Veno-Venous ECMO (VV ECMO) has been an important therapeutic option for severe respiratory failure in patients with COVID-19, particularly in cases where conventional mechanical ventilation and prone positioning fail to maintain adequate oxygenation. VV ECMO provides extracorporeal respiratory support, allowing time for the lungs to heal while bypassing the damaged respiratory system. This is particularly critical for COVID-19 patients with severe acute respiratory distress syndrome (ARDS), which can result from the viral infection causing widespread lung inflammation and significant hypoxemia (low blood oxygen levels).

Why VV ECMO for COVID-19?

COVID-19 infection, caused by the SARS-CoV-2 virus, can lead to severe ARDS in some patients, particularly those who are elderly, have comorbidities, or have severe pneumonia. In such cases, the lung’s ability to exchange gases is impaired, leading to severe hypoxemia (low oxygen levels in the blood) and hypercapnia (high carbon dioxide levels).

VV ECMO can be used as a last-resort intervention when lung-protective mechanical ventilation fails and the patient’s respiratory status continues to deteriorate, especially when oxygenation cannot be sufficiently restored with ventilator settings.

Indications for VV ECMO in COVID-19

1. Refractory Hypoxemia:
• Severe hypoxemia (PaO2/FiO2 ratio < 100 despite maximum ventilator settings or FiO2 > 0.9).
• Persistent low oxygen levels, even with high levels of positive end-expiratory pressure (PEEP) and prone positioning, indicate the lungs cannot adequately oxygenate the blood.
2. Inability to Tolerate High Ventilator Settings:
• When conventional mechanical ventilation leads to high airway pressures and risk of ventilator-induced lung injury (VILI) (e.g., barotrauma, volutrauma), VV ECMO can help unload the lungs and prevent further injury.
3. Hypercapnic Respiratory Failure:
• When hypercapnia (high carbon dioxide) and respiratory acidosis persist, despite optimal ventilator settings, VV ECMO can help to remove carbon dioxide and improve acid-base balance.
4. Failure of Non-Invasive Ventilation:
• In cases where non-invasive mechanical ventilation (e.g., high-flow nasal cannula or BiPAP) or intubation with mechanical ventilation fails to support the patient’s oxygenation and respiratory needs, VV ECMO can serve as a bridge to recovery.
5. Severe ARDS with Reversible Cause:
• COVID-19-related ARDS is often reversible with time, so VV ECMO can act as a temporary support to allow the lungs time to heal as the viral infection resolves and the inflammation subsides.
6. No Contraindications for ECMO:
• Patients should not have severe multisystem organ failure, irreversible neurological injury, or other conditions that make survival unlikely, as ECMO is typically used in patients with a reversible condition that is expected to improve with time.

How VV ECMO Works in COVID-19

VV ECMO provides extracorporeal respiratory support, bypassing the lungs and taking over the critical functions of oxygenating the blood and removing carbon dioxide.

1. Venous Cannulation:
• Blood is drained from a large vein (usually the femoral vein, internal jugular vein, or subclavian vein) using a cannula.
2. Oxygenator:
• The blood flows through the ECMO circuit, where it is passed through an oxygenator. This component of the circuit adds oxygen to the blood and removes carbon dioxide, mimicking the lung’s role in gas exchange.
3. Venous Return:
• The oxygenated blood is then returned to the venous circulation, usually through a second cannula placed in another vein. The oxygenated blood flows back to the heart, where it is pumped to the lungs and the rest of the body.
4. Ventilator Settings:
• Since VV ECMO is supporting the lungs, the patient may still need to be on a ventilator, but the settings can be lung-protective (i.e., low tidal volumes, low pressures), reducing the risk of ventilator-induced damage to the lungs.
5. Management of Blood Flow and Gas Exchange:
• The ECMO pump adjusts the blood flow to optimize oxygenation, while the ventilator settings and ECMO parameters are adjusted based on the patient’s oxygenation status and overall condition.

