Posted by Bengt Redfors, Chief Physician in Anesthesia & Intensive Care. Div of Cardiothoracic section. Sahlgrenska University Hospital.
Patients whose respiration is so affected that optimal respiratory treatment can not maintain oxygenation or carbon dioxide removal, can be treated with venovenous extracorporeal membrane oxygenation (VV-ECMO) temporarily. Venoarterial ECMO (VA-ECMO), on the other hand, helps patients whose circulation is not consistent with continued life despite maximum inotropic support 1,2. Both treatments can maintain the life of the patient for a limited time and the goal is generally to restore the lungs and the heart within this time. While VV-ECMO can work for months, VA-ECMO’s treatment time is limited to one week. If the lungs do not recover within that period, single selected patients can be assisted by pulmonary transplantation, whereas single patients who do not allow discontuation from their VA-ECMO can be assisted by a mechanical long-term assistant device (Figure 1). This mechanical heart (most common HeartMate 3) permit survival for many years but it is usually used as a bridge for a future heart transplant.
Figure 1. Advanced circulatory treatment
Figure 2. The ECMO circuit consists of cannulas that are connected to large vessels in the body, hose leading the blood, a pump that drives the blood forward and an oxygenator that oxygenates the blood. With a sweep gas flow controller, to the right, the oxygenator’s respiration (carbon dioxide removal and oxygenation) is controlled.
There are many reasons why a patient’s respiration becomes so poor that it needs to be treated with VV-ECMO, pneumonia with ARDS development is the most common cause. Often, Murray score >3 is used as the limit for when ECMO treatment is indicated (Table 1), usually in combination with hypercapnoic limit, eg pH <7.2. Peek et al found that 15% of the patients who initially met the ECMO criteria became so respiratory stable that they did not need ECMO after being treated according to a strict protocol with, among other things, prone positioning, increased urinary output and transfusion to EVF of 40% 3 which emphasizes that patients should have had optimal respiratory treatment before decisions on placement of ECMO. It has been shown in the same randomized multicenter study that patients with the above inclusion criteria had better 6 month survival without invalidity if treated with ECMO than with respiratory therapy only 3.
|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|
Table 1. Add individual scores and share the number of components used (ie all patients do not need all measurements done). PaO2 is multiplied by 7.5 because the original measures in mmHg instead of kPa. (From Murray et al. Am Rev Respir Dis 138 (1988), 720-723).
A Murray Score calculator is available on: http://cesar.lshtm.ac.uk/murrayscorecalculator.htm
Different types of cannulation
The problem with extremely severe pulmonary failure is that the lungs do not oxygenate the blood satisfactorily. The goal of VV-ECMO is therefore to oxygenate as much of the blood as possible before it passes from the right atrium to the right ventricle. This can be done either with two catheters or with a double-lumen catheter (such as Avalon®).
Figure 4. Two different ways of catherization with double-lumen cannulation
Figure 5. The Avalon catheter in an optimal position. Blood from inferior and superior caval vein returns oxygenated blood in a jet stream towards the tricuspid valve.
In the case of a double lumen cannulation, the patient receives a wide-bore cannula via the femoral and the jugular internal vein with the tip positioned in inferior caval vein and superior caval vein/right atrium (Figure 4). One withdraws blood in one catheter and returns it in the other. However, there are two major problems that may reduce the VV-ECMO’s effectiveness. One is, if the cannula that withdraws blood has insufficient flow, it creates a shunt where non-oxygenated blood flows directly to the right ventricle. In addition, already oxygenated blood will flow into the ECMO a second time, so-called recirculation, which further reduces ECMO’s effectiveness.
An Avalon catheter is a double lumen catheter that withdraws blood in both superior and inferior caval vein. After oxygenation, the blood is returned to the right atrium. The advantage of this catheter is that you can have a smaller shunt of non-oxygenated blood and thus better oxygenation of the patient. The disadvantage is that it is harder to position correctly as the tip must be stably positioned in the inferior caval vein (Figure 5).
Oxygenation via VV-ECMO
Whatever cannulation you choose, some of the blood will shunt through the right atrium without oxygenation. Therefore, you may accept lower arterial saturation than in other patients. Since oxygen supply to the body is not only due to saturation but also Hgb and cardiac output, a little higher Hb and good cardiac output may keep oxygen supply adequate despite the lower saturation.
In order for the lungs to recover, it is important to use low-pressure lung protection ventilation, PEEP adapted to that patient and not too high FiO2 in the respirator.
The ECMO activates the coagulation cascade, therefore heparin infusion must be continuous. Patients should be kept as “dry” as possible to minimize edema in the lungs. It is a great advantage if they can wake up during respiration treatment, as it is of the utmost importance that muscle strength is maintained and treatment can be prolonged. This means that either tracheostomy or patients should be extubated early in selected cases.
When the lung begins to recover, you can switch off the airflow in the oxygenator to see if the lung can take over complete respiration itself. If the lung can handle it for a few hours, the ECMO can be switched off and the ECMO catheters may be pulled out.
