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MCS Series: ECMO Basics



ECMO is rapidly becoming a popular therapy, with more and more hospitals and institutions establishing ECMO programs. With this growth will inevitably come a higher rate of ECMO transfers. EMS providers in the future may even encounter this technology in the field, although in my personal opinion, we are still quite a few years away from this becoming a regular occurrence outside of the hospital environment. In this blog we will explore the basic goals of ECMO and the four main components seen in every ECMO circuit. Let’s get started!


My first real job at age 15 was at Burger King, and I'm pretty sure my first car still smells like fryer grease.


During orientation, we were given a binder with the ingredients for each menu item. This was a great training aid, but I quickly found out that BK's motto encouraged … off-menu creations… Regardless, the basic components of the sandwiches remained the same and served as the foundation for the custom items.


ECMO is somewhat similar. The basic components must remain the same to achieve the therapeutic goal. At its most basic form, ECMO or Extracorporeal Membrane Oxygenation is a way to artificially support the patient’s heart and or lungs by removing the blood from the body, oxygenating it, and returning it to the patient in real-time. However, each patient is different and may require circuit modifications to achieve the optimal level of support. These are the off-menu creations that plagued my mid-teen years. We will explore these customizations in the next MCS blog post.


 

For now, let’s concentrate on the following four items that make up our basic ECMO Sandwich:

  • Cannulas to access the patient’s vessels

  • Tubing to carry the blood outside of the body

  • A pump to circulate the blood

  • An oxygenator to exchange O2 and CO2


The Cannulas:

A cannula is a broad term for an elongated tube with holes on each end placed into the patient's vasculature to move blood out of and into the body. These are similar to peripheral IV cannulas, but they are much bigger and usually reinforced with flexible circumferential wiring. They can flow multiple liters per minute.

 

Traditionally, these cannulas were described as "Arterial" and "Venous," named for the type of blood they carried. You may still find these cannulas referred to as Arterial / Venous, but the terminology is being shifted to "Inflow" and "Outflow" in relation to the patient, not the ECMO pump. Except for dual-lumen cannulas, Inflow and Outflow cannulas usually follow some distinct characteristics.

 

Inflow:

  • Deliver oxygenated blood back to the patient.

  • Usually shorter than an outflow cannula, designed to reach proximal to the femoral bifurcation when placed femoral 

  • Typical adult sizes from 17-22fr

 

Outflow:

  • Retrieve deoxygenated blood from the patient

  • Usually longer, designed to reach the IVC/SVC Junction when placed femoral

  • Typical adult sizes 21-27fr

 

Dual Lumen:

  • Retrieves deoxygenated blood and delivers oxygenated blood through separate but adjoined lumens.

  • Use in V-V or hybrid V-A-V ECMO only

  • Typical sizes adult from 27-34fr

 

Placement of Cannulas requires the assistance of a guidewire and dilators and can be done under ultrasound guidance or fluoroscopy (x-ray). Upon removing the cannulas, surgical repair of the vessel will be required to achieve hemostasis.

  • CIT: It's common for oozing to occur around the cannulation site. Assess these areas frequently for increased bleeding and the development of large hematomas, which may kink the cannula or, in some circumstances, indicate displacement.


Circuit Tubing:

The first successful heart-lung machine used beer tubing from a local brewery to transport blood from the cannulas to the circuit pump. The advancements in plastic products have thankfully given us a much better solution. Most ECMO tubing is made of polyvinyl chloride (PVC) with the addition of plasticizers to allow the PVC to become pliable. Most modern-day tubing is also coated with a proprietary chemical from the manufacturer to achieve the best possible biocompatibility when the blood interfaces with the tubing's walls.

 

The tubing generally comes wrapped in a sterile container in a single continuous loop to allow the circuit to be recirculated with a priming solution (usually normal or playmate-A) before being cut and connected to the cannulas. When the cannulating physician is ready to connect the tubing to the cannulas, they will cut the tubing with scissors or a scalpel and make a wet-to-wet connection to the cannula.

  • CIT: The connection between the tubing and the cannula is held by friction. Some institutions will place "tie-bands" or other devices to further secure these connections. Still, special attention should be paid to this area of your assessment. If any connections appear loose or have moved from your previous assessment, notify the pump operator immediately.

 

The Size and Length of tubing is often an overlooked variable. While the length of tubing should provide the ability to transport the patient and manipulate the ECMO controller safely, unnecessary length can produce significant resistance within the circuit which may lead to sub-optimal pump performance. Assess the length of tubing and formulate a plan with your crew to move the patient safely. Generally speaking, once the length of tubing has been established, you got what you got. Changing the tubing length will require the pump to be temporarily turned off and additional wet-to-wet connections to be made.


ECMO tubing usually comes in two diameter sizes: 1/4 inch for pediatrics and 3/8 inch for adults. The size of the tubing greatly affects a patient's delusional hematocrit when placed on ECMO. That is to say, a circuit with 3/8-inch tubing will require twice the priming volume of a 1/4-inch circuit of the same length. The diameter of the tubing also effects the achievable flow rate.



The Pump:

As discussed in the previous MCS blog, "Go With the Flow," Most pumps used with ECMO today are centrifugal pumps. These pumps use a spinning impeller to generate pressure that pushes the blood through the circuit and body. Some of these pumps are separate from the oxygenator, but others, such as the Maquet CardioHelp © System, use an integrated pump and oxygenator.

 

Regardless of which system you encounter, the centrifugal pump works the same. An inlet at the pump's center allows blood to flow into the pump head. As blood encounters the spinning impeller, it is forced along the curved vanes of the impeller outward to the outside wall. When the blood meets the resistance of the outside wall, pressure is generated, and forward flow is achieved through the outlet port on the side of the pump head.

 

The pump operator can change the speed of the pump, which subsequently generates more or less pressure. This translates to more or less flow. It's important to remember that these pumps depend on preload and afterload. Adequate preload must be established to prevent the cavitation of air emboli, and forward flow depends on the afterload pushing back against the pump.

 

Please check out the previous blog, "Go With the Flow," to learn more about how the centrifugal pump works.


The Oxygenator:

After the blood leaves the pump, it enters the oxygenator for gas exchange. Oxygenators come in various forms, shapes, and sizes, but their goal is the same: Remove CO2 and deliver O2.

 

Most oxygenators today are considered "Hollow Fiber Membrane Oxygenators, " a general term used to describe how gas exchange occurs within the oxygenator. Generally speaking, gas (oxygen) is carried along the inside of these hollow fibers, which are made from a material permeable to gas but not fluid. Oxygen and CO2 move freely across this membrane while blood remains in the appropriate channels. As blood moves along this membrane, CO2 can diffuse out of the blood and into the hollow fibers. O2 is allowed to diffuse in the opposite direction.

 

For more information on how an oxygenator works, check out the first blog in the MCS Series "SWEEP"!



Thanks for making it to the end of the Blog! Make sure to follow @FOAMFrat on social media for updates on future blog posts. As always, I welcome any feedback, questions, comments, or suggestions for future blog posts. You can reach me at bcress@foamfrat.com


Brian Cress, CCP, LP, FP-C, NRP


References: Brogan, Thomas V., et al. Extracorporeal Life Support: The Elso Red Book. Extracorporeal Life Support Organization, 2023.


Lich, Bryan V., and D. Mark Brown. The Manual of Clinical Perfusion. Perfusion.Com, 2004.

Miller, G. Wayne. King of Hearts: The True Story of the Maverick Who Pioneered Open Heart Surgery. Crown Publishers, 2000.



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