Extracorporeal Membrane Oxygenation (ECMO)

What Is ECMO?

• An inflow cannula - drains blood from the patient
• A centrifugal pump - maintains blood flow (magnetically levitated, preload-dependent, afterload-sensitive)
• A membrane oxygenator - gas exchange surface (~2 m² vs. ~143 m² in the human lung)
• An outflow cannula - returns oxygenated blood to the patient
• A heat exchanger - prevents heat loss across the oxygenator
• Continuous heparin infusion to prevent thrombosis

Basic ECMO circuit - Fishman's Pulmonary Diseases and Disorders

Two Main Types

1. Veno-Venous (VV) ECMO

• Purpose: Lung support only
• Cannulation: Venous drainage and venous return (e.g., femoral vein in, internal jugular vein out via dual-lumen cannula)
• Indication: Severe acute respiratory failure (ARDS, pneumonia, bridging to lung transplant)
• Advantage: Preserves native cardiac output; lower risk of limb ischemia
• Limitation: Does not provide hemodynamic (cardiac) support

2. Veno-Arterial (VA) ECMO

• Purpose: Heart AND lung support
• Cannulation: Venous drainage (femoral/internal jugular), arterial return (femoral artery, or via subclavian graft)
• Indications: Cardiogenic shock, cardiac arrest, refractory hypoxemia, bi-ventricular failure, myocarditis, myocardial stunning
• Key concern: Can cause pulmonary edema and LV fluid overload because it bypasses the LV without unloading it. Often requires additional strategies: inotropes, vasodilators, IABP, or Impella to unload the LV
• Bridges to recovery, VAD placement, or heart transplant

3. Hybrid Configurations

• VAV ECMO and Oxygenated RVAD (Protek Duo): Used when RV failure accompanies respiratory failure (e.g., ARDS with cor pulmonale)

Indications

Circuit Physiology & Key Parameters

• To achieve adequate O2 delivery (~260 mL O2/min) with post-oxygenator PaO2 >300 mmHg, blood flow of ~4 L/min must be maintained
• CO2 removal is efficient even at low flow rates (<1 L/min with high sweep gas)
• Transmembrane pressure drop should not exceed 30 mmHg (inlet ~250 mmHg, outlet ~220 mmHg)
• Tubing length/surface area matters: activates the inflammatory cascade, consumes clotting factors, and alters pharmacokinetics of antibiotics, opioids, and sedatives

Anticoagulation

Complications

Hemorrhagic

• Epistaxis, GI bleeding (most common)
• Rare: intracranial hemorrhage, pulmonary hemorrhage
• Mechanism: thrombocytopenia, platelet destruction, clotting factor consumption, anticoagulation

Thrombotic

• Circuit thrombosis (clot in oxygenator or tubing)
• Arterial embolism (especially with VA ECMO)

Hemodynamic (VA ECMO specific)

• LV distension and pulmonary edema (LV cannot eject against the increased afterload from arterial return)
• North-South syndrome (mixing of deoxygenated native LV output with oxygenated ECMO return)

Circuit/Technical

• Pneumothorax and arterial injury during cannulation
• Hemopericardium from guidewire cardiac injury
• Air embolism (especially with awake VV ECMO cannulation)
• Hemolysis (increases exponentially if a second extracorporeal circuit, e.g., CRRT, is added)
• Oxygenator failure (monitor by pressure drop and post-membrane gas values)

Systemic

• Inflammatory cascade activation (cytokine storm)
• Altered drug pharmacokinetics (sequesters antibiotics, sedatives, opioids in tubing)
• Acute kidney injury requiring CRRT (associated with OR 6.5 for in-hospital mortality)

Monitoring & Imaging

• TEE (Transesophageal echocardiography): Before and during cannulation to assess RV/LV function, detect PFO/ASD/VSD, rule out pericardial effusion, guide cannula tip position
• Fluoroscopy: Guides stiff wire and cannula placement
• Ultrasound: Linear surface ultrasound for percutaneous vascular access
• Pre/post-membrane pressure sensors + flow monitoring: Continuous circuit surveillance; transmembrane pressure >30 mmHg signals oxygenator failure

Bridge Strategies

Limitations

• VA ECMO does not compensate for severely impaired LV function and may worsen pulmonary congestion
• Not a permanent solution; prone to infection and complications with prolonged use
• Highly institution-dependent: deployment, monitoring, and troubleshooting require dedicated, experienced teams
• Large prime volume causes acute hemodilution at initiation
• Allosensitization risk if blood-primed in pre-transplant patients

Recent Evidence

• ECMO for interstitial lung disease (2024 meta-analysis) - Systematic review in ASAIO J examining outcomes of ECMO in ILD patients [PMID: 38810214]
• Mechanical circulatory support in cardiogenic shock (Lancet 2024) - Individual patient data meta-analysis of temporary MCS (including ECMO) in infarct-related cardiogenic shock at 6-month follow-up [PMID: 39236726]