What are the indications for extracorporeal membrane oxygenation (ECMO) and how does it work?

ECMO Indications and Mechanism

ECMO is a life-saving mechanical circulatory support system that should be deployed for severe refractory cardiogenic shock (INTERMACS level 1), cardiac arrest requiring extracorporeal CPR, refractory ventricular arrhythmias, acute right heart failure, and severe acute respiratory failure with refractory hypoxemia (PaO₂/FiO₂ <80 for >6 hours) or hypercapnia (pH <7.25, pCO₂ >60 mmHg) despite optimal protective ventilation. 1

Mechanism of Action

ECMO functions by removing venous blood from the patient, passing it through a membrane oxygenator where it is oxygenated via high-concentration sweep gas (pO₂ up to 600 mmHg) and carbon dioxide is eliminated, then returning the blood to the circulation 2, 3. This process allows the failing heart and/or lungs to rest and heal while maintaining adequate oxygen delivery to vital organs 3.

There are two primary configurations:

• Veno-venous (VV) ECMO: Provides respiratory support only by draining and returning blood to the venous system
• Veno-arterial (VA) ECMO: Provides both cardiac and respiratory support by draining venous blood and returning it to the arterial system 4, 3

Cardiac Indications (VA-ECMO)

For cardiac failure, initiate VA-ECMO when patients have:

• Severe refractory cardiogenic shock despite multiple vasoactive medications or other mechanical circulatory support devices (INTERMACS level 1)
• Active cardiopulmonary resuscitation for cardiac arrest (ECPR)
• Refractory ventricular arrhythmias unresponsive to conventional therapies
• Acute or decompensated right heart failure in the context of pulmonary hypertension or massive pulmonary embolism 1

The evidence supporting these indications remains limited, with practices varying widely across centers 1. The key principle is that ECMO should serve as a bridge—either to recovery, to definitive therapies (durable VAD or transplantation), or to allow time for reversible conditions to resolve 1.

Respiratory Indications (VV-ECMO)

For respiratory failure, initiate VV-ECMO when:

• PaO₂/FiO₂ ratio <80 mmHg for at least 3-6 hours despite maximal conventional therapies
• Refractory hypercapnia with pH <7.25 and pCO₂ >60 mmHg with respiratory rate >30/min
• Despite optimal protective mechanical ventilation (driving pressure <14 cmH₂O, plateau pressure <30 cmH₂O, tidal volume <6 mL/kg ideal body weight) 5, 2, 6

The primary goal is to prevent ventilator-induced lung injury while supporting gas exchange in severe ARDS 2, 6.

Absolute Contraindications

Do not initiate ECMO in patients with:

• Severe irreversible non-cardiac organ failure limiting survival (e.g., severe anoxic brain injury)
• Irreversible cardiac failure if transplantation or long-term VAD will not be considered
• Severe aortic insufficiency (for VA-ECMO)
• Aortic dissection (for VA-ECMO) 1

Relative Contraindications

Exercise caution with:

• Severe coagulopathy or contraindication to anticoagulation
• Limited vascular access
• Severe peripheral arterial disease
• Life-limiting comorbidities 1

Critical Implementation Considerations

ECMO is a high-risk, complex, and resource-intensive therapy that requires rigorous patient selection 1, 4. The major challenge is the frequent need for emergent deployment with limited time to assess patient appropriateness 1.

Key organizational requirements:

• Multidisciplinary team of experts to guide institutional use and patient care
• Minimum case volume of 30 ECMO cases per year, with substantial proportion for cardiac failure, to optimize outcomes 1
• Seamless transport pathways to comprehensive care centers capable of providing heart replacement therapies for patients stabilized on ECMO but unable to wean 1
• Continuous access to facilities, equipment, and staffing for both routine management and emergent complications 1

Common Pitfalls

The most frequent complications include bleeding, thrombus formation, and cannula-associated infections 2. Advanced patient age significantly impacts survival rates, particularly in COVID-19-associated ARDS 2.

Critical caveat: ECMO does not cure the underlying disease—it provides temporary support to allow organ recovery or serve as a bridge to definitive therapy 2, 6. Centers with limited resources should establish collaborative relationships with high-volume comprehensive care centers rather than attempting to manage complex cases independently 1.

The emergency clinician is uniquely positioned to affect the timeliness of ECMO referrals and should maintain a low threshold for early consultation with ECMO centers when patients meet physiologic criteria 5.