Let’s start our discussion of the fundamentals of extracorporeal membrane oxygenation (ECMO) by returning to our graph. We have spent the last few chapters developing and unpacking the simple but essential concept that the amount of support that is needed for critically ill patients is associated with a dose-related toxicity that increases at higher levels.
The majority of the concept was developed in the context of optimizing oxygen delivery, but the same concept exists for any intervention that is performed in the intensive care unit (ICU). The more sedation, antibiotics, and medications that are needed to support the patient, the greater the potential for drug interactions as well as progressive weakness, delirium, and immobility. It is not simply that sick patients need more support – the support itself can lead to a progressive downward spiral with diminishing positive returns and progressive toxicity that can lead to decompensation and death.
Let’s propose now that for specific patients, we can support them with ECMO, a modality that may be associated with some increased risk, but which can hypothetically decrease the degree of dose-related toxicity associated with conventional care ( Fig. 6.1 ).
So what is ECMO?
Extracorporeal membrane oxygenation, or ECMO, is a form of extracorporeal life support that functionally involves a blood pump and membrane lung or oxygenator, to support the heart and/or lungs. Let’s look at a basic schematic.
The mechanism is deceivingly simple – pumping the blood out of the body, oxygenating the blood, and returning that blood back to the body ( Fig. 6.2 ).
As straightforward as that mechanism is, the physiology of the body that it interacts with can be complex, and forms the basis of the discussions to follow, to truly understand the concepts, physiology, and management principles behind ECMO.
Why does the ECMO circuit seem more complicated when I look at it?
Fig. 6.3 is an example of what might be seen in an ECMO circuit.
It is important to first understand the basics of what ECMO is trying to accomplish. From there, we can fill in the other components that are part of the ECMO circuit. ECMO circuits have become more straightforward in recent years, but the complexity of the circuit that you are managing can differ widely and is largely determined by institutional protocols, preferences, and processes.
Each component has serves a specific function. Often if this is difficult to distinguish, it can be helpful to trace the circuit in the direction of the blood, reminding ourselves that the basic design is drainage of deoxygenated blood through a pump, through a membrane oxygenator, with the return of oxygenated blood back to the patient.
Let’s now better familiarize ourselves with all of the other components, using Fig. 6.4 to help discover various parts of the circuit. We will start in the direction of blood proceeding from the drainage cannula, through the circuit, and finally to the return cannula.
Drainage Cannula
The drainage cannula is the entry point into the ECMO circuit. It is a large-bore access tube that is implanted into the venous circulation through a variety of techniques, including surgical through a graft sewn onto the blood vessel, percutaneously, or through cutdown. It can usually be distinguished from the return cannula in that it is draining dark, deoxygenated blood ( Fig. 6.4 ).
Drainage Line
This is usually a set of clear plastic tubing that is connected to the drainage cannula. The convention is for drainage tubing to be 3/8 of an inch, but this can differ. The drainage line is usually distinguished by the presence of blue tape; however, if this tape is not present, then the color of the blood can be revealing ( Fig. 6.4 ).
Pump
The pump is probably one of the most important components of the ECMO circuit – indeed there is no extracorporeal membrane oxygenation without the ability to circulate blood extracorporeally! However, despite this relative importance of the pump, it can be difficult to notice at first. If you do not know what you are looking for, then you may miss it. Note the pump, where the blood is entering and where it is exiting. Also at this point, note the backup for the pump, which should always be within arms reach, whether a hand crank, backup console, or backup circuit altogether ( Fig. 6.4 ).
Console
The console is like the dashboard of your car – even though it has nothing to do with actually moving the vehicle forward, it is the interface that you are most likely to interact with. Some consoles can be very straightforward, denoting the speed of the pump and the rate of blood flow only, while others can have a lot of bells and whistles with other data points. The console will usually have some mechanism for altering the revolutions per minute (RPMs), whether a dial or push button ( Fig. 6.4 ).
Oxygenator
The oxygenator is where the membrane oxygenation of ECMO occurs. There are many different types of oxygenator, the common thread being that they have a mechanism for oxygenating the blood through diffusion, and usually a mechanism for temperature regulation ( Fig. 6.4 ).
Blender/Oxygen Source
The oxygenator will be connected to some source of oxygen, usually in the form of a blender, which can be mixed with medical grade air to allow for differing concentrations of oxygen to be delivered. The blender will have the ability to alter the FiO 2 of the gas delivered as well as the rate of the gas flow (known as sweep gas flow). In cases of transport or in emergencies, this oxygen connection can take the form of an oxygen tank, which can deliver 100% FiO 2 at differing sweep gas flow rates ( Fig. 6.4 ).
Heater/Cooler
As we will cover later, the oxygenator is formed by the interface of three phases – blood, oxygen/sweep gas, and water. The water is used to temperature regulate the blood that is going back to the patient, either warming it to account for ambient loss of temperature in the circuit or cooling it in the case of fever or targeted hypothermia ( Fig. 6.4 ).
Return Line
The return line looks almost identical to the drainage line, with the notable exception that it is filled with oxygenated/bright red blood. The line may also be denoted with red tape, which can be especially important when being connected as a new circuit ( Fig. 6.4 ).
Return Cannula
The destination of the return line is a second cannula that is inserted into the venous or arterial circulation, depending on the type of support being offered. In some cases, the return line can be connected to the return port of a dual lumen cannula that returns blood. We will go into much more detail on the types of drainage/return cannulas when we discuss configurations in Chapter 8 .
Overall, that’s it! Those are the primary components common to almost every ECMO circuit that you will encounter. There are other components, many of which we will cover in more detail in Chapter 9 , but these are the common components that achieve the primary objective of ECMO – drain blood, oxygenate/remove CO 2 , temperature regulate, and return the blood to the body. Keep the overall flow straight and you will have a systematic approach to any circuit that you come into contact with.
Now let’s go into more detail about what is happening with the fundamental component of ECMO – the oxygenator.
Understanding the oxygenator: the fundamental mechanism of ECMO
The essence of ECMO is returning oxygenated blood to the body. The oxygenator is where all of this happens. When blood flows through the oxygenator, it enters though an inlet and then follows a specific path, in order to allow for blood to easily pass through while minimizing the entrainment of any air that may come through the circuit ( Fig. 6.5 ).
