Extracorporeal Membrane Oxygenation Weaning

Anatomy and Physiology

Anatomy

When discussing ECMO weaning and decannulation, we must first establish the correct anatomical placement and removal of the ECMO circuit. The anatomical placement of the ECMO cannulas is determined by the type of ECMO that will be necessary to provide the correct type of support to the patient. Cannulas can be placed either via percutaneous placement or by surgical cutdown to the vessels. The percutaneous approach is the most commonly used technique, with the Seldinger technique being the most common approach for placement of the cannulas.

Regarding respiratory failure with the patient maintaining cardiac output, the patient will be able to undergo venovenous cannulation.  Venovenous cannulation can occur by two different cannulas into either the jugular veins or into the femoral veins. A single, dual-lumen cannula can be inserted into the right internal jugular vein, which will allow drainage and reinfusion through the same cannula. A single, dual-lumen cannula into the right internal jugular vein will allow the patient to be out of bed to ambulate and require less sedation.

Venoarterial ECMO is most needed when the cardiac system cannot provide enough output to perfuse the organs in the body. VA ECMO cannulation occurs either through the open technique with placement into the femoral artery and either the jugular or femoral vein. It can be performed via the percutaneous approach. VA ECMO cannulation can also occur via central cannulation into the aorta and the right atrium/superior and inferior vena cava.

Physiology

Every life form on Earth needs oxygen for survival except a few forms of bacteria. We rely on the exchange of oxygen for energy sources for the cells. The physiology of the ECMO circuit acting as an artificial lung and heart is very complex. We will briefly discuss the oxygenation and removal of carbon dioxide from the patient with attention to the patient's physiology during weaning.

ECMO is basically a circuit that oxygenates the blood and decarboxylates its contents. The blood is then returned to the patient, fully saturated with oxygen via a venous line near the heart or an arterial line. The VA function of ECMO provides both cardiac and lung support, while VV only provides respiratory support. Running an ECMO circuit is not only an art but undertaking human physiology as we know it.

During the exchange of gases, oxygen must be added while carbon dioxide is removed. The oxygenator on the ECMO circuit is where gas exchange occurs. This area is a long microporous tube that allows the sweep gas to flow through the lumen of the membrane while the patient’s blood can flow on the outside of the membrane. This area is the oxygen exchange of the artificial lung. In a normal functioning human lung, the alveolar-capillary membrane exchange area is much larger, and the membrane for exchange is much thinner for the gases to travel. With the ECMO circuit being a fixed thickness and surface area, the only factor the perfusionist must modify is the patient’s blood flow and the sweep flow through the circuit to help modify the oxygen and carbon dioxide is delivered to the patient. One of the most important parameters is the pressure gradient between the patient's blood flow and the sweep gas flow to maintain physiological normalcy.[1]

When the normal lung can no longer oxygenate the blood, but the patient's normal cardiac function remains, VV ECMO can be initiated. The poorly oxygenated venous blood is returned to the ECMO circuit and reoxygenated. The blood is then transferred back to the patient via a venous line placed near the heart in the caval system. The returned blood, which is oxygen-rich, is then mixed with the poorly oxygenated blood. This will improve the arterial concentration of oxygen and allow the heart to distribute oxygen-rich blood throughout the body. The majority of oxygen is transported throughout the body on hemoglobin. Therefore hemoglobin concentration is essential when discussing the oxygenation of a patient on ECMO.

The partial pressure of the gradient between the patient’s blood and the sweep gas when running through the circuit is set by the FiO2, which determines the partial pressure of oxygen in the sweep gas. As mentioned earlier, when the blood flow increases in the circuit, this will also allow a greater increase in the patient's arterial oxygenation. When running on VV ECMO, not only will oxygenation be important, you must think about the patient's cardiac output. Higher cardiac output will cause increased tissue demands, increasing your use of oxygen from the ECMO circuit.

The goal of VV ECMO support is to allow the patient’s injured lung to rest and be exposed to lower lung volumes, peak end-expiratory pressures, and lower FiO2 support. A great deal of thought must go into the physiology of VV ECMO and the ventilatory management of the lungs while on VV ECMO, as all these settings will affect the ECMO circuit all the way down to the size of the cannulas that are placed.

