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 contain oxygenation or carbon dioxide removal, can be treated with venovenous extracorporeal membrane oxygenation (VV-ECMO). Venoarterial ECMO (VA-ECMO), on the other hand, helps patients whose circulation is not consistent with continued life despite maximum inotropic treatment [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 come from their VA-ECMO can be assisted by a mechanical long-term assistant (Figure 1). This mechanical heart (most common HeartMate 3) allows you to live 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 bad that it needs to be treated with VV-ECMO, but pneumonia with ARDS development is the most common. Often, Murray score> 3 is used as the limit for when ECMO treatment is indicated (Figure 3), usually in combination with hypercapic limit, eg pH <7.2. Peek et al found that 15% of the patients who initially met the ECMO criteria became so respiratory that they did not need ECMO after being treated according to a strict protocol with, among other things, boots, increased diures and transfusion to the EVF 40%  which emphasizes that patients should have had optimal respiratory treatment before decisions on ECMO are taken. 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 .[table “96” not found /]
Figure 3. 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 Murrayscore calculator is available on: http://cesar.lshtm.ac.uk/murrayscorecalculator.htm
Different types of cannulation
The problem with extreme grave lung failure is that the lungs do not oxygenate the blood. The goal of VV-ECMO is therefore to acidify 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 needles or with a double-chamber catheter (Avalon®).
Figure 4. Two different ways to cannula in two-channel cannulation
Figure 5. Avalon catheter in optimal position. Blood in v cava inferior and superior and gives oxygenated blood in a jet towards the tricuspid opening.
In the case of a two chanelled canula cannulation, the patient receives a coarse cannula via v femoralis and an internal vaginal internal with tips in v cava inferior and cava superior / right atrium (Figure 4). One draws the blood in one needle and returns in the other. However, there are two major problems that reduce the VV-ECMO’s effectiveness. Secondly, the cannula that does not absorb blood can not do it, creating a shunt where non-oxygenated blood flows directly to the right-hand chamber. In addition, already oxygenated blood will be sucked into the ECMO a second time, so-called recirculation, which further reduces ECMO’s effectiveness.
An Avalon catheter is a double chamber catheter that draws blood in both v cava superior and inferior. After oxygenation, the blood is returned to the right atrium. The advantage of this needle 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 as the tip must be stably lowered in v cava inferior (Figure 5).
Oxygenation via VV-ECMO
Whatever cannulation you choose, however, some of the blood will shunt beyond 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 Hb 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, low-pressure, low-pressure lung protection, 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, as it is of the utmost importance that muscle strength is maintained and treatment can be long. This means that either tracheotomy patients should be extubated early or in selected cases.
When the lung begins to recover, you can turn off the airflow in the oxygenator to see if the lung can take over the respiration itself. If the lung can handle it for a few hours, the ECMO can be switched off and the ECMO hoses are pulled out.
Patients who have such an affected circulation that they believe they have minutes left to live can regain their circulation using a VA-ECMO. The causes of the circulatory failure may vary, but myocardial infarction, dilated cardiomyopathy, myocarditis and pulmonary embolism are some conditions in which patients may be so bad that ECMO treatment is needed.
When you need to quickly start a VA-ECMO, the venous and arterial femoralis are usually eroded because they are large vessels and often quite easy to find. To speed up any possible cannulation further, patients with high risk of ECMO may be well prepared with arthritis and thin CVK in the artery and vena femoralis, respectively. VA-ECMO provides a partial cardiopulmonary bypass. As the blood is drawn from the right atrium and is given back into the femoral artery, a retrograde flow is obtained in the aorta (Figure 6). In parallel with this, the heart continues and blood flow through the heart and the ECMO flow is encountered in the aorta. This causes a large part of the body to get blood oxygenated by the ECMO, while coronary artery, right arm and right brain are receiving blood that is oxygenated by the lungs, even if the heart only turns out a bit. If the heart does not break out at all, the whole body is acidified by the ECMO. In order to properly handle these patients, one must first understand the double circulation and which part of the body, such as the heart / lungs and the ECMO, perfuses and oxygenates.
Figure 6. Peripheral cannulated VA-ECMO with extra needle downwards to avoid ischemia in the bone. The blood from the ECMO flows in the retrograde direction of the aorta until it meets the blood as the heart pumped
Circle of Death
The blood that the right chamber pumps on, as well as blood from, for example, bronchial circulation and possible aortic insufficiency, the left ventricle must pass the pump to the aorta. The problem is that patients are usually placed in ECMO because of cardiogenic shock caused by very severe left ventricular failure. When the ECMO is started, the circulation is restored, but the tired left ventricle (which did not even manage to maintain low blood pressure) must now look for a constant high afterload. If it does not pass the bloodstream of the bloodstream into the lung cycle, greatly elevated pressure builds up causing lung edema, lung bleeding and lung destruction. Furthermore, the left ventricle is gradually filled and can be overexpressed with chamber 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 break out but with heavily elevated fill pressure (wedge pressure (PCWP)) the lungs can still be injured. The blood that comes from the heart becomes poorly oxygenated and can cause ischemia in the organs it perfuses (coronaries, right brain). This evil circle is called the circle of death (Figure 7).
Figure 7. Circle of Death: risk if left ventricle fails to smash with low fill pressure.
In order to avoid the circle of death and formation of thrombosis, one can provide inotropy to help the heart to pump the blood and put an aortic pallon pump to reduce the heart’s afterload. If this is not enough, you can either switch to a cannula where you also suck blood from the left side of the heart (eg central cannulated VA-ECMO with left atrial vent) or put an Impella, 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 entrance canal of most patients. Patients who fail to resume their circulation after a cardiac surgery may instead need a needle placed directly to 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 has a high risk of thrombosis. However, many patients are often at the same time prone to bleeding because the anticoagulant is a balance between bleeding and thrombus risk.
Since patients with peripheral cannulated VA-ECMO have a double circulation, double pulse oximeters are good. This in order to assess the oxygenation of the lungs and the oxygenation of the lung and oxygen tower; On the right hand it is measured first if the patient knocks out enough, on the foot it later. In addition, general intensive care guidelines apply.
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 pallon 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.