VENO-VENOUS ECMO (VV ECMO)
INTRODUCTION OF VV ECMO
DEFINITION
VV ECMO stands for Veno-Venous Extracorporeal Membrane Oxygenation. It is a medical technique used to provide life support for patients with severe respiratory failure. VV ECMO involves temporarily diverting a patient's blood from a large vein, passing it through an external artificial lung (oxygenator) to remove carbon dioxide and add oxygen, and then returning the oxygenated blood back into the patient's body. This process allows the patient's lungs to rest and heal while maintaining adequate oxygenation and carbon dioxide removal.
HOW DOES THE VV ECMO WORKS
COMPONENTS OF VV ECMO
- Cannula
- Tubings
- Pump
- Oxygen membrane
- Heat exchanger
CANNULATION SITES FOR VVECMO
There are 3 types of cannulation sites in VV-ECMO
- Femero-femoral vein
- Internal jugular-femoral vein
- Internal jugular vein (dual lumen catheter)
CANNULATION SIZES OF ECMO
ECMO CONFIGURATION | CANNULA SIZE (FRENCH) | CANNULA LENGTH (cm) | BLOOD FLOW (L/min) |
Femero-jugular vein | Inflow:23-29 outflow:15-23 | Inflow: 38-64 Outflow: 15-23 | 2-7L/min |
Femero-femoral vein | Inflow:23-29 | Inflow:38-68 outflow:55-68 | 2-4L/min |
Dual lumen cannula | 23-31 | 31 | 2-5L/min |
X-RAY OF VV ECMO
INDICATIONS OF VV-ECMO
VV ECMO is used for respiratory support in those who do not respond to mechanical ventilation or any acute potentially reversible respiratory failure.
Acute respiratory distress syndrome secondary to either severe bacterial or viral pneumonia, including COVID-19 or aspiration pneumonitis. ECMO bypasses the compromised activity of the lungs and maintains oxygenation and ventilation with the removal of CO2.
Covid-19 Severe Respiratory Failure: ARDS due to SARS-CoV-2 infection when prolonged mechanical ventilatory support fails. In some cases, when ventilation fails, ECMO support (venovenous ECMO) has been initiated.
Extracorporeal assistance to support lung in cases of airway obstruction, pulmonary contusion (barotrauma), smoke inhalation, drowning, air leak syndrome, hypercapnia, or hypoxic respiratory failure
Status asthmaticus
Massive hemoptysis or pulmonary hemorrhage
Bridge to lung transplant
Support for lung resections in unstable patients.
PATIENT MANAGEMENT IN VV ECMO
Patient selection
pre-ECMO assessment
Cannulation
Initiation of ecmo
monitoring
Anticoagulation management
Ventilator management
Nutritional support
Complication management
Weaning and decannulation
Post-ecmo care
rehabilitation
OUTCOMES OF VV ECMO
Survival
Lung recovery
- Bridge to recovery or transplant
WEANING PROTOCOL OF VV ECMO
When weaning from VV ECMO we should see all respiratory function should be recovered.
Before weaning from ECMO support, the patient’s respiratory function should be thoroughly inspected. The patient's native lung should be able to support 50 to 80% of the total gas exchange within the native lung. The majority of severe lung disease patients can begin weaning when their native lungs have gained 80% of their oxygen delivery function. The patient's respiratory mechanics must demonstrate significant improvement. The gas exchange when considering a weaning trial on moderate ventilator settings should have a PF ratio >100. The FiO2 should be less than 50% with reasonable PEEP levels < 10.
As the native lung function improves, the ECMO support can be decreased. The extracorporeal blood flow can be slightly decreased, but with VV ECMO, the sweep gas flow controls the oxygenation to the native lungs through the ECMO circuit.
The majority of patients on VV ECMO have a sweep gas at 100%. As their lung's oxygenation and ventilation improved, we recommend decreasing the ventilator FiO2 before decreasing the sweep gas oxygen concentration. This will help avoid any major oxygen toxicity to the healing lungs. Some centers do recommend weaning VV ECMO sweep gas prior to weaning FiO2 on the ventilator.
When weaning, there are no particular ventilator settings that are recommended for better outcomes. Some centers use controlled mechanical ventilation when on support, then transition to assist control with pressure support when weaning off. The vent settings should be changed to values that are acceptable to non-ECMO patients. Sedation will also have to be managed accordingly.
As the sweep gas and FiO2 are decreased, an hourly arterial blood gas should be performed within 30 minutes of the adjustment to check for oxygenation and ventilation.
If the patient tolerates the wean with tolerable arterial blood gas levels and vent settings, the sweep gas can continue to be titrated. Some ECMO programs report that decreasing flow rates less than 2.5 L/min puts the patient at a higher risk for blood clot formation in the circuit or cannulas. Some physicians wean the flow rate to 2.5L/min, then decrease the sweep gas down while monitoring the patient, while other physicians will lower the flow rate to 1-1.5 L/min while weaning the circuit once the sweep gas has been titrated to off. As mentioned above, the flow rate does not matter nearly as much during VV ECMO. The sweep gas is the major determinant of oxygenation and ventilation of the patient. Therefore higher or lower flow rates are per physician preference regarding clot formation.
Around 20 minutes after the sweep gas has been turned off, the patient’s O2 saturation and arterial blood gas will confirm if the patient can support oxygenation and ventilation while being off VV ECMO. The ECMO flow rate can be continued with no adjustment in the heparin dose.
If the patient shows signs of distress or hemodynamic instability at any point, the weaning trial should be aborted, and the patient should be put back on full ECMO support. Weaning failure is indicated by an SPO2 level of less than 88% or an increase in the patient's respiratory rate greater than 30 to 35 breaths per minute.
Some centers recommend a trial of 1 hour or up to 6 hours to prove that the patient has passed weaning from VV ECMO. During this time, the patient must be monitored closely for any signs of hemodynamic instability, arterial blood gas analysis to confirm the adequacy of oxygenation and ventilation, and the patient’s ventilator settings and respiratory mechanics must be carefully assessed as well. If the patient remains hemodynamically stable with normal lab values during this time, the ECMO support can be discontinued, and the patient can undergo decannulation.
Regarding VV ECMO decannulation, the circuit can be disconnected once the patient is weaned, and the cannulas can be left in place for up to 48 hours in case the patient needs to be restarted on ECMO. If the cannulas are to be left in place, they require periodic flushing with heparin to avoid thrombosis up the cannula with the continuation of the patient anticoagulation. Once the cannulas are removed, pressure can be held over the site for around 30 to 45 minutes. Some centers then perform venous Doppler ultrasound of the cannulated vessels' limbs to ensure there is no thrombosis present.
