The circuit consists of a pump, blender, oxygenator, control console, heater/cooler and two cannulas. The blender mixes oxygen with CO2. This mixture of gas usually consists of 95 % oxygen and 5 % CO2 and is referred to as the ‘sweep gas’. The pump is usually a centrifugal or roller pump and is used to transport blood throughout the circuit. The heater/cooler aids in temperature regulation of extracorporeal blood. The cannulas provide direct transport of oxygenated and deoxygenated blood to and from the patient.
The initial oxygenators consisted of venous blood pools that were oxygenated with oxygen bubbles. These first clinical attempts at extracorporeal life support began in the 1950s and were largely unsuccessful beyond several hours as direct exposure of blood to oxygen led to life-threatening coagulopathy, haemolysis, and multi-organ failure. In 1959, Burns developed the definitive solution involving a silicone membrane that separated blood from oxygen and allowed for indirect oxygenation of blood . This was a major step in extracorporeal life support. In the years following (1972–1976) the first published reports of the successful use of extracorporeal life support emerged . Modern membranes are largely made from polymethyl-pentene, which is lower in resistance and causes less consumption of blood products .
Indications for initiation of adult ECMO
VA ECMO is used for both cardiac and pulmonary support such as acute cardiac failure or failure to wean from cardiopulmonary bypass after cardiac surgery . VV ECMO is used for reversible respiratory failure with normal cardiac function. The most common use is in acute respiratory distress syndrome, which may be secondary to severe pneumonia or influenza . More specifically, eligibility was defined in the CESAR trial by a Murray score >3, a pH of less than 7.20 and having a reversible disease process.
The Murray score consists of four factors including (1) the PaO2/FiO2 ratio, (2) PEEP (positive end expiratory pressure), (3) dynamic lung compliance in ml/cmH20 and (4) number of quadrants infiltrated on radiographs  (Fig. 1b). Each element is graded on a scale of 0–4. The scores are then averaged and the mean score is used to grade the severity of respiratory disease; 0 implies no significant disease and 4 implies the most severe disease. Normal for the above elements are PaO2/FiO2 ≥ 40 kPa, PEEP ≤ 5, compliance ≥ 80 and 0 quadrants infiltrated on the radiograph .
Contraindications to ECMO
Relative contraindications include high pressure ventilation, high FiO2, limited vascular access and organ dysfunction that would lead to poor quality of life. In patients with pre-existing disease such as metastatic cancer or severe irreversible brain injury, ECMO may be contraindicated. Absolute contraindications include any conditions that preclude anticoagulation .
Imaging modalities used in evaluation of ECMO
Transthoracic echocardiography (TTE) and transesophageal echocardiography (TEE) play crucial roles during initial cannula placement. A full discussion on the role of echocardiography in ECMO is discussed by Platts et al. in “The Role of Echocardiography in the Management of Patients Supported by Extracorporeal Membrane Oxygenation” .
Radiographs of the chest and abdomen are useful initial examinations for cannula position. They may reveal malposition or unintended migration. They often offer the first clue towards complications such as haemothorax, pneumothorax or mediastinal fluid collections.
Ultrasound is a portable, focussed and readily available modality that can be used to evaluate for insertion site haematomas, peri-cannula thrombus or deep venous thrombus. Spectral colour Doppler is a powerful tool to evaluate distal limb perfusion as the arterial return cannula can lead to obstruction/occlusion with decreased forward flow in the access artery. It can also be used for evaluating thrombus in the access vein. Interpretation of spectral Doppler waveforms with comparison of flow in the contralateral vessel can help in differentiating slow flow, which may be due to a systemic cause such as poor cardiac output, from slow flow in the access artery from either the access cannula itself or thrombus.
