Extracorporeal Membrane Oxygenation

1 What is extracorporeal membrane oxygenation (ECMO)?

ECMO is a means to provide peripheral oxygenation, ventilation, and circulation to a variety of patients with diseases of the heart and/or lungs for periods of days or weeks. The lungs of patients with severe respiratory failure have an impaired ability to perform the necessary functions of the thoracic cavity, primarily oxygenation and ventilation. Data show that patients with respiratory failure who cannot receive oxygenation or ventilation adequately with use of low tidal volumes without high plateau pressures can be treated with ECMO successfully. Certain patients with hemodynamic failure who lack the ability to meet the metabolic demands of the organs of the body despite pharmacologic support may also benefit from ECMO.

2 What is the history of ECMO?

ECMO was developed as an extension of cardiopulmonary bypass. The first reported use of ECMO for respiratory failure in an adult patient was reported in 1972 in a young man with adult respiratory distress syndrome after a motor vehicle accident. This was followed in 1976 by the first use of ECMO in neonatal patients. Since its development ECMO has been shown to improve survival in neonates with respiratory failure from a variety of causes compared with more traditional therapy. The benefits in adult patients are more controversial.

3 How is ECMO different from cardiopulmonary bypass?

Cardiopulmonary bypass was developed as a means to provide short-term support to patients undergoing cardiac surgery. The setup necessary for cardiopulmonary bypass is also designed to accomplish several other tasks including suction and venting of the field and cardiac chambers and administration of cardioplegia (Table 13-1).

^{Table 13-1} Differences and Similarities Between ECMO and Cardiopulmonary Bypass

4 What are clinical situations where ECMO may be beneficial?

ECMO has been used for the short-term hemodynamic and respiratory support for numerous conditions (Box 13-1).

5 Are there any diagnostic modalities that may indicate when emergent ECMO is indicated?

Transesophageal echocardiography (TEE) allows real-time assessment of cardiac function. Acute pulmonary thromboemboli, amniotic fluid embolism, and air emboli cause an acute pressure overload on the right ventricle. Acute right ventricular pressure overload results in ventricular dilatation, tricuspid regurgitation, and a shift of the interventricular septum toward the left, further impairing filling of the left ventricle and a reduction in cardiac output. TEE may also be used to guide proper placement of the venous return cannula into the right atrium when ECMO is initiated.

6 What does the literature show regarding ECMO?

Dr. Warren Zapol performed the first randomized study to evaluate the effectiveness of ECMO in adult patients with severe acute respiratory failure. This study, published in 1979, determined that ECMO could support respiratory gas exchange but did not increase long-term survival. Subsequent studies have generally demonstrated survival rates of approximately 50% for patients with primary respiratory failure. Older patients, those having complications while receiving ECMO, and those having prolonged ventilation before ECMO were predictors of higher mortality. Survival for patients with primary cardiac failure also tends to be less.

7 Are there any contraindications to ECMO?

Few absolute contraindications to ECMO exist. Patient or proxy refusal is an absolute contraindication. Patients with “do not resuscitate” orders should not be subject to ECMO. Irreversible respiratory failure in patients who are not candidates for lung transplantation should preclude the use of ECMO. Severe bleeding and peripheral vascular disease increase the risk of complications with ECMO. Some have argued against the use in elderly patients, although one study demonstrated that 41.7% of patients older than 75 years with cardiogenic shock treated with ECMO survived to discharge.

8 What are the components of an ECMO circuit?

An ECMO circuit contains several key components including the vascular access, tubing, driving force (pump), gas exchange unit (oxygenator-ventilator), and interface or console. Vascular access is most commonly established in the femoral artery and veins. The cannulas are sized between 21F and 28F for adults and smaller for newborns and children. The venous or drainage cannula may be positioned into the right atrium under echocardiography or fluoroscopic guidance. The arterial or return cannula is placed in the other femoral vein (venovenous [VV] ECMO) or femoral artery (venoarterial [VA] ECMO).

