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|Extracorporeal membrane oxygenation|
|Extracorporeal membrane oxygenation|
In intensive care medicine, extracorporeal membrane oxygenation (commonly abbreviated ECMO) is an extracorporeal technique of providing both cardiac and respiratory support oxygen to patients whose heart and lungs are so severely diseased or damaged that they can no longer serve their function. Initial cannulation of a patient receiving ECMO is performed by a surgeon or anaesthetist and maintenance of the patient is the responsibility of the Perfusionist or ECMO Specialist who gives 24/7 monitoring care for the duration of the ECMO treatment.
There are several forms of ECMO, the two most common of which being veno-arterial (VA) and veno-venous (VV). In both modalities, blood drained from the venous system is oxygenated outside of the body. In VA ECMO, this blood is returned to the arterial system and in VV ECMO the blood is returned to the venous system. In VV ECMO, no cardiac support is provided.
In veno-arterial ECMO – a venous cannula is usually placed in the right common femoral vein for extraction and an arterial cannula is usually placed into the right femoral artery for infusion. The tip of the femoral venous cannula should be maintained near the junction of the inferior vena cava and right atrium, while the tip of the femoral arterial cannula is maintained in the iliac artery. In adults accessing the femoral artery is preferred because the insertion is simpler. Central VA ECMO may be used if cardiopulmonary bypass has already been established (with cannulae in the right atrium and ascending aorta)
In Veno-venous ECMO – venous cannulae are usually placed in the right common femoral vein for drainage and right internal jugular vein for infusion. Alternatively, a dual lumen catheter is inserted into the right [internal jugular vein], draining blood from the superior and inferior venue cavae and returning it to the right atrium
The clinical outcomes of patients undergoing ECMO can be categorized according to the indication for the ECMO: severe acute respiratory failure or cardiac failure.
With acute respiratory failure use of ECMO has been shown to improve survival rates. Survival rates from 50—70 percent have been reported in observational and uncontrolled clinical trials. The survival rates reported are better than historical survival rates. In the United Kingdom, respiratory (VV) ECMO is concentrated in designated ECMO centres to ensure top quality care.
Venoarterial (VA) ECMO is a bridge to further therapy, either a ventricular assist device, transplant or recovery.
Guidelines that describe the indications and practice of ECMO are published by the Extracorporeal Life Support Organization (ELSO). Criteria for the initiation of ECMO include acute severe cardiac or pulmonary failure that is potentially reversible and unresponsive to conventional management. Examples of clinical situations that may prompt the initiation of ECMO include the following:
Most contraindications are relative, balancing the risks of the procedure (including the risk of using valuable resources that could be used for others) vs. the potential benefits. The relative contraindications are:
ECMO should be performed only by clinicians with training and experience in its initiation, maintenance, and discontinuation. Once it has been decided that ECMO will be initiated, the patient is anticoagulated 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.
Cannulae are usually placed percutaneously by the Seldinger technique. The largest cannulas that can be placed in the vessels are used in order to maximise flow and minimise pressures bernoulli equation
ECMO required for complications of cardiac surgery can be placed directly into the appropriate chambers of the heart or great vessels.
Following cannulation, the patient is connected to the ECMO circuit and the blood flow is increased until respiratory and hemodynamic status is stable.
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.
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.
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.
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.
For patients with respiratory failure, improvements in radiographic appearance, pulmonary compliance, and arterial oxyhemoglobin saturation indicate that the patient may be ready to be taken off of ECMO support. For patients with cardiac failure, enhanced aortic pulsatility correlates with improved left ventricular output and indicates that the patient may be ready to be taken off of ECMO support. Once the decision has been made to discontinue ECMO, the cannulae are removed.
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 as indicated by arterial and venous blood gas results.
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. In general, VA ECMO trials are shorter in duration than VV ECMO trials because of the higher risk of thrombus formation
A common consequence in ECMO-treated adults is neurological injury, which may include subarachnoid hemorrhage, ischemic watershed infarctions, hypoxic-ischemic encephalopathy, unexplained coma, and brain death. Fatal sepsis may occur when the large catheters inserted in the neck provide fertile field for infection. Additional risks include bleeding. In adults, ECMO survival rates are around 60%. ECMO has yet to have proven survival benefit in adults with acute respiratory distress syndrome (ARDS). In VA ECMO, patients whose cardiac function does not recover sufficiently to be weaned from ECMO may be bridged to a ventricular assist device (VAD) or transplant.
In infants aged less than 34 weeks of gestation, several physiologic systems are not well-developed, especially the cerebral vasculature and germinal matrix, resulting in high sensitivity to slight changes in pH, PaO2, and intracranial pressure. Preterm infants are at unacceptably high risk for intraventricular hemorrhage (IVH) if administered ECMO at a gestational age less than 32 weeks. Also later, given the risk of IVH, it has become standard practice to ultrasound the brain prior to administering ECMO.
Bleeding occurs in 30 to 40 percent of patients receiving ECMO and can be life-threatening. It is due to both the necessary continuous heparin infusion and platelet dysfunction. Meticulous surgical technique, maintaining platelet counts greater than 100,000/mm3, and maintaining the target ACT reduce the likelihood of bleeding. Bleeding tendency can be identified using techniques like Sonoclot, which helps in giving ACT & Platelet Function in one single test.
Systemic thromboembolism due to thrombus formation within the extracorporeal circuit is an infrequent complication that can be devastating. Its impact is greater with VA ECMO than VV ECMO because infusion is into the systemic circulation. Heparin infusion that achieves its target ACT and vigilant observation of the circuit for signs of clot formation successfully prevents thromboembolism in most patients.
A variety of complications can occur during cannulation, including vessel perforation with hemorrhage, arterial dissection, distal ischemia, and incorrect location (e.g., venous cannula within the artery). These complications are rare (<5 percent). A skilled and experienced surgeon is important to avoid or address such complications.
Heparin-induced thrombocytopenia (HIT) is increasingly common among patients receiving ECMO. When HIT is suspected, the heparin infusion is usually replaced by a non-heparin anticoagulant.
Pulmonary hemorrhage can occur in patients receiving ECMO.
There is retrograde blood flow in the descending aorta whenever the femoral artery and vein are used for VA ECMO. Stasis of the blood can occur if left ventricular output is not maintained, which may result in thrombosis.
During VA ECMO, fully saturated blood infused into the femoral artery from the ECMO circuit will preferentially perfuse the lower extremities and the abdominal viscera. Blood ejected from the heart will selectively perfuse the heart, brain, and upper extremities.
Applications for ECMO may expand in the future to include percutaneous temporary left ventricular assistance and low flow ECMO for CO2 removal (ECOOR)]. In addition, new technologies will improve the simplicity and safety of ECMO, including new oxygenators, pumps, and surface coatings.
Extracorporeal Life Support Organization (ELSO) - www.elso.org