Extracorporeal life support can be viewed as a spectrum of modalities based on modifications of a cardiopulmonary bypass circuit to provide cardiac and respiratory support, which can be used for extended periods, from hours to several weeks. Extracorporeal membrane oxygenation (ECMO) is among the most frequently used forms of extracorporeal life support. It can be configured for venovenous blood flow, to provide adequate oxygenation and carbon dioxide removal in isolated refractory respiratory failure, or in a venoarterial configuration, when support is required for cardiac and/or respiratory failure. Echocardiography plays a fundamental role throughout the entire journey of a patient supported on ECMO. It provides information that assists in patient selection, guides the insertion and placement of cannulas, monitors progress, detects complications, and helps in determining cardiac recovery and the weaning of ECMO support. Although there are extensive published data regarding ECMO, particularly in the pediatric population, there is a paucity of data outlining the role of echocardiography in guiding the management of adult patients supported by ECMO. ECMO is likely to become an increasingly used form of cardiorespiratory support within the critical care setting. Hence, clinicians and sonographers who work within echocardiography departments at institutions with ECMO programs require specific skills to image these patients.
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Components of an Extracorporeal Membrane Oxygenation Circuit
An ECMO circuit typically consists of large-bore tubing with
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a cannula for drainage from the venous system,
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a blood pump and control unit,
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an oxygenator for the addition of oxygen and removal of carbon dioxide,
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a heater and cooler unit, and
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a cannula to return blood to the venous or arterial system.
ECMO circuits can also be used in series with renal replacement devices, allowing fluid removal dialysis and plasmapheresis during cardiopulmonary support. In adults, the arterial cannulas used typically range in size from 17 to 23 Fr, and the venous cannulas range in size from 19 to 29 Fr if placed percutaneously and 32 to 36 Fr if placed centrally. Cannula sizes are determined by flow rate requirements, patient and vessel size, and the flow characteristics of the cannula. Roller pumps have been used extensively, but the use of centrifugal pumps is increasing, with blood flows up to 7 to 10 L/min at a maximal speed of 5,000 rpm. Figure 1 depicts the components of a typical ECMO circuit.
Components of an Extracorporeal Membrane Oxygenation Circuit
An ECMO circuit typically consists of large-bore tubing with
- •
a cannula for drainage from the venous system,
- •
a blood pump and control unit,
- •
an oxygenator for the addition of oxygen and removal of carbon dioxide,
- •
a heater and cooler unit, and
- •
a cannula to return blood to the venous or arterial system.
ECMO circuits can also be used in series with renal replacement devices, allowing fluid removal dialysis and plasmapheresis during cardiopulmonary support. In adults, the arterial cannulas used typically range in size from 17 to 23 Fr, and the venous cannulas range in size from 19 to 29 Fr if placed percutaneously and 32 to 36 Fr if placed centrally. Cannula sizes are determined by flow rate requirements, patient and vessel size, and the flow characteristics of the cannula. Roller pumps have been used extensively, but the use of centrifugal pumps is increasing, with blood flows up to 7 to 10 L/min at a maximal speed of 5,000 rpm. Figure 1 depicts the components of a typical ECMO circuit.
Modes of Extracorporeal Membrane Oxygenation
Two types of support are commonly used in adults: venovenous (VV) ECMO and venoarterial (VA) ECMO. VV ECMO is used for gas exchange in patients with isolated refractory respiratory failure and requires adequate native cardiac function, as it provides no direct circulatory support. Large-bore cannulas are placed in the inferior vena cava and/or superior vena cava, via the femoral and/or the internal jugular vein, to drain blood into the ECMO circuit. Gas exchange occurs in the oxygenator, and blood is returned through a large-bore cannula placed in another large vein close to the right atrium. The oxygenated blood from the ECMO circuit mixes with any blood not passing through the circuit and is pumped by the right heart through the lungs to the left heart and systemic circulation. Thus, right-heart function, pulmonary vascular resistance, and left-heart function need to be adequate to ensure systemic oxygen delivery.
