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ECMO Explained

What started as a so called minor infection has now escalated to massive sepsis and acute respiratory distress syndrome. The young patient is on the ventilator at Wake Forest Baptist Medical Center (WFBMC), and his vent settings have rapidly escalated. Conventional therapy is not providing the support the patient needs. The attending has decided to make an extracorporeal membrane oxygenation (ECMO) consult.

ECMO is essentially a modification of the bypass machine that is used routinely in cardiac surgery. Many of ECMO's advances have come from the operating room arena along with a better understanding of  perfusion in cardio-thoracic surgery. Unlike cardio-thoracic surgery where the patient is supported for hours, ECMO supports a patient for days to weeks.

ECMO is used in patients where conventional therapy (using a mechanical ventilator) has failed. ECMO in and of itself is not therapeutic. It provides gas exchange as an alternative to high ventilator settings and so allows the lungs and heart to rest until the patient can recover.

History of ECMO

The heart/lung machine development is contributed primarily to John Gibbon, MD, in 1939. During Dr. Gibbon's surgical residency, he spent all night caring for a patient who was dying of a massive pulmonary embolism. This experience led him to the development of the heart and lung machine and the first successful heart operation in 1954.  This allowed the establishment of the specialty of cardiac surgery.1

The first successful use of ECMO was on an adult trauma patient suffering from ARDS.  In 1971, J. Donald Hill, MD, and associates placed this patient on ECMO for three days, and he survived.2 In 1975, Robert Bartlett, MD, successfully supported a neonatal patient on ECMO.  The nurses named this child Esperanza, which is Spanish for hope. Some years ago I had the pleasure of meeting this young lady during one of Dr. Bartlett's lectures.  It was inspirational to be able to spend time talking with her.

Because of these early successes with ECMO, the National Institute of Health conducted a multi-center clinical trial for adults with ARDS comparing ECMO and mechanical ventilation in 1975. The study had abysmal results with 10 percent survival in both groups and was stopped early. The results of the study were published in 1979.3 There were many problems with this study including inexperienced teams, continuing the high ventilator strategy, and not using lung protective strategies that we use today and now know is the primary cause of lung injury in these sick patients.  

Despite this early setback, some centers continued to treat and perfect ECMO.  After 1975, Dr. Bartlett continued his work with ECMO and treated 40 neonatal patients with 50 percent survival. By 1986, 18 neonatal centers had experienced ECMO teams with 80 percent survival in the neonatal population. As success with neonatal patients grew, so did the desire to use ECMO for pediatric and adult patients who failed to improve with conventional therapy. ECMO is the only system that can replace cardiac function in the pediatric population, thus bridging patients to recovery or heart transplant.

Currently, there are 132 centers internationally that perform ECMO. More than 44,000 patients have been treated.4


VA and VV ECMO

The ECMO program at WFBMC was started in 1996. We have treated all patient populations from neonatal to adult, and at the time of this article we have treated more than 510 patients. We are one of the busiest adult programs on the East Coast.  Here are guidelines that we use when selecting patients for ECMO:

• No significant coagulopathy or uncontrolled bleeding

• No major intracranial hemorrhage (> grade 1 intracranial hemorrhage)

• Mechanical ventilation for 10 to 14 days or less

• Reversible lung injury

• No lethal malformations

• No major untreatable cardiac malformation

• Failure of maximal medical therapy.

There are two types of ECMO: veno-venous (VV) and veno- arterial (VA). VV ECMO is for pulmonary support. VA ECMO is for cardiac and pulmonary support.

ECMO consists of a pump, an oxygenator, vascular access, and circuitry. The pumps can be a rollerhead or centrifugal pump. Many centers are decreasing the length of the circuitry by using centrifugal pumps because this pump head can be placed in the bed, which decreases the length of tubing needed to reach the patient cannulas.  

 In VV ECMO there are two ways the patient can be cannulated.  They can have two cannula sites or one cannula site with a double lumen cannula. Most neonatal and small pediatric patients are cannulated using a double lumen cannula. Adults can be placed on VV ECMO using two cannulas or one double lumen cannula. In using two cannulas for the adult patient, the internal jugular (IJ) and the femoral veins are chosen as cannulation sites. The IJ vein cannula is between 17Fr. and 21Fr. The cannula tip sits in the right atrium. This is the return site from the oxygenator and rollerhead so that oxygenated blood goes to the right atrium. Even though this cannula is in a vein, we call this the arterial cannula because it brings oxygenated blood from the circuit to the patient. The femoral vein cannula is between 22Fr. and 28Fr. This cannula is placed in the femoral vein, and the tip sits in the inferior vena cava.

One recent advance in ECMO is the advent of the adult BiCaval cannula by Avalon Laboratories. In the adult patient the BiCaval cannula is placed via the IJ through the right atrium, and the tip sits in the inferior vena cava. This cannula is usually 27Fr. or 31Fr. This allows for only one cannulation site. When using a femoral cannula for drainage, the patient needs to keep their legs still so as not to impede flow. With the Avalon cannula, the patient can move their legs more in the bed. In fact, our goal is to have the patient awake, comfortable, and watching TV while on ECMO.  

ECMO flow with a roller head is dependent on gravity (siphon) drainage from the venous cannula. This is why the bed will be raised as high as it can go, and the largest cannula will be placed in the patient.  The blood drains from the patient to the roller pump to the oxygenator where oxygen is delivered and CO2 is removed. If a centrifugal pump is used, the height of the bed is not as important. The centrifugal pump can be placed in the bed with the patient.

