V-V ECMO is used for respiratory failure refractory to mechanical ventilation, in the context of adequate cardiac function. It provides gas exchange support only. It does not provide direct cardiac support.
The primary aims of V-V ECMO are to provide adequate gas exchange to support life and thereby facilitating a significant reduction in mechanical ventilation, reducing ventilator-associated lung injury and allowing the best chance of lung recovery.
The effects of V-V ECMO on patient arterial O2 and CO2 are interesting and quite complex as outlined in the learnECMO gas exchange podcast. However, the following general principles apply:
During V-V ECMO, arterial O2 is primarily determined by the relationship between the ECMO pump flow and the patient’s cardiac output. Therefore, despite PO2 of 400-500 mmHg in the ECMO blood post-oxygenator, if the ECMO pump flow is low compared to the patient’s cardiac output the resultant arterial O2 will still be low. In this circumstance, increasing the ECMO pump flow will increase the proportion of well-oxygenated blood in the mixed venous circulation and therefore increase the arterial O2.
A flow rate of approximately 66% of cardiac output is necessary to achieve patient saturation of > 90%. For patients on V-V ECMO it is common for the maximum achievable arterial PO2 to only be in the range 55-90 mmHg.
Carbon Dioxide Removal
Removal of CO2 by the ECMO circuit is very efficient and is proportional to the “sweep” gas flow through the oxygenator. Therefore, the higher the gas flow through the oxygenator the more CO2 is cleared.
- Age: > 70 years
- Active malignancy
- Severe brain injury
- Previous Bone marrow transplant
- Previous heart, lung, renal transplant (>30 days)
- AIDS defining illness despite antiretroviral therapy
- End-stage renal failure (dialysis)
Specific Exclusions for VV ECMO:
- Chronic lung disease not eligible for transplant
- Profound shock with multiorgan failure
- Severe Pneumonia
- Status Asthmaticus
- Pulmonary Vasculitus
- ARDS from primary lung injury
- Acute graft failure following lung transplantation
- Pulmonary contusion or haemorrhage
- Bridge to lung transplantation
- Secondary ARDS (pancreatitis, burns etc.)
Cannulation & Circuit Configurations
Veno-Venous ECMO cannulation is commonly performed percutaneously with ultrasound guidance using Seldinger technique although surgical cut down may occasionally be required. Echocardiography or fluroscopy are used to confirm wire placement and cannula position. The access cannula is usually placed in the inferior vena cava via the femoral vein. The tip of the return cannula should sit close to the right atrium and it may be placed via the femoral or internal jugular vein. The Avalon Dual luma catheter provides both drainage and return via a single puncture via the internal jugular vein.
The CESAR study supports ECMO as a valid treatment option for the management of patients with severe respiratory failure. However, it does not show that ECMO is better than conventional ventilation. With incomplete follow up data in nearly half of the patients and 24% of patients in the ECMO group not actually receiving ECMO, we are left with insufficient evidence as to whether ECMO is better or worse than protocolised ARDSnet ventilation in adults patients with severe respiratory failure. The study does highlight the importance of involving specialist units, effective lung protective ventilation and ECMO as an option in refractory respiratory failure. Read the summary via the Bottom Line