# Extracorporeal Membrane Oxygenation (ECMO)

## Introduction

Extracorporeal membrane oxygenation (ECMO) provides prolonged cardiopulmonary support for patients with severe cardiac or respiratory failure refractory to conventional management. Originally adapted from cardiopulmonary bypass technology, modern ECMO circuits offer improved biocompatibility, reduced priming volumes, and enhanced gas exchange, allowing support durations measured in weeks rather than hours.

## ECMO Configurations

### Venovenous (VV) ECMO

VV ECMO provides respiratory support only with no direct hemodynamic augmentation. Blood is drained from a central vein, oxygenated, and returned to the venous system proximal to the right atrium. Common cannulation strategies include femoral vein drainage with internal jugular vein return, or alternatively a dual-lumen bicaval cannula via the right internal jugular vein. Indications include severe ARDS, bridge to lung transplant, and primary graft dysfunction after lung transplant.

### Venoarterial (VA) ECMO

VA ECMO provides both respiratory and hemodynamic support. Blood is drained from the venous system and returned to the arterial circulation. Peripheral cannulation from the femoral vein to femoral artery is the most common emergent approach, while central cannulation from the right atrium to the ascending aorta is typically used in the post-cardiotomy setting. Indications include cardiogenic shock, post-cardiotomy failure, bridge to decision for advanced heart failure therapies, and cardiac arrest (ECPR).

### Hybrid Configurations

VAV ECMO combines arterial and venous return to address concurrent cardiac and pulmonary failure. VA ECMO with left ventricular venting incorporates an Impella or atrial septostomy to decompress the left ventricle.

### VV ECMO vs. VA ECMO Comparison

| Feature | VV ECMO | VA ECMO |
|---------|---------|---------|
| Support type | Respiratory only | Respiratory + hemodynamic |
| Drainage site | Central vein (femoral or IJ) | Central vein (femoral vein or RA) |
| Return site | Central vein (IJ or femoral) | Artery (femoral artery or aorta) |
| Cannulation options | Femoral-IJ; dual-lumen bicaval (IJ) | Peripheral (femoral-femoral); central (RA-aorta) |
| Hemodynamic augmentation | None (relies on native cardiac function) | Full circulatory support |
| Key indications | Severe ARDS; bridge to lung transplant; PGD after lung transplant | Cardiogenic shock; post-cardiotomy failure; ECPR |
| LV afterload effect | No direct effect | Increased (retrograde aortic flow) |
| Harlequin syndrome risk | No | Yes (peripheral VA; monitor right hand SpO2) |
| Limb ischemia risk | Low | High (femoral arterial cannula; requires distal perfusion catheter) |
| Recirculation risk | Yes (if cannulae too close) | Minimal |

## Circuit Components

The ECMO circuit consists of a large-bore drainage cannula (21-29 Fr multi-stage venous), a centrifugal pump generating continuous non-pulsatile flow (typically 3-6 L/min), a membrane oxygenator using polymethylpentene hollow-fiber technology for gas exchange, an integrated heat exchanger for temperature regulation, flow and pressure sensors for continuous monitoring of pre- and post-oxygenator pressures, and heparin-bonded tubing to reduce systemic anticoagulation requirements.

## Cannulation Techniques

### Peripheral Cannulation

Peripheral cannulation is performed at the bedside using the Seldinger technique under ultrasound and fluoroscopic guidance. Femoral artery cannulation requires a distal perfusion catheter (6-8 Fr) to prevent limb ischemia. Wire position must be confirmed in the appropriate vessel with transesophageal echocardiography or fluoroscopy before dilation.

### Central Cannulation

Central cannulation requires sternotomy or thoracotomy for direct right atrial and aortic cannulation. This approach provides superior flow and reduced recirculation and is reserved for post-cardiotomy shock or when peripheral access is inadequate.

## Management on ECMO

### Anticoagulation

Unfractionated heparin infusion is titrated to an activated clotting time (ACT) of 180-220 seconds or anti-Xa of 0.3-0.7 IU/mL. Bivalirudin is used for heparin-induced thrombocytopenia. Careful monitoring for circuit thrombosis, oxygenator dysfunction, and coagulopathy is essential.

### Hemodynamic Optimization

Target flows of 60-80 mL/kg/min are maintained for VA ECMO. Native cardiac function is assessed with serial echocardiography. Left ventricular distension on VA ECMO is a critical complication that requires venting strategies.

### Respiratory Management on VV ECMO

Ventilator settings are reduced to lung-protective parameters with FiO2 below 0.4, PEEP of 10-15, and tidal volume of 4 mL/kg. Sweep gas flow controls CO2 removal while FiO2 on the circuit controls oxygenation. Persistent hypoxemia despite adequate ECMO flows should prompt a differential diagnosis including recirculation and cannula malposition.

## Complications

Hemorrhage is the most common complication, occurring in 30-50% of patients, with cannulation sites and surgical wounds being the most frequent sources. Thromboembolism includes circuit clots, stroke, and limb ischemia. Hemolysis produces elevated plasma-free hemoglobin from shear stress. Infection at cannula sites and bloodstream infections increase with duration. Limb ischemia affects 10-20% of patients with peripheral VA ECMO and should be monitored with near-infrared spectroscopy and serial examinations. Neurological injury includes stroke, seizures, and intracranial hemorrhage.

## Weaning and Decannulation

VV ECMO weaning involves reducing sweep gas to zero and observing gas exchange on ventilator alone for 1-6 hours. VA ECMO weaning requires gradual flow reduction (typically to 1-1.5 L/min) with echocardiographic assessment of ventricular function. Peripheral cannulas are decannulated with direct pressure or surgical repair of vessels, while central cannulas require operative removal.

## Key Clinical Pearls

A distal perfusion catheter should always be placed at the time of femoral arterial cannulation to prevent limb ischemia. On VA ECMO, Harlequin syndrome (differential hypoxemia) must be monitored by placing a pulse oximeter on the right upper extremity. Left ventricular distension on VA ECMO worsens myocardial recovery and should prompt early intervention with venting or Impella. A rising transmembrane pressure gradient across the oxygenator signals impending oxygenator failure and the need for circuit exchange. Daily assessment of neurological status and end-organ perfusion is essential throughout the ECMO run.

## References

1. Extracorporeal Life Support Organization (ELSO). *ELSO Guidelines for Cardiopulmonary Extracorporeal Life Support*. Version 1.4, 2017.
2. Combes A, Hajage D, Capellier G, et al. Extracorporeal membrane oxygenation for severe acute respiratory distress syndrome. *N Engl J Med*. 2018;378(21):1965-1975.
3. Lorusso R, Shekar K, MacLaren G, et al. ELSO interim guidelines for venoarterial extracorporeal membrane oxygenation in adult cardiac patients. *ASAIO J*. 2021;67(8):827-844.
4. Cheng R, Hachamovitch R, Kittleson M, et al. Complications of extracorporeal membrane oxygenation for treatment of cardiogenic shock and cardiac arrest. *J Am Coll Cardiol*. 2014;63(23):2601-2610.