Neal D. Kon, MD, professor of surgical sciences-cardiothoracic surgery at Wake Forest Baptist Medical Center, explains how ECMO is used in his facility's program.

 Frequently, the patients are on high ventilator settings before initiation of ECMO. Once on ECMO, ventilatory support can be decrease to allow the lungs to rest (the primary benefit of ECMO). Our typical ventilator settings are pressure vontrol mode with FiO2: 40 percent rate: 10 bpm, PIP: 24 cmH2O, and PEEP 10 cmH2O. Using  these settings for the first couple of days we may see no tidal volumes and on chest X-ray; the lungs will be completely "whited out." We will accept oxygen saturations on the pulse oximeter around 85 percent.

A typical VV run for an adult patient with ARDS is two to three weeks. As the patient's lung function improves, the arterial saturations improve. This is not because we are doing better supporting the patient with ECMO, it is because the patient is contributing to the oxygenation and ventilation with their native lungs.  When the patient has improved to the point that ECMO can be discontinued, the ventilator settings are increased to a reasonable level. We stop the flow of oxygen to the oxygenator, but the blood flow through the circuit remains the same. At this point, all the patient's gas exchange is from the lungs, and the patient is ready for decannulation.

 Veno-arterial ECMO is for cardiac and pulmonary support but is usually reserved for those patients without  adequate cardiac function. In veno-arterial support, the venous cannula can be in the IJ or the femoral vein, and the arterial cannula is in the artery. It can be placed in the femoral artery or the carotid artery.  Carotid artery cannulation results in a good supply of oxygenated blood to the brain because of its placement in the aortic arch. Care must be taken to ensure adequate oxygen delivery to the brain during VA ECMO with femoral artery cannulation because the oxygenated blood flow needs to travel retrograde up the descending aorta to supply the brain. This is why it is important to have the lungs maintain adequate function with femoral artery cannulation.

If the heart is not generating a cardiac output, this becomes less of a problem because all of the cardiac output is supplied by ECMO. As the heart recovers and begins to generate more of a squeeze, then potentially deoxygenated blood will supply the brain if the lungs are not functioning properly. If the lungs are not functioning properly, then the patient can be cannulated in the right atrium via the internal jugular, and blood can be directed to both the right atrial (venous) cannula and femoral artery cannula. Thus the patient is on VV and VA ECMO at the same time. This is known as veno-arterial venous access (VAV). This supplies oxygenated blood to the aortic arch.

Once the lungs improve to supply adequate oxygenation without ECMO support, then the right atrial cannula can be converted to an additional drainage cannula.5 The patient will contribute more to cardiac output as his cardiac function improves. The ECMO flow will be decreased slowly allowing the patient's own cardiac output to take over. Once the patient is able to maintain his own cardiac output, then the patient is ready for decannulation.

Complications

A recent review of the ELSO Registry data found component failures occurred in close to 15 percent of ECMO runs.6 The ECMO specialist is trained to assess, manage, and intervene with the ECMO circuit. The goal is constant vigilance in the hope of circumventing problems before they occur. Mechanical complications can include thrombosis (clots in the circuit), membrane oxygenator failure, air entrainment, tubing rupture, mechanical failure, cannula problems resulting in vascular damage, and accidental decannulation.  Complications also are associated with the use of heparin. When blood is exposed to a foreign surface such as the ECMO circuit, the coagulation pathways are activated. Heparin is used keeping a close range in inhibiting coagulation and the risk of excess bleeding. Thus there is a risk with surgical site bleeding and intracranial hemorrhage.  

ECMO specialists attend training drills at least twice a year to practice how to quickly recognize emergencies and intervene in a way that obtains a positive outcome. These drills can be in a laboratory setting or in a patient simulator lab to mimic a real-time environment.

The young patient described in the introduction was cannulated with the Avalon cannula. He remained on ECMO for six days. After discontinuation of ECMO, he was extubated three days later and discharged home five days later.  It truly is a team concept to enable such a wonderful success story from the ECMO specialist, registered nurses, and ECMO attending who are caring for the patient on ECMO, to the referring physicians and health care team who care for the patient after decannulation.

References

1. Gibbon JH.  Application of a mechanical heart and lung apparatus to cardiac surgery. Minn Med 37:171, 1954.

2. Hill JD, Obrien TG, Murray JJ, et al. Extracorporeal oxygenation for acute post-traumatic respiratory failure: Use of the Bramson Membrane Lung. N Engl J Med 286:629-634, 1972.

3. Zapol WM, Snider MT, Hill JD et al. Extracorporeal membrane oxygenation in severe acute respiratory failure: A randomized prospective study. JAMA 242:2193-2196, 1979.

4. ELSO (Extracorporeal Life Support Organization):  January 2011 ECLS Registry Report, International Summary.

5. MeursKV, Lalley KP, Peek G, Zwischenberger J. ECMO Extracorporeal Cardiopulmonary Support in Critical Care 3rd Edition: Management of ECLS in Adult Respiratory Failure. Extracorporeal Life Support Organization; Ann Arbor, Michigan:  2005. 

6. Short BL, Williams L, ECMO Specialist Training Manual. 3rd Edition: ECMO Mechanical Complications.  Extracorporeal Life Support Organization; Ann Arbor, Michigan: 2010.

Scott Copus, BS, RRT, is ECMO coordinator and PICU supervisor at Wake Forest University Baptist Medical Center and Brenner Childrens Hospital, Winston-Salem, N.C.


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