VA ECMO can provide cardiac and respiratory support. In adults with preserved cardiac function and isolated respiratory failure, VV ECMO is usually preferred to VA ECMO, because it avoids the risks associated with large-bore arterial access. In VA ECMO, blood is withdrawn from the right atrium either by direct surgical cannulation or through a cannula placed in a major vein with the tip sitting in the right atrium. Oxygenation and carbon dioxide removal proceed via the oxygenator as previously described, before being pumped back through a cannula placed centrally in the ascending aorta or peripherally in a large artery. Peripheral cannulas may be placed percutaneously or surgically, depending on patient anatomy, clinical circumstances, and operator preference. Surgical placement may be by cannulation under direct vision or by a tube graft anastomosed to the artery with the cannula placed inside the graft. Specific cannula configurations in VA ECMO have advantages and disadvantages. Central cannulation (right atrium and ascending aorta) allows the use of larger cannulas, providing higher flows and reliable coronary and cerebral perfusion at the expense of requiring a sternotomy. It is thus used after cardiac surgery for patients who are unable to separate from cardiopulmonary bypass despite high-dose inotropes with or without intra-aortic balloon pump support. Peripheral cannulation avoids a sternotomy, but the flows are lower because cannula size is usually smaller. Specific issues arise with the femoral arterial return cannula position in VA ECMO. If lung function is severely impaired and cardiac function is preserved, blood passing through the lungs may not be oxygenated, and hypoxic blood ejected from the left ventricle will preferentially flow to the coronary and cerebral circulations. Adequacy of central oxygen delivery is estimated by placement of an arterial line and pulse oximeter in the right radial artery. Because flow is dependent on cannula diameter, large cannulas are used, which may completely obstruct distal flow down the leg, and the placement of a smaller backflow cannula is necessary to provide adequate blood flow distal to the cannula insertion point. Figure 2 shows a backflow cannula in the right common femoral artery to allow perfusion to the leg distal to the cannula.
The axillary artery can be used as the return site to improve coronary and cerebral oxygenation. However, this artery is smaller than the femoral artery, and this may adversely affect flow rates. Multiple other circuit configurations are possible. Low-flow VA ECMO is a temporizing resuscitative mode in which smaller cannulas are placed emergently to facilitate resuscitation and/or transport, before definitive management and/or full VA ECMO. An ECMO circuit can be used as a temporary right ventricular (RV) assist device after the insertion of a left ventricular (LV) assist device when there is unexpected RV failure. Blood can be withdrawn from the right atrium through a percutaneous cannula and returned to the pulmonary artery, bypassing the right heart and thus allowing right-heart recovery. An oxygenator can be added for gas exchange and temperature control depending on native lung function in the perioperative period. Figure 3 demonstrates the use of ECMO as a temporary right VAD to treat right heart failure following insertion of a left VAD. Other possible configurations include venoarterial-venous (access from the venous circulation and return to both the arterial and venous circulation) and hybrid central peripheral combinations. New systems are also being developed, including specific transport systems and pumpless AV ECMO systems in which the arterial pressure drives blood across the oxygenator with blood return to a large peripheral vein.
Indications
Indications for ECMO can be subdivided into cardiac and respiratory indications. The role and timing of VV ECMO in adult respiratory failure secondary to acute respiratory distress syndrome are debated. Patients may be considered for VV ECMO when there is refractory hypoxemia (and/or respiratory acidosis, with pH < 7.2) despite maximal conventional mechanical ventilation and treatment of reversible contributing factors. The exact values chosen in patients with acute respiratory distress syndrome will vary but would typically be partial pressure of oxygen < 60 mm Hg with fraction of inspired oxygen = 1.0 despite >15 cm H 2 O positive end-expiratory pressure, because this is associated with poor outcomes in patients with acute respiratory distress syndrome. The oxygenation index ([fraction of inspired oxygen × mean airway pressure × 100]/partial pressure of oxygen [mm Hg]) is used to describe the severity of respiratory failure. An oxygenation index > 35 to 40 represents failure of conventional ventilation and should trigger consideration of rescue therapies. The Conventional Ventilation or ECMO for Severe Adult Respiratory Failure trial used a lung injury (Murray) score of >3.0.