Most ECMO circuits have a membrane gas exchange unit. This gas exchange unit consists of a series of hollow fibers through which the blood passes. Gas is passed in a countercurrent manner allowing exchange across the membranes. The pump system may be either a roller system or a centrifugal system. A roller pump system consists of flexible tubing in a track. The roller compresses the tubing and forces the blood forward with each turn. Such flow is independent of systemic vascular resistance, and high pressures can develop. Centrifugal pumps have been popular in Europe and are becoming more popular in the United States. Centrifugal pumps contain a magnetically driven impeller, which cycles at several thousand rotations per minute generating a pressure gradient across the pump head and propelling blood forward. It is preload and afterload dependent. If flow into the pump is decreased, then flow and pressure will be decreased. The CentriMag blood pump is a magnetically levitated centrifugal device that produces unidirectional flow and may generate flows up to 10 L/min (Figs. 13-1 and 13-2).

Figure 13-1 CentriMag ECMO console.

Figure 13-2 CentriMag pump and gas exchange system.

9 How is vascular access established for ECMO?

Several potential sites exist for both venous and arterial cannulation for patients requiring ECMO. The decision as to which cannulation strategy is necessary depends on the goals (Table 13-2). The venous or drainage cannula for adult patients should be 23F to 25F and the return cannula 17F to 21F.

10 What does one monitor while using ECMO?

Monitoring patients subject to ECMO is a multidisciplinary task. Surgeons, intensivists, perfusionists, anesthesiologists, respiratory therapists, and nurses may all be involved in the care of these patients. Maintenance of normal physiologic parameters is the goal (Table 13-3). Flow rates, sweep speed, and FiO₂ are adjusted to attain such measure.

^{Table 13-3} Goals for Management During ECMO

11 How is ventilation managed while a patient is receiving ECMO?

The avoidance of barotrauma is a prime benefit of ECMO. Most patients will be given rest settings to avoid complications associated with high volume and high pressure ventilation. Inspiratory pressures are often limited to 20 to 25 cm H₂O with a positive end-expiratory pressure of 10 to 15 cm H₂O and a low respiratory rate. Terragni demonstrated that ECMO allows tidal volumes less than 6 mL/kg to be used. An improvement in morphologic markers of lung protection and a reduction in pulmonary cytokines are observed.

12 What problems are commonly encountered during the clinical management of ECMO?

Acute problems tend to fall into three categories:

13 How does one wean a patient from ECMO?

The technique of weaning a patient from ECMO support depends on the type of ECMO (VV or VA) and the purpose of the support. In general the lungs and heart assume more responsibility for oxygenation, ventilation, and circulation. Sidebotham et al. describe their technique for weaning ECMO (Table 13-4).

^{Table 13-4} Weaning a Patient from ECMO Support

ABG, Arterial blood gases; CXR, chest radiograph.

Modified from Sidebotham D, McGeorge A, McGuinness S, et al: Extracorporeal membrane oxygenation for treating severe cardiac and respiratory failure in adults: part 2—technical considerations. J Cardiothorac Vasc Anesth 24:164-172, 2010.

14 What are complications of ECMO?

Most adult ECMO is established with peripheral cannulation of the femoral artery. Lower limb ischemia may occur because of occlusion of arterial inflow and may be treated with decannulation or supplemental cannulation of the superficial femoral artery. Neither body mass index, body surface area, nor cannula size predicts limb ischemia. Abdominal compartment syndrome may occur in both adult and pediatric patients receiving ECMO and is likely due to massive fluid resuscitation in an effort to achieve adequate ECMO flows. Other complications include bleeding, clot formation, stroke, renal failure, and nosocomial infection. Mechanical mishaps such as failure of the pump and oxygenator must be considered in planning support.

15 How does one transfer a patient receiving ECMO?

The transfer of patients with severe respiratory failure and hemodynamic failure to large medical centers is increasingly common. The University of Michigan reviewed all patients transferred between 1990 and 1999 while receiving ECMO. Patients were transferred over a range of 2 to 790 miles. No patients died during transport. Complications noted included power failure, circuit tubing leakages, circuit rupture, membrane lung thrombosis, membrane lung leakage, and hyperventilation.