Benefits of VV ECMO in COVID-19

1. Improved Oxygenation and CO2 Removal:
• VV ECMO helps restore oxygen levels in the blood and remove carbon dioxide, allowing the body’s organs to function properly while the lungs heal from COVID-19-induced inflammation.
2. Lung Rest and Protection:
• ECMO reduces the work the lungs must do by facilitating gas exchange externally and allowing the lungs to rest, which can help prevent further damage from mechanical ventilation (e.g., barotrauma or volutrauma).
3. Reduced Risk of Ventilator-Induced Lung Injury (VILI):
• By allowing the lungs to rest, VV ECMO reduces the need for high-pressure ventilation, thereby decreasing the risk of VILI and minimizing the chances of further injury to the lungs.
4. Bridge to Recovery:
• VV ECMO serves as a bridge to recovery, providing critical support until the patient's lungs recover from COVID-19-induced ARDS. It allows time for viral clearance, reduction of pulmonary inflammation, and eventual lung healing.
5. Avoiding Intubation-Associated Damage:
• In some severe cases, ECMO can be a viable option when intubation or invasive mechanical ventilation has failed or if the patient is unable to tolerate these interventions due to airway trauma or barotrauma.

Risks and Challenges of VV ECMO in COVID-19

1. Bleeding Complications:
• Anticoagulation is required for ECMO to prevent clot formation in the circuit, but this increases the risk of bleeding, including at the cannulation site or intracranial hemorrhage. Patients with COVID-19 may also have coagulopathy, which increases bleeding risks.
2. Infection:
• Prolonged ECMO support increases the risk of infection, especially in critically ill patients who already have immunosuppression, multiple central lines, or prolonged hospital stays. This can include infections of the ECMO circuit, cannulation sites, or hospital-acquired infections.
3. Vascular Complications:
• The process of cannulation for VV ECMO can lead to vascular damage or thrombosis, potentially affecting the femoral, subclavian, or jugular veins, leading to limb ischemia or thrombosis.
4. Organ Dysfunction:
• Prolonged ECMO use can contribute to multi-organ failure, particularly in patients with pre-existing comorbidities such as kidney dysfunction, liver dysfunction, or cardiac complications.
5. Technical Issues:
• The ECMO circuit requires constant monitoring for potential problems, such as clotting, pump failure, or oxygenator malfunction, and requires rapid intervention to prevent adverse outcomes.
6. Duration and Weaning:
• Weaning from ECMO can be challenging and requires careful monitoring of lung recovery. Extended periods on ECMO are associated with increased complications and worse outcomes. Early initiation and timely weaning are critical to improving survival chances.

Considerations for ECMO in COVID-19

1. Timing of ECMO:
• Early identification of candidates for VV ECMO is key. Studies suggest that early intervention (before the onset of multi-organ failure) leads to better outcomes. However, ECMO should only be used after other therapies have been exhausted and should be considered in patients with reversible ARDS.
2. Patient Selection:
• Not all patients with COVID-19-related ARDS are candidates for ECMO. Ideal candidates typically have severe hypoxemia or hypercapnia that is refractory to conventional therapy but still have a reasonable chance of recovery. Patients with irreversible organ damage, advanced age, or poor prognosis may not benefit from ECMO.
3. Multidisciplinary Team:
• The decision to initiate ECMO in COVID-19 patients requires input from a multidisciplinary team, including intensivists, ECMO specialists, respiratory therapists, and cardiologists, who will carefully evaluate the patient’s prognosis, the reversibility of ARDS, and the likelihood of recovery.

Outcomes and Prognosis

• Survival Rates: The prognosis for patients on VV ECMO for COVID-19 varies, with early initiation generally leading to better outcomes. Studies suggest that the survival rate for patients receiving ECMO for COVID-19-related ARDS can range from 50% to 70%, depending on various factors, including the severity of ARDS, age, comorbidities, and timing of ECMO initiation.
• Lung Recovery: In many cases, if the underlying COVID-19 infection is resolved and pulmonary inflammation subsides, the lungs can recover with ECMO support serving as a bridge. However, some patients may require lung transplantation if recovery is not possible.

Conclusion

VV ECMO has proven to be a valuable tool in the management of severe ARDS caused by COVID-19, offering critical support when conventional mechanical ventilation fails. By allowing the lungs to rest and providing extracorporeal gas exchange, VV ECMO gives patients time to recover while preventing further damage. However, ECMO is associated with risks such as bleeding, infection, and vascular complications, making careful patient selection and monitoring essential for achieving the best outcomes. As the pandemic has evolved, ECMO has become an important therapy for patients with severe COVID-19, but its success relies on early intervention, careful management, and multidisciplinary care.