VA ECMO oxygenation is very similar to VV ECMO, but the cannulas are positioned differently, with VA ECMO having an arterial catheter. The oxygenation of all extremities must be watched. This is performed best when a right radial arterial line is placed to monitor right-sided O2 concentration. Left ventricular function and venting require close monitoring. Other interventions may be necessary when these functions become impaired, potential placement of an intra-aortic balloon pump or other left heart support devices. This discussion is beyond the scope of this article.[2][3]

We must understand the oxygenation of the tissues and understand one of the most important factors- removing carbon dioxide from the patient. Carbon dioxide removal during ECMO support replaces the normal ventilatory function of the human lungs. When comparing the removal of carbon dioxide to oxygenation of the patient, removal of carbon dioxide is a much easier task to achieve. Carbon dioxide removal with VV ECMO is utilized when acute respiratory distress syndrome causes lung injury. The patient may have hypercapnia during a COPD exacerbation, or CO2 may need to be removed while the patient is on a bridge to lung transplantation. The patient's CO2 removal is easily acquired through the low flow on the ECMO circuit.

The amount of blood flow through the ECMO circuit has little effect on CO2. The removal rate is relatively independent of the flow. The sweep gas flow rate is one of the major ways to remove CO2. Since the sweep gas flow can remove CO2, the patient’s tidal volumes and respiratory rate on the ventilator can be turned down to allow the lungs to recover and have minimal involvement when the lungs are at their most severe state. When it is time to wean the patient, the lungs can then be recruited, and you can titrate the sweep gas flow to see how well the lungs can ventilate. Regarding VA ECMO, the lungs are basically a physiologic dead space since the heart’s ejection fraction is decreased and can’t perfuse the lungs through the pulmonary artery. The lungs will be less ventilated during VA ECMO but will still require some amount of PEEP to avoid complete atelectasis.[4][5]

Indications

Indications for Weaning VV ECMO

• The patient has recovered from their pulmonary disease process, and the lungs are ready to be tested for adequate performance of oxygenation and ventilation.

Indications for Weaning VA ECMO

• The patient has recovered from their cardio-pulmonary disease process. The heart and lungs are ready to be tested for the adequate performance of ejection and perfusion throughout the body.

Contraindications

The patient continues to have signs and symptoms of cardiopulmonary failure; therefore, the patient is not ready to be weaned. Depending on the disease process, the patient may need more time to heal their heart or lung problem. They may need a ventricular assist device or potentially a transplant of their heart or lungs.

Equipment

• ECMO machine with an oxygenator and artificial lung (cardiopulmonary bypass machine with all tubing, cannulas, and monitoring devices)
• Intensive care unit
• Fully functioning lab with arterial blood gas sampler
• Transthoracic echocardiography and transesophageal echocardiography

Personnel

• Critical care physicians
• Cardiothoracic, vascular, or trauma/acute care surgeon
• ECMO coordinator and technicians
• Perfusion team
• ICU nurses
• ECHO technicians 
• Respiratory therapists 
• Physical therapists
• Occupational therapists

Preparation

No matter how prepared a patient is to wean from ECMO, whether VV or VA, successfully weaning them from ECMO does not signify that the patient will survive.

• When preparing for weaning trials, you must have all necessary personnel to go directly back on full ECMO support if needed.
• The patient must be hemodynamically stable for weaning trials to begin.
• If the patient shows signs of continued cardiac or respiratory failure requiring full support, weaning attempts should be avoided.
• Patients who are deemed not good candidates for weaning should be discussed for transplant candidacy or assist device placement.
• The patient should be evaluated daily with clinical, laboratory, and transthoracic/transesophageal echo data to establish the potential for weaning. Cardiac markers require monitoring for normalization. Echocardiography plays a large role in monitoring for recovery of cardiac function along with any new problem that may arise.
• Echocardiographic findings when preparing for weaning that may indicate the patient will be difficult to wean: cardiac abnormalities with systolic or diastolic dysfunction, right or left heart ventricular failure, wall motion abnormalities, pericardial effusions or tamponade, hypovolemia, pulmonary artery hypertension.
• Patients should have a cardiac ejection fraction of greater than 25% to 30% with an aortic velocity-time interval greater than 12 cm and a lateral mitral valve peak systolic velocity greater than 6 cm while the patient is on minimal ECMO support.

