How does Extracorporeal Membrane Oxygenation (ECMO) work in patients with severe cardiac or respiratory failure?

How ECMO Works

ECMO is a mechanical life support system that temporarily replaces heart and/or lung function by draining blood from the body, pumping it through an artificial membrane that adds oxygen and removes carbon dioxide, then returning the oxygenated blood back to the patient's circulation. 1

Basic Components and Mechanism

ECMO consists of three essential elements that work together to support failing organs 2, 3:

• Drainage cannula: Removes deoxygenated blood from the venous system (typically via femoral vein or internal jugular vein) 1
• Membrane oxygenator: A hollow-fiber membrane where oxygen is added to blood and carbon dioxide is removed through a concentration gradient created by sweep gas flowing on the opposite side 4, 3
• Centrifugal pump: Actively pumps blood through the circuit and returns it to the patient's circulation, providing continuous non-pulsatile flow 2, 3

Two Primary Configurations

Venovenous (VV) ECMO: Respiratory Support Only

VV ECMO provides isolated lung support for patients with severe respiratory failure but adequate cardiac function. 5

The circuit works as follows 1:

• Blood is drained from the venae cavae via femoral or internal jugular venous cannula
• After oxygenation in the membrane, blood returns to the venous system (femoral vein or internal jugular vein)
• A single bicaval double-lumen cannula inserted in the internal jugular vein can be used for both drainage and return
• Blood flow rates can reach up to 7 L/min, providing full or partial pulmonary support 1

Venoarterial (VA) ECMO: Combined Cardiopulmonary Support

VA ECMO provides both heart and lung support for patients with combined cardiopulmonary failure or cardiogenic shock. 5

The circuit differs critically from VV ECMO 1:

• Blood is drained from the right atrium via femoral/internal jugular venous cannula or directly from the right atrium in open-chest patients
• After oxygenation, blood is actively pumped into the arterial system via peripheral artery (usually femoral or subclavian) or directly into the aorta
• This provides hemodynamic stability in addition to gas exchange 5
• Can pump up to 7 L/min, providing full or partial cardiopulmonary support 1

Gas Exchange Mechanism

The membrane oxygenator functions through diffusion gradients 4, 3:

• Oxygenation: Determined primarily by blood flow rate through the circuit; higher flow = more oxygen delivery 4
• CO2 removal: Controlled by sweep gas flow rate through the membrane oxygenator, independent of blood flow 4
• Sweep gas (typically 100% oxygen or air) flows opposite to blood, creating concentration gradients that drive gas exchange 4

Critical Management Point: Sweep Gas Titration

Sweep gas flow must be adjusted carefully based on arterial blood gases to target PaCO2 35-45 mmHg, avoiding rapid drops >20 mmHg within 24 hours, as this is associated with acute brain injury and intracranial hemorrhage. 4

Extracorporeal CO2 Removal (ECCO2R): Low-Flow Variant

ECCO2R represents a distinct approach using much lower blood flow rates 1:

• Operates at 200-1,500 mL/min (compared to up to 7,000 mL/min for standard ECMO)
• Provides substantial CO2 removal but minimal oxygenation
• Uses venovenous or arteriovenous configuration
• Potentially simpler to implement in general ICUs 1

Key Physiological Principles

ECMO differs fundamentally from operating room cardiopulmonary bypass 1:

• Designed for prolonged support (days to weeks) rather than hours
• Provides temporary support for potentially reversible cardiac or respiratory failure 2, 6
• Creates continuous, non-pulsatile blood flow rather than pulsatile flow 2
• Requires continuous anticoagulation, leading to acquired von Willebrand syndrome in almost all patients within hours 5

Common Pitfall: Configuration Selection

The critical decision is whether the patient needs cardiac support or only respiratory support—this determines VV versus VA ECMO. 5

• Echocardiography is essential to assess cardiac function before cannulation 5
• VA ECMO is indicated when patients require significant inotropic support and/or norepinephrine >0.5 µg/kg/min 5
• VV ECMO suffices when cardiac function is adequate despite severe respiratory failure 5

Institutional Requirements for Safe Implementation

ECMO should only be performed at centers managing at least 20-25 cases annually, as higher-volume centers demonstrate significantly better outcomes. 1, 5

Essential infrastructure includes 1, 5:

• 24/7 availability of multidisciplinary ECMO team (physicians, nurses, perfusionists, ECMO specialists)
• Nurse-to-patient ratio of at least 1:1 to 1:2 for ECMO patients
• Mobile ECMO teams for patient retrieval from referring hospitals
• Quality assurance review procedures