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11 September, 2024
Here, the principles and techniques of extracorporeal circulation (ECMO) in severe circulatory failure are described. Cannulation and treatment methods for both veno-venous and veno-arterial ECMO are detailed.
ECMO – General Principles
Patients whose respiration is so severely impaired that optimal ventilator support cannot maintain adequate oxygenation or carbon dioxide removal can be treated with venovenous extracorporeal membrane oxygenation (VV-ECMO). On the other hand, venoarterial ECMO (VA-ECMO) helps patients whose circulation is incompatible with continued life despite maximal inotropic support [1,2]. Both treatments can sustain life for a limited time, with the goal that the lungs or heart recover within this period. While VV-ECMO can function for months, the treatment duration for VA-ECMO is limited to about a week. If the lungs do not recover within that time, select patients may be helped through lung transplantation, while some patients who cannot be weaned off VA-ECMO may be supported with long-term mechanical assist devices (Figure 1). This mechanical heart (most commonly HeartMate 3) can allow patients to live for many years, but it is usually used as a bridge to future heart transplantation.
The ECMO circuit consists of cannulas connected to large blood vessels, tubes to transport the blood, a pump to propel the blood, and an oxygenator to oxygenate the blood. A sweep gas flow regulator, on the right, controls the oxygenator’s respiratory function (carbon dioxide removal and oxygenation).
VV-ECMO (Venovenous ECMO)
Indication
There are many reasons a patient’s respiration may become so compromised that they require treatment with VV-ECMO, but pneumonia leading to ARDS is the most common. Often, a Murray score > 3 is used as the threshold for ECMO treatment (Figure 3), typically combined with criteria for hypercapnia, such as pH < 7.2. However, Peek et al. found that 15% of patients who initially met ECMO criteria improved enough that they did not need ECMO after being treated with a strict protocol, including prone positioning, increased diuresis, and transfusion to maintain a hematocrit of 40% [3], highlighting that patients should receive optimal respiratory treatment before deciding on ECMO. The same randomized multicenter study showed that patients with the above inclusion criteria had better six-month survival without disability if treated with ECMO rather than only with ventilator support [3].
Murray Score
| Score | 0 | 1 | 2 | 3 | 4 |
|---|---|---|---|---|---|
| Number of quadrants with alveolar consolidation on X-ray | None | 1 | 2 | 3 | 4 |
| Hypoxemia PaO2 x 7,5/FiO2 | >300 | 225-299 | 175-224 | 100-174 | <100 |
| PEEP cm H2O | ≤5 | 6-8 | 9-11 | 12-14 | ≥15 |
| Lungcompliance ml/cm H2O | ≥80 | 60-79 | 40-59 | 20-39 | ≤19 |
Add individual scores and divide by the number of components used (i.e., not all patients need to have all measurements taken). PaO2 is multiplied by 7.5 since the original uses mmHg instead of kPa.
From Murray et al., Am Rev Respir Dis 138 (1988), 720-723.
The Murray score calculator is available at: http://cesar.lshtm.ac.uk/murrayscorecalculator.htm
Types of Cannulation
The problem with severe lung failure is that the lungs do not oxygenate the blood. The goal of VV-ECMO is therefore to oxygenate as much blood as possible before it passes from the right atrium to the right ventricle. This can be done with two cannulas or a dual-lumen catheter (Avalon®).
Two different methods of cannulation using two cannulas
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In two-cannula cannulation, the patient receives a large cannula through the femoral vein and another through the internal jugular vein, with the tips located in the inferior vena cava and the superior vena cava/right atrium (Figure 4). Blood is drawn from one cannula and returned through the other. However, there are two significant issues that reduce the effectiveness of VV-ECMO. First, the venous cannula cannot capture all the blood, resulting in a shunt where deoxygenated blood flows directly to the right ventricle. Additionally, oxygenated blood can be drawn back into the ECMO circuit for a second time, known as recirculation, which further decreases the efficiency of ECMO.
An Avalon catheter is a dual-lumen catheter that draws blood from both the superior and inferior vena cava. After oxygenation, the blood is returned to the right atrium. The advantage of this cannula is that it minimizes the shunt of deoxygenated blood, thereby improving patient oxygenation. The drawback is that it is more challenging to position because the tip must be stably placed in the inferior vena cava (Figure 5).
Oxygenation in VV-ECMO
Regardless of the type of cannulation chosen, some blood will still shunt past the right atrium without being oxygenated. Therefore, a lower arterial oxygen saturation must be accepted in these patients compared to others. However, since oxygen delivery to the body depends not only on saturation but also on hemoglobin levels and cardiac output, a slightly higher hemoglobin level and good cardiac output can maintain adequate oxygen delivery despite the lower saturation.
For the lungs to recover, it is crucial to use lung-protective ventilation with low peak pressures, PEEP tailored to the patient, and not too high FiO2 in the ventilator.
General Guidelines
ECMO activates the coagulation cascade, so continuous heparin infusion is required. Patients should be kept as dry as possible to reduce the risk of pulmonary edema. It is highly beneficial if the patients can remain awake, as maintaining muscle strength is crucial, and the treatment can be prolonged. This means early tracheostomy should be considered, or in selected cases, extubation.