Technique or Treatment

Weaning from VV or VA ECMO is one of the most challenging aspects of ECMO management. Each patient has their own algorithm and should be individualized each time a weaning attempt is tried. There is not an exact algorithm for weaning every patient from ECMO.

VA ECMO Weaning

• The initial cause of cardiogenic shock must have resolved or been corrected prior to weaning from VA ECMO. Depending on the initial cause of VA ECMO initiation, the weaning process will start at different times for different disease processes. Every patient will have a different weaning strategy and an individual plan.
• To begin a weaning trial, the patient should have MAP >70 with or without inotropic or device support, low vasopressor/inotropic support, oxygen saturation greater than 95%, central venous oxygen concentration greater than 70%, normal ventilation and oxygenation of the patient's lungs with improving chest Xrays, echocardiographic data with an ejection fraction greater than 25% to 30%.
• Weaning is started by reducing the overall pump flow on the ECMO circuit.
• The patient will need to have inotropic drugs at reasonable levels for support. The patient may also have an aortic balloon pump or a left ventricular device in place for added support. Weaning attempts can still be started with these mentioned drugs/devices in place. Some centers recommend weaning ECMO before removing the left ventricular support device.
• When the pump is slowly titrated down, the patient will start to develop more pre-load, and the heart can be monitored for how well the left ventricle can eject.
• Each time the ECMO flow level is decreased, the cardiac function should be continuously monitored using echocardiography. Noninvasive cardiac function monitoring can be used, or a pulmonary artery catheter can be placed to monitor second to second cardiac function changes.
• Strict attention to the ventilator settings and respiratory support must always be accounted for. Pulmonary blood flow will significantly increase, thus changing your PEEP and tidal volume settings.
• The maximum flow rate on the majority of ECMO machines is around 6 liters per minute. When weaning VA ECMO, we recommend weaning at increments of 0.5 liters per minute to 1.0 liter per minute changes when decreasing the flow rates. Flow rates should not drop below 2.0 to 2.5 l/min as this will cause clotting in the cannulas and circuit.
• Each time a decrease in flow rate is made, this rate should be maintained for at least 60 minutes to monitor the patient's decompensation. If the patient shows any signs of failure to have adequate cardiac output, signs of inadequate tissue perfusion, increasing blood lactate levels, or any echocardiographic findings of ventricular demise should prompt the physician to place the patient back on full support and monitor the patient for recovery of their cardiac function on full ECMO support.
• When weaning from VA ECMO, the patient may show signs of stability with a left ventricular ejection fraction greater than 25% with a normal cardiac index greater than 2.5 L/min. These are good signs for ECMO weaning.
• If the patient shows signs of hemodynamic instability or signs of distress at any time, the patient should be changed to full ECMO support. When monitoring with a transesophageal echo or transthoracic echo when weaning, we need to look for signs of rising left or right-sided filling pressures, progressive ventricular dilation, worsening or new signs of mitral or tricuspid regurgitation, any sign of hypoxia or hypercarbia on arterial blood gas, any sign of ventilator changes with elevated peak pressures or plateau pressures. Also, look for signs of increasing vasopressors support when the patient has signs of hypotension with a map less than 60
• Once the patient has completed weaning from the ECMO circuit, the heparin infusion can be stopped, and the pump flow on the ECMO machine will be raised. This is performed to avoid any clot accumulation. Some institutions will administer a positive inotrope at this time to help facilitate cardiac output after the cannulas have been clamped. If the patient is deemed liberated/weaned from ECMO support at this time, the patient can be decannulated with the removal of the cannulas. This could be performed at the bedside if the percutaneous placement of the cannulas was performed. If central cannulation or surgical cutdown were performed, the cannulas would need to be removed in the operating room.
• Once the patient has been completely weaned from ECMO, the patient will need continuous monitoring of their cardiac output, oxygen saturation levels, lactic acid levels, pH, urine output, and vent settings to confirm the patient can maintain perfusion of their organs.
• After liberating the patient from ECMO support, patients sometimes have signs of an inflammatory response requiring an increase in their inotropic support. This is not uncommon when weaning from ECMO.[6]

VV ECMO Weaning

• When weaning from VV ECMO as described above with VA ECMO, 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.[7][8]
• 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.[9] Sedation will also have to be managed accordingly.[10][11]
• 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.[12]
• 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.

