Let’s now continue our exploration of the fundamentals of extracorporeal membrane oxygenation (ECMO) support by discussing cannulation configurations. To remind ourselves, the basic function of the ECMO circuit is to drain deoxygenated blood, remove CO 2 /oxygenate in the membrane oxygenator, and return that blood back to the body ( Fig. 8.1 ).
Where this blood is drained from and where it returns to is exactly what we will go over in this chapter. We will cover a lot of configuration possibilities and when possible, the relevant physiologic rationale. This chapter is neither a comprehensive compilation, nor is every configuration mentioned used by every cannulating team, as some may be related to the preferences of the team. If anything, the breadth of this list underscores the many possibilities that exist for configuring the ECMO circuit to the specific physiologic needs of the patient.
Cannula selection
The first consideration is size and type of cannulas. Cannulas are large catheters that are inserted into heart or blood vessels by which blood is drained and returned. Most of the time, one cannula drains the blood while the other cannula returns the blood. These two cannulas often differ by where holes are placed for drainage and return of blood. There are primarily two types of cannulas that should be distinguished, referred to as multistage and single stage ( Fig. 8.2 ).
Multistage cannulas are usually used as the drainage cannula. When draining blood, you want to have the ability to pull in blood over a large surface area so that a sufficient volume of blood can be run through the circuit without excessive negative pressure to do so. Thus, multistage cannulas have multiple holes for draining blood.
Compare this to the single-stage cannula, which has only one set of holes at the top. This carries the advantage of allowing for directional flow of returned blood. The single-stage return cannula can differ in length, usually being around 15 cm for short return cannulas and 50 cm for long return cannulas.
The other type of cannula is the dual lumen cannula. Dual lumen cannulas embed a return cannula inside an outer, drainage cannula as illustrated in Fig. 8.3 . There is an outlet that allows for the return of oxygenated blood that can be aimed in a manner so that it does not get pulled back into the ECMO circuit. Dual lumen cannulas allow for both drainage and return of blood from a single access site, which can allow for improved mobility and ambulation.
The final consideration for cannula selection is width and length. The length can vary but for the most part is fairly standard for multistage cannulas, all being around 50 cm for adults. The length of the return cannula depends on where the cannula is being implanted and for the most part remains as a long return (~50 cm) or a short return (~15 cm) ( Fig. 8.4 ).
Width is an important consideration for cannula selection, as the width is an important determination of overall flow – the wider the cannula, the more flow you will be able to obtain. We will talk about the concept of flow in much greater detail in Chapter 10 . The benefits of greater flow have to be weighed against the possibility of greater obstruction of the blood vessel, more damage to the blood vessels, and whether or not the vessels can accommodate a larger cannula.
The standard convention for width is French, which is three times the diameter of the cannula in millimeters. This can be helpful when sizing cannulas to the size of the blood vessel. If the blood vessel is 6 mm, for example, then it likely could not accommodate a 21-French cannula, which would have a diameter of 7 mm.
Naming convention
First off, let’s review the basic naming convention. Typically, the name of the configuration follows the direction of blood. If you are draining blood from the venous system and returning it to the arterial system, then this is referred to as V eno- A rterial ECMO or VA ECMO ( Fig. 8.5 ).
Contrast this to the following configuration, where we are draining from the venous system and returning back to the venous system. This is referred to as V eno- V enous ECMO, or VV ECMO ( Fig. 8.6 ).
Now let’s look at some configurations, starting with VV ECMO.
Femoral jugular VV ECMO
Femoral-jugular involves a drainage cannula that is implanted into the inferior vena cava (IVC) via the common femoral vein and a short return cannula implanted into the superior vena cava (SVC)/right atrial junction via the internal jugular vein ( Fig. 8.7 ). The drainage cannula is usually a multistage cannula to allow for optimal drainage and positioning. Ideally, the drainage cannula should be inserted into the intrahepatic IVC right under the diaphragm. The intrahepatic IVC position allows for the liver parenchyma to keep the IVC open and helps to optimize flow without collapse of the IVC around the negative pressure generated from the drainage ports.
The return cannula, by contrast, should be a short, single-stage cannula, implanted into the right or left internal jugular vein, although the right IJ vein allows a somewhat straighter path to the optimal position in the RA/SVC junction. Placing the return cannula in the SVC/RA junction, with the drainage cannula lower at the intrahepatic IVC just below the diaphragm, separates the two cannulas and prevents oxygenated blood that is being returned from the ECMO circuit to be sucked back into the circuit, a concept known as recirculation, which we will cover in more detail later on.
Femoral-femoral VV ECMO
Femoral-femoral VV ECMO involves a multistage drainage cannula placed into the intrahepatic IVC just like in the femoral-IJ configuration; however, in this configuration, the return cannula is a long, single-stage cannula implanted into the IVC/RA junction from below, via that contralateral femoral vein ( Fig. 8.8 ). Since the return cannula is a single-stage cannula, and is inserted above the top of the drainage cannula, blood can theoretically be returned without being sucked back into the drainage cannula in the form of recirculation. However, this risk of recirculation can be more significant with this configuration than with femoral-jugular.
Additional venous drain
Intermittently, you may come across a circuit with two drainage cannulas. To confirm it is two drainage cannulas as opposed to two return cannulas, ensure that both cannulas are draining dark blood.
What is the advantage of an additional drainage cannula? Some providers use a second cannula if obtaining adequate flows are insufficient. Usually, if a second cannula is added for improved flow, it is a drainage cannula, as the flow limitations on VV ECMO are often dictated by the drainage side ( Fig. 8.9 ).
