Extracorporeal Membrane Oxygenation (ECMO) in Acute Respiratory Distress Syndrome

Introduction:

Since the 1970s, extracorporeal membrane oxygenation (ECMO) has been widely used to treat severe respiratory failure. However, significant complication rates, primarily due to technological limitations, result in poor prognosis early on. Advances in technology and treatment have reportedly increased survival with lower complication rates in recent years, culminating in an increase in the use of ECMO for severe acute respiratory distress syndrome (ARDS). As the technology continues to develop, ECMO has increased the potential to boost how ARDS is managed, most notably by facilitating lung-protective breathing and reducing ventilator-associated damage to the lungs.

What Is Extracorporeal Membrane Oxygenation (ECMO)?

ECMO is a system that uses a pump to collect blood from the body by means of a tiny tube, then passes the blood across a membrane oxygenator, where both the delivery of oxygen and removal of carbon dioxide take place before reinfusing the oxygen-rich blood back into the body using a thin tube, providing extracorporeal (outside the body) gas exchange.

Blood passes from the right side of the heart to the heart-lung machine's membrane oxygenator, where it is rewarmed and returned to the body. This technique enables blood to "bypass" the heart and lungs chambers, giving these organs time to rest and recuperate. ECMO is utilized in critical care conditions when the lungs and heart require assistance to continue to heal. It could help treat COVID-19, ARDS, and other infections.

What Are the Types of Extracorporeal Membrane Oxygenation?

ECMO designs include two types.

• Venovenous involves draining deoxygenated blood from a central vein and reinserting oxygenated blood into the same vein. Most ARDS cases include severe respiratory failure alone, necessitating a venovenous design.

• Venoarterial involves draining blood from a central vein and reinserting it into a central artery.

• Venovenous ECMO offers respiratory support, whereas venoarterial ECMO also provides hemodynamic support. Most ARDS cases include extreme respiratory distress, for which a venovenous arrangement is ideal.

• Some episodes of ARDS may be accompanied by severe cardiac shock. In such cases, a hybrid technique may be required to offer circulatory support and appropriate upper-body oxygen, in which blood is removed from a vein and replenishment is split between a central artery and a vein.

A Two-Site Strategy for Venovenous ECMO Cannulation: The venous drainage cannula (a thin tube inserted into the vein) is usually inserted into a femoral vein (a large blood vessel in the thigh) and extended into the inferior vena cava (a big vein that transports oxygen-deprived blood from the lower and middle bodies to the heart's right atrium). A pump is used to extract blood from the cannula. This blood is pushed ahead via an oxygenator and reinfused into the body. The venous reinfusion cannula generally goes into an internal jugular vein (a vein that carries blood from the brain and parts of the face and neck). It extends into the right atrium, where reinfusion takes place.

Cannulation of the Venovenous-Arterial System for ECMO: The cannula (a small tube inserted into the vein) for venous drainage penetrates a femoral vein and continues into the inferior vena cava. This blood is subsequently introduced into a pump, pushed through an oxygenator beforehand, and reinfused into the patient's body. A venous cannula, often put in an internal jugular vein, and an arterial cannula, typically implanted in a femoral artery, is used for blood reinfusion.

Technique to Venovenous ECMO Cannulation at a Single Location: A dual-lumen cannula is inserted into the internal jugular vein and ends in the inferior vena cava. The pump draws blood into the drainage lumen via openings in the inferior and superior vena cava. This blood is pushed ahead using the oxygenator before getting reinfused by the cannula's second lumen, which has a link in the right atrium, and the flow of blood is channeled through the tricuspid valve.

How Is Extracorporeal Membrane Oxygenation Managed in ARDS?

When a patient is ready for venovenous ECMO, a gas supply, often a mixture of oxygen and air, is linked to the membrane oxygenator, and the fraction of supplied oxygen (FDO2) is set using a gas blender. This gas, known as sweep gas, flows across one aspect of a membrane that is semipermeable. In contrast, blood flows across the other, with the barrier allowing oxygen and carbon dioxide to diffuse along the appropriate gradient. At high blood flow rates, the principal determinant of carbon dioxide removal is the sweep gas flow rate, which can be titrated to PaCO2 or pH.

Oxygenation and Carbon Dioxide Removal Parameters:

• Oxygenation Parameters

  1. The proportion of extracorporeal blood flow to total cardiac output.

  2. Percentage of oxygen provided using oxygenator.

  3. Hemoglobin.

  4. Membrane characteristics.

  5. Recirculation.

  6. Local gas exchange.

• Carbon Dioxide Removal Parameters

  1. The flow rate of gas (sweep gas) via the oxygenator.

  2. Lung function's contribution.

  3. Extracorporeal circulation of blood.

  4. Membrane characteristics.

  5. Local gas exchange.

Anticoagulants: To reduce the danger of blood clot development, all ECMO circuits must employ systemic anticoagulation, having heparin being the most routinely used anticoagulant. Although there are no commonly acknowledged anticoagulation objectives for ECMO, several centers have chosen an activated partial thromboplastin time of 40 to 60 seconds as a target that offers appropriate circuit anticoagulation while avoiding any possible bleeding problems.

What About Mobilization When Receiving Extracorporeal Support?

Occupational and physical therapy interventions have been found to improve functioning, decrease delirium, and increase ventilator-free days in patients with acute respiratory failure.

While individuals needing ECMO for ARDS are frequently too severely sick to participate in active rehabilitation efforts, it could be achievable in suitable individuals at multimodal physical therapy clinics. The positive effect of moving ARDS patients on ECMO is not well documented, and it must be balanced against the dangers of rehabilitation in this group of people.

What Are the Complications of Extracorporeal Support?

Some of the possible risks or problems of ECMO include, among other things, bleeding, thrombosis, hemolysis, and infection, which must be evaluated against the prospective benefits while selecting suitable individuals for ECMO treatment. Many institutions have established a lower anticoagulant objective to reduce the risk of bleeding while preserving circuit integrity. Although improvements to extracorporeal technology and procedures and more experience have reduced the incidences of these problems over time, the risk of ECMO persists significantly.

Conclusion:

Despite an absence of rigorous, high-quality evidence, modern-day ECMO is rapidly being regarded as a feasible salvage therapy for individuals with severe ARDS and can potentially enhance lung-protective oxygenation in these individuals, but with questionable benefits.