Complications

Weaning patients when they are not deemed candidates to be weaned or potential candidates that wean well for the first several hours can still have associated complications.

• Bleeding
• Ischemia to limbs
• End organ hypoperfusion
• Neurological complications
• Infections

Clinical Significance

Factors That Predict Successful Weaning from ECMO 

• Normal laboratory findings of lactic acid
• Echocardiographic evidence of complete cardiac recovery with EF >25%
• An aortic velocity-time interval of >10 cm on minimal ECMO support settings (1.5 L/min)
• Lateral mitral annulus peak systolic velocity (TDSa) > 6 cm on minimal ECMO support settings
• Tricuspid annular plane systolic excursion (TAPSE) >12mm on minimal ECMO support settings
• Mean pulse pressure- the higher the pulse pressure the more likely the patient can be successfully weaned. 
• No signs of ventricular dilation with stable left and right-sided filling pressures[13]

 Factors That Predict Mortality in Weaned Patients

• Age >60 years old
• Prolonged ECMO with associated complications
• Low pH with high lactate levels
• Elevated sequential organ failure assessment score

When patients have a failure to wean from ECMO they should be properly identified with the discussion of long-term support or if the patient may need transplantation of their organs. The discussion should be performed to determine if the patient is a candidate for a ventricular assist device for a bridge to possible transplantation or if the patient or needs destination therapy with a ventricular assist device for the remainder of their life.

Enhancing Healthcare Team Outcomes

ECMO has allowed many patients to have life-saving care, which will ultimately give them time to heal their own hearts or lungs. The entire initiation of ECMO with cannulation, day-to-day management, and weaning requires a vast team of professionals with direct lines of communications. Patients who are on ECMO should have daily discussions regarding their potential for possible weaning trials.

All members of the interprofessional healthcare team (clinicians, mid-level practitioners, nurses, respiratory therapists) caring for the patient must be aware of attempted trials and be ready for any possible trial, whether it is successful or not. Incorrect or improper communication between the team members could lead to morbidity or mortality when weaning patients from ECMO. When teams are in constant communication, this will enhance patient care and provide better outcomes. Having a detailed plan for weaning and alternative plans if the patient fails the wean or emergently needs to go back on ECMO should have been discussed in depth with all team members with open communication lines to provide the best care. Being prepare for the weaning trials will provide the patient with the best possibility for weaning and decrease morbidity and mortality. [Level 5]

Nursing, Allied Health, and Interprofessional Team Interventions

An interprofessional team is needed to care for patients on ECMO. Once the patient has undergone cannulation with the initiation of ECMO, a multi-facet team must step in. There are many moving parts to running VA or VV ECMO. The perfusion team monitors all the ECMO circuit parameters and makes small changes to better oxygenate and remove CO2 on the ECMO circuit. Surgical or medical intensivists monitor the progression of the patient on ECMO and also monitor the ventilator.

The intensivists must decide along with other team members when the patient is ready for weaning trials or if they need more time to heal their damaged organs. ICU nurses and ECMO technicians are crucial in their role with the management of the patient as they are at the bedside every day. Rehab specialists, along with PT and OT, can work with the patients to help them regain their strength and potential for weaning from ECMO. Weaning patients from ECMO is not an easy task. Every member of the team is crucial to the success of weaning the patients from the ECMO circuit. Interprofessional interventions and communication are vital to the success and outcome of the patient.

Nursing, Allied Health, and Interprofessional Team Monitoring

Interprofessional team monitoring is critical to the outcomes of the patient. While a patient is on ECMO, they should be monitored continuously by an ECMO technician who constantly monitors the circuit and looks at the vital signs. They can make small changes to help facilitate better hemodynamic stability for the patient. They must remain in constant contact with the ECMO coordinator and ICU nursing staff to alert the team if any changes arise.

All team members must be able to monitor the patient from the perspective of their field and be able to communicate to provide the patient with the best possible outcomes. The patient could have a complication arise at any time. Therefore all team members must be ready to do their part to help with the care of the patient. When all team members are prepared, the patients have better outcomes with less morbidity and mortality.