Extracorporeal Membrane Oxygenation
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Direct answer
Days to weeks of extracorporeal support — a pump driving venous blood through a membrane oxygenator that removes carbon dioxide and adds oxygen — is what extracorporeal membrane oxygenation (ECMO) provides, returning blood either to the venous system (venovenous, for respiratory failure) or the arterial system (venoarterial, for cardiac failure and selected arrests). In severe acute respiratory distress syndrome it is offered when optimized ventilation fails, using EOLIA trial thresholds — a PaO2/FiO2 below 50 for three hours, below 80 for six hours, or a pH below 7.25 with a PaCO2 of 60 mmHg or more for six hours — with systemic anticoagulation and bleeding as the dominant risks.
What you must remember
- Configurations: venovenous drains and returns within the venous system — it oxygenates and decarbonates but provides no circulatory support; venoarterial drains venous blood and returns it arterially — it supports both circulation and gas exchange, at the cost of arterial cannulation complications.
- Respiratory support (venovenous): indicated in severe, potentially reversible acute respiratory distress syndrome refractory to optimized lung-protective ventilation (EOLIA thresholds above), severe status asthmaticus and bridge-to-lung-transplant strategies; the circuit allows ultraprotective tidal volumes and plateau pressures, resting the injured lung while it heals.
- Cardiac support (venoarterial): refractory cardiogenic shock from myocardial infarction, myocarditis or post-cardiotomy failure, intractable arrhythmia, and extracorporeal cardiopulmonary resuscitation in selected in-hospital arrests; configuration is femorofemoral, central, or axillary depending on need and anticipated duration.
- The Harlequin (differential hypoxaemia) phenomenon of femoral venoarterial support: retrograde circuit blood supplies the lower body while the failing lungs desaturate blood ejected by the heart to the brain and coronaries — managed by improving native lung oxygenation, adding venous return (hybrid configuration), or repositioning the return cannula.
- Circuit and patient management: continuous systemic anticoagulation (unfractionated heparin, monitored by activated clotting time or aptt) balanced against bleeding; blood flow rates set to support; sweep gas flow controls carbon dioxide removal; native lung ventilation continues at ultralow settings with periodic recruitment assessment.
- Complications: bleeding at cannula and other sites including intracranial haemorrhage, circuit and patient thrombosis, haemolysis, limb ischaemia after femoral arterial cannulation (mitigated by distal perfusion catheters), infection, air embolism, and circuit component failure.
- Weaning and contraindications: venovenous weans as native lung function returns, trialled by reducing or stopping the sweep gas; venoarterial weans with flow reduction under echocardiographic assessment of ventricular recovery; ECMO is contraindicated without a plan for definitive therapy (irreversible disease unsuitable for transplant), by uncontrolled bleeding or anticoagulation contraindication, and relatively by prolonged high-pressure ventilation, advanced age and multiorgan failure.
Choosing the configuration, then running the circuit
Two referrals arrive the same night. The first is severe ARDS: prone positioned, on optimized ventilation, yet PaO2/FiO2 below 80 for six hours — an EOLIA threshold crossed — so venovenous ECMO follows. Venovenous drains and returns within the venous system: it oxygenates and decarbonates but adds no flow or pressure, permitting ultraprotective tidal volumes that rest the injured lung — and never treating shock. The second is cardiogenic shock: venoarterial ECMO, returning blood arterially to support circulation and gas exchange, also serving post-cardiotomy failure, arrhythmia and extracorporeal resuscitation. Running either circuit means continuous heparin by activated clotting time or APTT, sweep gas controlling carbon dioxide removal, and vigilance for bleeding, thrombosis, haemolysis, limb ischaemia and infection. In the femoral venoarterial patient, watch for the Harlequin phenomenon — circuit blood supplying the lower body while the failing lungs desaturate blood to the brain. ECMO is a bridge, not a treatment.
Where students slip
Circulatory support is attributed to venovenous circuits, but the venous return adds no flow — shock belongs to venoarterial. The Harlequin phenomenon confuses because lower-body saturations read normal while the brain is hypoxaemic. The EOLIA number set (50, 80, pH 7.25 with 60) is the most testable element, with bleeding the leading complication, and treating ECMO as therapy rather than bridge completes the errors.
Frequently asked questions
What is the difference between venovenous and venoarterial ECMO?
Venovenous supports gas exchange only, returning blood to the venous system; venoarterial supports gas exchange and circulation by returning blood to an artery.
What EOLIA criteria trigger venovenous support in severe ARDS?
PaO2/FiO2 below 50 for over three hours, below 80 for over six hours, or pH below 7.25 with PaCO2 of 60 mmHg or more for over six hours, despite optimized ventilation.
What is the Harlequin phenomenon?
Differential hypoxaemia during femoral venoarterial support in which the failing lungs desaturate blood ejected to the brain while the circuit supplies oxygenated flow below.
Why is anticoagulation required?
Contact of blood with the artificial surfaces of the circuit activates clotting, so unfractionated heparin prevents thrombosis — balanced against bleeding risk.
What are the leading complications?
Bleeding (including intracranial), thrombosis, haemolysis, infection, limb ischaemia after arterial cannulation and circuit failure.
How is venovenous ECMO weaned?
By reducing and then stopping the sweep gas while monitoring native gas exchange, alongside a spontaneous breathing assessment on minimal ventilator support.