CT is reserved for evaluation of complications that cannot be fully evaluated by radiographs, ultrasound or Doppler imaging. While lacking the functional data of spectral Doppler, CT provides excellent anatomic detail. This may be used in evaluating cannula position or malposition, haematoma formation, haemothorax, stroke or arterial/venous thrombus in large vessels. A full summary of the different imaging modalities and their indications are provided in Table 2.
VV ECMO is usually placed percutaneously and peripherally. Several configurations are seen. In the ‘femoroatrial’ configuration the drainage cannula is introduced via the femoral vein and advanced to the level of the diaphragm, below the hepatic veins . The return cannula is introduced via the internal jugular vein and advanced to the level of the superior vena cava (SVC)-atrial junction (Fig. 1). The tip is directed towards the tricuspid valve. This particular configuration is optimal to minimise recirculation , a phenomenon in which a portion of the returning oxygenated blood is prematurely siphoned back to the ECMO circuit. Recirculation will result in less oxygenated blood returning to the pulmonary circulation and thus the systemic circulation.
A configuration in which the drainage cannula and return cannula are introduced via the femoral veins is termed 'femorofemoral ECMO'. In this configuration the drainage cannula may be placed on one side and the return cannula on the other. Alternatively, both cannulas may be introduced on the same side. In either case the drainage cannula is advanced to the distal inferior vena cava (IVC) and the return cannula is advanced to the right atrium .
Another configuration of VV ECMO is the single cannula, dual lumen ECMO (Fig. 2a) . This cannula is usually inserted via the right internal jugular vein with the tip advanced to the IVC. Alternatively this cannula may be inserted via the femoral vein with the tip advanced to the SVC. The atrial return port is identified on radiographs as a lucency in the cannula (Fig. 2b, c). Rarely, this dual lumen cannula may be inserted via the left internal jugular vein if the right is scarred or thrombosed. The distal tip of this cannula drains deoxygenated blood and a separate opening at the level of the right atrium allows for oxygenated blood return.
Compared with VV ECMO, the VA variation is associated with a higher incidence of bleeding, a greater need for transfusion, an increased need for reoperation and greater resource utilisation . Despite this, VA ECMO is preferred when cardiac support is required in addition to pulmonary support and also after sternotomy. The two main types are central and peripheral. Central refers to cannula placement in the mediastinum and peripheral VA ECMO refers to placement outside of the mediastinum . A summary comparison between VA and VV ECMO can be found in Table 3.
Also known as mediastinal VA ECMO, central VA ECMO involves direct placement of cannulas into the aorta (return cannula) and right atrium (drainage cannula) through an open sternum (Fig. 3). This is usually done when there is failure to wean from cardiopulmonary bypass in the operating room as direct access is readily available. If left ventricular (LV) function is poor, a drainage catheter may be placed in the LV (Fig. 4) to aid in decompression . Left ventricular unloading is indicated when there is an extremely poor ejection fraction with a closed aortic valve . This predisposes the patient to stasis of blood in the ventricle and subsequently thrombus formation. Patients with aortic regurgitation may also have a LV drain as retrograde flow of blood may compromise ventricular recovery after the initial insult . Finally, patients with severe pulmonary oedema may also require an LV drain to help decrease the pulmonary afterload . The drain can be introduced directly into the left ventricle after sternotomy. Alternatively, the LV drain can also be introduced via the femoral artery or right upper pulmonary vein. The LV drain is connected to the ECMO circuit along with the drainage cannula.
In peripheral VA ECMO, the arterial cannula may be seen projecting over the femoral or axillary arteries. At our institution, carotid artery cannulation is less commonly used in adults. The arterial cannulas are usually advanced to the level of the iliac arteries or abdominal aorta , and usually multiple surgical clips are used for anchoring that are readily visualised on radiographs. The venous cannulas may be seen on the same side as the arterial cannula but are advanced closer to the right atrium, terminating in the SVC or IVC (Fig. 5). Often a distal perfusion catheter is placed in the access artery to prevent distal perfusion defects and limb ischaemia (Fig. 6).