Veno venous extracorporeal membrane oxygenation (VV ECMO) is an absolute rescue therapy for patients suffering from severe respiratory failure. The therapy has many advantages, but hypoxia is the most challenging to control. If not treated in time, hypoxia can lead to dangerous complications in a patient due to the low level of blood oxygen. This blog post will talk about the causes of hypoxia under VV ECMO, its effects, and how it can be controlled effectively.

What is VV ECMO?

VV ECMO is an extracorporeal life-support modality for patients with severe, potentially reversible respiratory failure. Unlike VA ECMO, which will support the heart and lungs, VV ECMO only supports the lungs. It is initiated by draining blood from a central vein, passing it through an oxygenator to remove carbon dioxide and add oxygen, and finally returning it to the venous system. VV ECMO can be initiated centrally or peripherally. Commonly used peripheral access includes femoral vein and internal jugular vein access.

Common Causes of Hypoxia in VV ECMO Many factors cause hypoxia in VV ECMO; the most found reasons include:

Problems with the Functioning of the ECMO Circuit: Any significant issue may arise in oxygenator failure, clot formation or kinking in the circuit that may affect optimal oxygenation.

Patient-related Factors: Severe lung pathology, high metabolic demands, and inadequate native lung function are other factors that could contribute to hypoxia.

Mechanical Ventilation Issues: Incorrect ventilator settings or ventilator-induced lung injury can exacerbate hypoxia.

Low Hemoglobin: Anemia leads to a reduction of oxygen-carrying capacity, causing hypoxia. A low haemoglobin level can shift the oxygen dissociation curve, indicating the haemoglobin has a reduced affinity for oxygen. This means that oxygen is released more easily to the tissues, but less oxygen is carried by the blood. This may lead to hypoxia.

Correction of Hypoxia

Hypoxia is a condition where there is insufficient oxygen in the blood to meet the body's needs. It can cause serious complications, particularly in critical care settings such as VV ECMO, where maintaining adequate oxygenation is crucial. Managing hypoxia effectively involves addressing multiple factors to ensure optimal patient outcomes.

- ECMO Circuit—Maximising Oxygenator Performance: The efficacy of the oxygenator must be checked at frequent intervals and replaced at the earliest. Pre- and post-oxygenator gas values can reflect the performance of the oxygenator.
- ECMO Circuit—Monitoring: An ECMO circuit must be routinely checked for clots, kinks, or any abnormality that may result in flow complications. Any abnormalities should be promptly addressed.
- Flow Rates: Blood flow rates can be manipulated to obtain the maximum oxygen delivery. High flow rates generally lead to high oxygenation, but this needs to be carefully treated against complications.

Improving Patient Care

Ventilator Settings: Mechanical ventilation remains essential even with using ECMO. Protective lung strategies that employ low tidal volumes and a reasonable level of PEEP are advised to avoid ventilator-induced lung injury.

Sedation and Paralysis: Adequate sedation and, when necessary, paralysis could decrease metabolic demands and improve patient-ventilator synchrony and thus could help improve oxygenation.

Management of Haemoglobin

Blood Transfusions: Adequate haemoglobin is needed to maintain adequate oxygen delivery. Consider blood transfusions if anemia is contributing to hypoxia.

Erythropoietin Therapy: Erythropoietin therapy may sometimes be used to stimulate red blood cell production to raise haemoglobin levels and thus increase oxygen-carrying capacity.

Alternate Ways to initiate VV ECMO

Dual Lumen Cannulas - Using dual-lumen cannulas can help improve the efficiency of VV ECMO by enhancing venous drainage and reinfusion dynamics.

Regulatory mechanisms for gas exchange on VV ECMO

Sweep gas flow across the oxygenator - When regulated, can allow changes in carbon dioxide removal and, hence, indirectly impair oxygenation. High flows of sweep gas improve CO2 removal and thus affect gas exchange maximally.

Infection

Infection aggravates hypoxia through increased metabolic demands and worsening lung functions; infection control and timely treatment of diseases are paramount in complication management.

Nutritional Support: Adequate nutritional support helps meet the metabolic needs of critically ill patients and can improve oxygenation, resulting in better overall outcomes.

Advanced Interventions

Recruitment Maneuvers - These are transient increases in airway pressure used to open collapsed alveoli and improve oxygenation. While possibly effective, such maneuvers must be performed cautiously to prevent barotrauma.
Inhaled Nitric Oxide and Prostacyclins - Inhaled nitric oxide and prostacyclins are delivered to enhance oxygenation by creating selective pulmonary vasodilation, reduce ventilation-perfusion mismatching, and improve gas exchange.
Extracorporeal CO2 Removal Device - Indications for using an extracorporeal CO2 removal device may include its addition to managing severe hypercapnia and indirectly improving oxygenation. ECCO2R can reduce high ventilatory pressures and decrease ventilator-induced lung injury.

Monitoring and Evaluation Continuous Monitoring

Blood Gas Analysis: Continuous monitoring is necessary regarding oxygenation and ventilation status through frequent blood gas analysis.
Haemodynamic Monitoring: Monitoring haemodynamic values such as cardiac output and mixed venous oxygen saturation also provides valuable data on oxygen delivery efficacy.
ECMO Parameters: Parameters of ECMO, such as circuit pressures and flows, should be continuously monitored so that any problem can be detected promptly.

Imaging and Diagnostics

Chest X-rays- Serial chest X-rays can be used to establish any lung pathology, placement of the cannula, and possible complications like pneumothorax or effusions.
Ultrasound and CT scans - High-end imaging tools can provide highly accurate information related to lung conditions, thereby drawing out a management plan.

Conclusion

Hypoxia on VV ECMO needs a multi-dimensional approach, including optimisation of the ECMO circuit, patient management, and treatment of underlying causes. If applied well together, the combined strategies will let the provider manage the patient properly, improving the outcome and ensuring success in VV ECMO therapy. Good management involves continuous monitoring, timely interventions, and understanding the underlying mechanisms for hypoxia.

References

https://link.springer.com/article/10.1007/s00134-014-3579-y
https://www.elsevier.es/en-revista-clinics-22-articulo-severe-hypoxemia-during-veno-venous-extracorporeal-S1807593222009383