Respiratory Indications
Respiratory indications are listed in Table 1 .
| Common indications |
| Severe bacterial or viral pneumonia |
| Acute respiratory distress syndrome |
| Aspiration syndromes |
| Primary graft failure after lung transplantation |
| Other indications |
| Smoke inhalation |
| Status asthmaticus |
| Airway obstruction |
| Alveolar proteinosis |
| Pulmonary contusion |
| Massive hemoptysis or pulmonary hemorrhage |
Cardiac Indications
Typical cardiac indications ( Table 2 ) include cardiac arrest, near cardiac arrest, cardiogenic shock and refractory low cardiac output (cardiac index < 2.2 L/min/m 2 ), and hypotension (systolic blood pressure < 90 mm Hg) despite adequate intravascular volume, high-dose inotropic agents, and an intra-aortic balloon pump.
| Common indications |
| Cardiogenic shock |
| Inability to wean from cardiopulmonary bypass after cardiac surgery |
| Primary graft failure after heart or heart-lung transplantation |
| Sepsis with profound cardiac depression |
| Drug overdose/toxicity with profound cardiac depression |
| Myocarditis |
| Other indications |
| Cardiac arrhythmic storm refractory to other measures |
| Chronic cardiomyopathy: as a bridge to longer term VAD support or as a bridge to decision |
| Pulmonary embolism |
| Isolated cardiac trauma |
| Acute anaphylaxis |
| Periprocedural support for high-risk percutaneous cardiac interventions |
An advantage of VA ECMO over the insertion of a ventricular assist device (VAD) is that it can be initiated more rapidly outside the operating room, in multiple environments. It can also be performed at peripheral centers before a patient is transported to a referral center for definitive management. Early consultation between the non-ECMO and the ECMO centers is vitally important to facilitate planning and transportation of a critically ill patient before the establishment of irreversible end-organ dysfunction. ECMO can then be used as a bridge to VAD implantation in patients who present in decompensated cardiac failure or those in acute cardiogenic shock. VA ECMO also allows support of cardiorespiratory failure, as opposed to isolated cardiac dysfunction. The Interagency Registry for Mechanically Assisted Circulatory Support data demonstrates a significant increase in mortality when VADs are inserted in patients with severe organ dysfunction. In this cohort, VA ECMO improved organ perfusion, allowing time for further information to be obtained regarding patient suitability for VAD implantation or transplantation (“bridge to decision”). A VAD can be subsequently inserted when organ dysfunction has diminished through the use of VA ECMO. Similarly, if a patient recovers sufficiently and a donor heart becomes available, the patient can be transplanted off ECMO. One benefit of ECMO in the bridge-to-recovery situation is the option of peripheral cannulation as opposed to VAD cannulation, which is generally central and intracardiac and therefore associated with further muscle damage, risk for arrhythmogenic foci, and where subsequent cardiac surgery requires a repeat sternotomy. Because of the difficulties involved with inserting a VAD in the pediatric population (with size being a limiting factor) and the ability of many pediatric respiratory conditions to resolve with appropriate treatment, ECMO is a significantly more common form of support than VADs in this group of patients.
Contraindications to Extracorporeal Membrane Oxygenation
Potential contraindications to ECMO include nonrecoverable disease in patients who are not candidates for bridging or transplantation. Patients with irreversible neurologic injuries, those with advanced multiple-organ failure, and those who have contraindications to anticoagulation may not be suitable candidates for ECMO. The upper age limit is debated and will vary with the reversibility of the underlying condition. Similarly, an upper weight limit in adults (such as 125 kg) is debated, because cannulation may be difficult and flows may be inadequate. Absolute ( Table 3 ) and relative ( Table 4 ) contraindications are also specific to each type of ECMO support.
