet counts should be maintained greater than 50,000/microliter, which may require platelet transfusion.
The ECMO circuit is often the only source of oxygen in patients with complete cardiac or pulmonary failure. Oxygen delivery depends on the amount of hemoglobin and blood flow. The risks of high blood flow outweigh the risk of transfusion, so hemoglobin is maintained over 12 g/dL in ECMO patients.
Ventilator settings are reduced during ECMO in order to avoid barotrauma, volutrauma (ie, ventilator-induced lung injury), and oxygen toxicity. Plateau airway pressures should be maintained less than 20 cm H2O and FiO2 less than 0.5. Reduction of ventilator support is usually accompanied by increased venous return, which improves cardiac output.
We perform early tracheostomy to reduce dead space and improve patient comfort. Patients typically require light sedation during ECMO, although we prefer to maintain patients awake, extubated, and breathing spontaneously.
Special considerations — VV ECMO is typically used for respiratory failure, while VA ECMO is used for cardiac failure. There are unique considerations for each type of ECMO, which influence management.
- Blood flow – Near-maximum flow rates are usually desired during VV ECMO to optimize oxygen delivery. In contrast, the flow rate used during VA ECMO must be high enough to provide adequate perfusion pressure and venous oxyhemoglobin saturation (measured on drainage blood), but low enough to provide sufficient preload to maintain left ventricular output.
- Diuresis – Since most patients are fluid overloaded when ECMO is initiated, aggressive diuresis is warranted once the patient is stable on ECMO. Ultrafiltration can be easily added to the ECMO circuit if patients are unable to produce sufficient urine for diuresis.
- Left ventricular monitoring – Left ventricular output must be rigorously monitored during VA ECMO because left ventricular output may worsen. The cause is usually multifactorial, including the underlying left ventricular dysfunction and insufficient unloading of the distended left ventricle due to ongoing blood flow to the left ventricle from the bronchial circulation and right ventricle. Left ventricular output can be closely monitored by identifying pulsatility in the arterial line’s waveform and by frequent echocardiography. Interventions that can improve left ventricular output include inotropes (eg, dobutamine, milrinone) to increase contractility and intra-aortic balloon counterpulsation to reduce afterload and facilitate left ventricular output. Immediate left ventricular decompression is essential to avoid pulmonary hemorrhage if left ventricular ejection cannot be maintained despite intra-aortic balloon counterpulsation and inotropic agents. This can be accomplished surgically or percutaneously. Methods of percutaneous left ventricular decompression include transatrial balloon septostomy or insertion of a left atrial or ventricular drainage catheter.
ECMO and the Kidney
AKI is a common complication in the adult ECMO patients. Using the risk, injury, failure, loss, and end stage or AKI Network criteria, 2 single-center studies showed an AKI incidence of more than 80% with close to half of affected patients requiring renal replacement therapy (RRT). Fluid overload (FO) in the general ICU patient with AKI is independently associated with higher mortality rate. FO similarly compromises the cardiac and/or pulmonary functions in the ECMO patient and therefore current guidelines recommend achieving and maintaining euvolemia once the hemodynamics are stabilized . An international survey reported that treatment and prevention of FO are critically important indications for using RRT in conjunction with ECMO.
Weaning from ECMO — For patients with respiratory failure, improvements in radiographic appearance, pulmonary compliance, and arterial oxyhemoglobin saturation indicate that the patient may be ready to be liberated from ECMO. For patients with cardiac failure, enhanced aortic pulsatility correlates with improved left ventricular output and indicates that the patient may be ready to be liberated from ECMO.
One or more trials of taking the patient off ECMO should be performed prior to discontinuing ECMO permanently:
- VV ECMO trials are performed by eliminating all countercurrent sweep gas through the oxygenator. Extracorporeal blood flow remains constant, but gas transfer does not occur. Patients are observed for several hours, during which the ventilator settings that are necessary to maintain adequate oxygenation and ventilation off ECMO are determined.
- VA ECMO trials require temporary clamping of both the drainage and infusion lines, while allowing the ECMO circuit to circulate through a bridge between the arterial and venous limbs. This prevents thrombosis of stagnant blood within the ECMO circuit. In addition, the arterial and venous lines should be flushed continuously with heparinized saline or intermittently with heparinized blood from the circuit. VA ECMO trials are generally shorter in duration than VV ECMO trials because of the higher risk of thrombus formation.
|NOTE TO REMEMBER–THE DIFFERENCE AND BENEFITS: Extracorporeal membrane oxygenation (ECMO) is used in critically ill patients presenting acute cardiac and/or pulmonary dysfunctions, who are at high risk of developing acute kidney injury and fluid overload. Continuous renal replacement therapy (CRRT) is commonly used in intensive care units (ICU) to provide renal replacement and fluid management. The combination of ECMO and CRRT might be a safe and effective technique that improves fluid balance and ameliorates electrolyte disturbances. There are three major methods for performing CRRT during ECMO: ‘independent CRRT access’, ‘introduction of a hemofiltration filter into the ECMO circuit (in-line hemofilter)’ and ‘introduction of a CRRT device into the ECMO circuit’. SO; the combination of ECMO and CRRT might be a safe and effective technique that improves fluid balance and ameliorates electrolyte disturbances. A variety of methods for combining ECMO and CRRT can be chosen. A prospective multicenter study would be beneficial in determining the potential of this technique to improve the outcome of critically ill patients.