Patients who have such an affected circulation that they probably have only minutes left to live can regain their circulation temporarily using a VA-ECMO system. The causes of the critical circulatory failure may vary, but myocardial infarction, dilated cardiomyopathy, myocarditis, cardiogenic shock, severe poisoning and pulmonary embolism are some conditions in which patients may be in such critical condition that ECMO treatment is needed urgently.
When you need to urgently initiate VA-ECMO, the femoral vein and artery are usually chosen for cannulation because these are large vessels and mostly quite easy to access. To speed up any possible cannulation further, patients with high risk of cardiovascular collapse may be prepared with preoperative cannulation using a single-lumen central venous line in both the femoral artery (left side) and vein (right side) used for quick exchange of catheters into larg bore ECMO catheters.
VA-ECMO provides a partial cardiopulmonary by-pass. As the blood is drawn from the right atrium and given back into the aorta, a retrograde flow is obtained in the aorta as shown in Figure 6. In parallel, the heart continues to beat and blood flows through the pulmonary circulation and the ECMO flow is disposed in the aorta. This causes a large part of the body to receive blood oxygenated by the ECMO, while the coronary arteries, right arm and right cerebral arteries are receiving blood oxygenated by the lungs, even if cardiac output is low. If the heart does not eject blood at all, the whole circulation is supplied by the ECMO. In order to properly handle these patients, one must understand the complex circulation and changed physiology such as which part of the body, heart/lungs is perfused by the ECMO system.
Figure 6. Peripheral cannulated VA-ECMO system with an extra cannula downwards on the left femoral artery to avoid ischemia in the left leg. The blood from the ECMO pump flows in the retrograde direction of the aorta until it meets the blood ejected by the heart.
Circle of Death
The blood received by the right ventricle, as well as blood from, for example, bronchial circulation and possible aortic valve insufficiency, the left ventricle must eject to the aorta. The problem is that patients are usually placed in ECMO because of cardiogenic shock caused by severe left ventricular failure. When the ECMO is started, the circulation is restored, but the failing left ventricle (which did not even manage to maintain low blood pressure) must now encounter a constant high afterload. If it does not pass the blood into the lung circulation, greatly elevated pressure builds up in the pulmonary circulation causing pulmonary edema, bleeding and progressive lung tissue destruction. Furthermore, the left ventricle is gradually filled and can be over distended with cardiac muscle disorder as a result. Another problem is that stagnant blood in the lungs, heart and aortic root is at great risk of being organized into large thrombus, especially unless anticoagulation is sufficient.
If the heart is able to eject with heavily elevated filling pressure (wedge pressure – PCWP) the lungs may still be injured. The blood pumped by the heart becomes poorly oxygenated and can cause ischemia in the organs it perfuses (coronaries, right carotid arteries). This evil circle is called the Circle of death (Figure 7).
Figure 7. Circle of Death: risk is prominent if left ventricle fails to contract with low filling pressure.
In order to avoid the circle of death and formation of thrombosis, one can provide pharmacological or mechanical inotropy to help the heart to pump the blood by for instance inserting an aortic ballon pump to reduce the heart’s afterload. If this is not enough, one can either switch to a cannula where you also aspirate blood from the left side of the heart (eg central cannulated VA-ECMO with left atrial position) or insert an Impella cannula, which pumps the blood from the left ventricle to the aorta ascendent.
Other cannulations in the case of circulatory failure
Peripheral cannulation is often the vascular access of most patients. Patients who fail to resume their circulation after cardiac surgery may instead need a cannula placed directly in the heart, this is called central cannulation. Furthermore, a number of other cannulas are used to relieve the right and left chambers individually or just one of them.
It is of utmost importance that patients are optimally anticoagulated with heparin, especially considering that blood in most cannulae often flows slowly through certain parts of the heart and thus constitutes a high risk of thrombosis formation. However, many patients are often prone to bleed because anticoagulation is a delicate balance between bleeding and thrombus formation.
Since patients with peripheral cannulated VA-ECMO have a double circulation, double pulse oximeters is useful. This in order to assess the oxygenation of the lungs and the oxygenation by the ECMO; On the right hand it is measured first if the patient ejects blood satisfactorily, on the foot to evaluate the ECMO system and to see if there is a divergence. In addition, general intensive care guidelines may be applied.
After a week, most hearts recover as much as possible, and therefore it is usually no idea to continue with VA-ECMO treatment anymore. You wean ECMO by slowing down the blood flow in the ECMO while giving the patient cardiac support in the form of inotropy and often also aortic ballon pump. When you see that the heart can manage and take over the circulation, the needles clamp on the ECMO and then turns off. The cannulas can then either be withdrawn with subsequent femostop treatment or the arterial cannula is removed surgically.
- 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 multicentre randomised controlled trial. Lancet 2009, 374(9698):1351-1363.