Extra-corporeal membrane oxygenation (ECMO) is a technique of providing total cardiac and/or respiratory support. It uses an extra-corporeal circuit similar to cardiopulmonary bypass. It remains the mainstay of short-term mechanical circulatory support in neonates, infants and children with cardiac and/or respiratory failure.
The two commonest modes of ECMO support are
Veno-venous ECMO, which provides respiratory support only. Blood is drained from the venous circulation, oxygenated and returned to the venous circulation (VVECMO). In VVECMO, no direct cardiac support is provided.
Veno-arterial ECMO, which provides cardiac and respiratory support. Blood is drained from the venous circulation, oxygenated and returned to the arterial circulation, thereby bypassing the heart and lungs (VAECMO).
The most frequent indication for ECMO remains the treatment of neonatal respiratory failure secondary to aetiologies such as persistent pulmonary hypertension of the newborn (PPHN), meconium aspiration, sepsis, respiratory distress syndrome and congenital diaphragmatic hernia (CDH). Internationally, there is an increasing use of ECMO in children with myocardial failure following cardiac surgery or as a consequence of myocarditis or cardiomyopathy.
The aim of support is to provide adequate oxygenation and preservation of end-organ function to allow time for either myocardial recovery (‘bridge to recovery’) or as temporary support before converting to longer-term support such as a ventricular assist device (VAD) (‘bridge to bridge’) or as support to transplantation (‘bridge to transplant’).
Data from the Extracorporeal Life Support Organisation (ELSO) 2012 international registry reports an overall survival rate to discharge for neonates and children of 68.5 per cent following ECMO. The best survival rates are seen in neonates and children following respiratory ECMO of 75 and 56 per cent, respectively. The survival to discharge following ECMO for cardiac support is 40 per cent in neonates and 49 per cent in children.
ECMO should be considered in patients with circulatory and/or respiratory failure that is refractory to maximal conventional treatment, providing that the underlying disease process is potentially reversible and that they do not have an absolute contra-indication.
In respiratory failure, the oxygen index (OI) can be used to assess the severity of the illness. An OI greater than 25 is associated with a mortality of 25 per cent with conventional respiratory support, and the mortality rises to 80 per cent with an OI of over 40.
An OI in excess of 40 is usually an indicator for the need for respiratory ECMO. However, the OI must be assessed as part of the overall condition of the child and his or her response to treatment. An OI of 43 that rapidly falls in response to high-frequency oscillation ventilation or nitric oxide is not necessarily an indication for ECMO in contrast to an OI of over 30 that gradually climbs regardless of all therapeutic interventions.
There is no equivalent to OI for cardiac support. VAECMO should be considered in patients preoperatively who either cannot be stabilized prior to corrective surgery or are in low cardiac output that is either thought to be recoverable or as a bridge to cardiac transplantation. Increasingly, following cardiac surgery, ECMO is being used in patients who cannot be separated from cardiopulmonary bypass or in whom there is refractory low cardiac output postoperatively. There is also an increasing use of ECMO following cardiac arrest (ECPR), but in most institutions this is limited to patients following cardiac surgery or during a witnessed cardiac arrest in hospital. The indicators for cardiac support are those of persistent low cardiac output such as persistent metabolic acidosis, high inotropic requirement, hypotension and oliguria in the setting of myocardial failure or dysrhythmias.
Brain death and irreversible multi-organ dysfunction are clear contra-indications to ECMO. Prolonged anti-coagulation and severely reduced long-term functional ability are similarly contra-indications. A corrected gestational age of less than 34 weeks is associated with a significant increase in morbidity and mortality, as well as presenting problems with cannulation and management and consequently is a contra-indication. Previously, prolonged mechanical ventilation at high pressure for greater than 10 days was a contra-indication, but this is increasingly becoming only a relative contra-indication.