| Absolute contraindications to all forms of ECMO |
| Progressive and nonrecoverable disease and not suitable for transplantation |
| Severe neurologic injury or intracerebral bleeding |
| Absolute contraindications to VA ECMO |
| Unrepaired aortic dissection |
| Severe aortic valve regurgitation |
| Absolute contraindications to VV ECMO |
| Severe cardiac failure |
| Cardiac arrest |
| Severe pulmonary hypertension (mean pulmonary artery pressure > 50 mm Hg) |
| Age > 75 y |
| Inability to anticoagulate |
| High-dose immunosuppression |
| Cardiopulmonary resuscitation duration > 60 min |
| Established multiple-organ failure |
| Multiple trauma with multiple bleeding sites |
Complications
ECMO is associated with significant complications related to the critically ill patient subset in which it is used and the therapy itself. Common complications include bleeding, thromboembolic events, and sepsis. Less common complications include limb ischemia, hemolysis, and mechanical failure (such as oxygenator or cannula or device thrombosis). Rarer but potentially catastrophic complications include intracerebral bleeding, circuit rupture, accidental decannulation, and air embolism.
Echocardiographic Imaging of Patients on Extracorporeal Membrane Oxygenation
Because ECMO is based on the principles of oxygenation and hemodynamic support via blood flow within large-bore cannulas placed in or near the heart in patients with cardiorespiratory failure, echocardiography would be expected to have a fundamental role throughout the care of patients supported on ECMO.
Echocardiography before ECMO Commencement
Echocardiography helps exclude new reversible pathology, which may account for a patient’s hemodynamic instability (such as cardiac tamponade, undiagnosed cardiac valve pathology, and LV dysfunction), avoiding the need for ECMO support. It also provides information regarding contraindications, such as aortic dissection. The presence of significant aortic valve regurgitation may have a detrimental impact on LV unloading in VA ECMO, in which LV afterload is increased. Pathology in the aorta, such as severe aortic atherosclerotic disease, may influence VA ECMO cannulation site (central vs peripheral) or technique (surgical vs percutaneous). The positioning of a venous cannula in the right atrium for VA and VV ECMO also dictates that right-heart anatomy be evaluated for any structural abnormality that may adversely affect the function and position of the cannula. Notable findings would include a prominent patent foramen ovale, atrial septal defect, interatrial septal aneurysm, prominent Chiari network, presence of a pacemaker or implantable cardioverter-defibrillator leads, and tricuspid valve pathology (such as tricuspid stenosis or a tricuspid valve replacement). By assessing cardiac function, echocardiography can help determine whether VV ECMO is sufficient or whether VA ECMO should be considered in conditions such as pneumonia with severe septic cardiomyopathy.
Echocardiography during ECMO Initiation and Cannulation
Echocardiography also has a key role during ECMO cannulation, as it assists in the correct placement of ECMO cannulas. It provides real-time feedback as to the degree of ventricular unloading and interventricular septal motion at the commencement of ECMO support. TTE may not provide the required spatial resolution to guide ECMO initiation and TEE may be required. There are multiple possible cannula configurations, so clear communication between the operator and the echocardiologist as to the intended cannula insertion strategy is vital. In VV ECMO, when one cannula is used for access and one to return blood, the optimal position of the access cannula tip is in the proximal inferior vena cava, before entry into the right atrium. The optimal position for the return cannula is in the mid right atrium and clear from the interatrial septum and tricuspid valve. Figure 4 , and Videos 1 and 2 (view video clips online) are TEE images of a VV ECMO return cannula positioned in the mid right atrium, with colour Doppler demonstrating flow out the end and side holes. If the access cannula is more proximally placed than the return cannula, or if the two cannula ends are too close together, recirculation will occur, resulting in little oxygenated blood passing into the pulmonary and systemic circulation. Figure 5 is a plain abdominal x-ray showing the appearance of the access and return cannulae in a VA ECMO circuit. Figure 6 is a plain chest demonstrating a return cannula in the right atrium in a VV ECMO circuit.
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