Weaning
When the lungs begin to recover, the airflow in the oxygenator can be turned off to see if the lungs can take over respiration independently. If the lungs manage this for a few hours, the ECMO can be turned off, and the ECMO cannulas removed.
VA-ECMO (Venoarterial ECMO)
Indication
Patients whose circulation is so impaired that it is deemed they have minutes left to live can regain their circulation with the help of VA-ECMO. The causes of circulatory failure vary, but heart attack, dilated cardiomyopathy, myocarditis, and pulmonary embolism are some conditions in which patients may become so critically ill that ECMO treatment is necessary.
Circulation
When VA-ECMO needs to be started quickly, the femoral vein and artery are usually cannulated, as these are large vessels and relatively easy to locate. To expedite potential cannulation, it is advisable to pre-prepare high-risk patients with an arterial line and a small central venous catheter (CVK) in the femoral artery and vein.
VA-ECMO provides partial cardiopulmonary bypass. Since blood is drawn from the right atrium and returned to the femoral artery, retrograde flow occurs in the aorta (Figure 6). In parallel, the heart continues to beat, and the blood flow from the heart and ECMO meet in the aorta. This means that much of the body receives blood oxygenated by the ECMO, while the coronary arteries, right arm, and right hemisphere of the brain receive blood oxygenated by the lungs, even if the heart only pumps a small amount. If the heart is not pumping at all, the entire body is oxygenated by the ECMO. To manage these patients properly, it is essential to understand the dual circulation and which parts of the body are perfused and oxygenated by the heart/lungs and which by the ECMO.
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The Circle of Death
The blood pumped by the right ventricle, along with blood from sources such as bronchial circulation and potential aortic insufficiency, must be pumped onward by the left ventricle into the aorta. The problem is that most patients are placed on ECMO due to cardiogenic shock caused by severe left ventricular failure. When ECMO is started, circulation is restored, but the weakened left ventricle (which previously could not maintain even low blood pressure) now faces the challenge of pumping against a constant high afterload. If it cannot manage, blood backs up in the pulmonary circulation, creating elevated pressures that lead to pulmonary edema, hemorrhage, and lung destruction. The left ventricle also becomes progressively overfilled, which can cause ventricular muscle destruction. Another issue is that stagnant blood in the lungs, heart, and aortic root poses a high risk of forming large thrombi, especially if anticoagulation is insufficient. If the heart continues to beat but with significantly elevated filling pressures (wedge pressure or PCWP), the lungs may still be damaged. The blood coming from the heart may then be poorly oxygenated, potentially causing ischemia in the organs it perfuses (coronary arteries, right hemisphere of the brain). This vicious cycle is referred to as the ‘circle of death’ (Figure 7).
To avoid the circle of death and thrombus formation, inotropes can be administered to help the heart pump blood forward, and an intra-aortic balloon pump can be used to reduce afterload on the heart. If this is not sufficient, the patient may require a cannulation that also draws blood from the left side of the heart (e.g., centrally cannulated VA-ECMO with left atrial venting) or the insertion of an Impella device, which pumps blood from the left ventricle to the ascending aorta.
Other Cannulations for Circulatory Failure
Peripheral cannulation is often the initial approach for most patients. Patients who cannot regain circulation after heart surgery may instead have cannulas placed directly into the heart, a procedure known as central cannulation. Several other cannulation methods are also used to unload either the right or left ventricle, or both.
General Guidelines
It is crucial that patients are optimally anticoagulated with heparin, especially considering that blood often flows slowly through certain parts of the heart in most cannulation setups, significantly increasing the risk of thrombus formation. However, many patients are also prone to bleeding, making anticoagulation a delicate balance between bleeding risk and thrombosis risk.
Since patients with peripherally cannulated VA-ECMO have dual circulation, it is advisable to use two pulse oximeters. This allows for the assessment of oxygenation provided by both the lungs and the ECMO. The pulse oximeter on the right hand measures oxygenation from the heart if it is pumping adequately, while the oximeter on the foot measures ECMO-derived oxygenation. Other standard ICU guidelines apply.
Weaning
After about a week, most hearts have recovered as much as they can, and there is typically little benefit to continuing VA-ECMO treatment. The weaning process involves gradually reducing the ECMO blood flow while providing cardiac support through inotropes and often an intra-aortic balloon pump. Once it is clear that the heart can take over circulation, the ECMO cannulas are clamped, and the ECMO machine is turned off. The cannulas can then either be removed, followed by femostop treatment, or the arterial cannula may be surgically removed.
References
- Annich GM et al. ECMO, Extracorporeal Cardiopulmonary Support in Critical Care, 4th Edition. ELSO; 2012. ISBN 978-0-9656756-4-2.
- Short et al. ECMO Specialist Training Manual, 3rd Edition. ELSO; 2010. ISBN 978-0-9656756-3-5.
- Peek GJ, Mugford M, Tiruvoipati R, Wilson A, Allen E, Thalanany MM, Hibbert CL, Truesdale A, Clemens F, Cooper N et al: Efficacy and economic assessment of conventional ventilatory support versus extracorporeal membrane oxygenation for severe adult respiratory failure (CESAR): a multicenter randomized controlled trial. Lancet 2009, 374(9698):1351-1363.
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