As ECMO continues to grow beyond the CVOR, a growing number of physicians outside cardiac surgery are being asked to cannulate. This expansion is an important step toward building more responsive, multidisciplinary ECMO programs, especially when timely cannulation can affect how quickly support is initiated.

But for physicians who do not routinely cannulate, gaining enough hands-on experience to feel prepared can be difficult. Cannulation is not a skill that can be fully learned by watching a video, reading a protocol, or observing someone else perform it. Understanding the fundamentals, cannula characteristics, flow dynamics, and how cannulas are actually selected, is a good place to start.

How Are Cannulas Selected?

Knowing that centrifugal ECMO pumps are preload dependent and afterload sensitive, the ability to achieve desired flow with reasonable line pressures will depend upon the characteristics of each cannula.

These include:

• Inner diameter
• Length
• Drainage and return ports
• Location of the cannula in the vessel

ECMO cannula flow is governed by Poiseuille’s Law, which states that the flow rate is directly proportional to the driving pressure multiplied by the fourth power of the radius, divided by the length. In other words…

• Doubling the length of the cannula decreases the flow by half at a given pressure
• Doubling the diameter of the cannula increases the flow 16 times!

So, the shorter the cannula and the larger the diameter, the more flow!

Factors that affect laminar flow and resistance in the ECMO circuit and cannulae are:

• Diameter
• Length
• Viscosity*

*most cannulae are tested for flow and pressure drop with water and not blood

The above images detail the testing done of the cannulas in water. The viscosity of blood is thicker than water, so we won’t see the pressures as low as they are listed here. These are the expected pressures we will see based on the cannula size. Remember that the biggest limiting factor to ECMO flow is the size of the venous (drainage) cannula, followed by preload, and afterload. Pressure drop refers to the difference between the pressure entering the cannula and the pressure exiting the cannula. According to the ELSO Red Book (6th ed.), clinicians should target a pressure drop of less than 100 mmHg, applying to both the drain and return cannulas.

Put it into practice:

If we have an adult patient and the goal flow based on patient BSA and ECMO configuration is 5 L/min, then using the above chart, we’d locate 5L/min for both the venous and arterial cannula and choose the smallest diameter option that keeps the pressure drop under 100 mmHg for each. That would allow a 21F, 23F, or 25F venous cannula and a 19F, 21F, 23F, or 25F arterial cannula.

**Keep in mind that the BIGGEST limiting factor to achieving adequate flow in the ECMO circuit is the SIZE of the venous cannula

Single Site Bicaval Dual Lumen Cannulae

These cannulas have an internal bifurcation that separates the internal lumen into two sides. The drainage lumen is larger and is about 2/3 of the total internal diameter, and the reinfusion lumen is smaller at about 1/3 of the internal diameter.

The cannulas drain from the SVC and IVC with the return port directed to the tricuspid valve. Radiopaque markers can be used to visualize placement on follow-up x-rays. Generally, placement is assisted with fluoroscopy or TEE in addition to ultrasound.

Some dual lumen cannulas are designed for right heart support, to rest the right ventricle (below).

Single Lumen Cannulae

Single lumen venous cannulae can be single stage (drain only from one area near the tip of the cannula) or multi-stage (designed to drain blood from the IVC, SVC, and hepatic vein).

Single Stage

Multi-Stage

Central Cannulation

For postcardiotomy patients, central cannulae may be left in place. Additionally, grafts may be utilized. Central cannulae are shorter and larger in diameter than peripheral cannulae. For grafts, sizing is listed in mm, not Fr. To approximate graft sizing in relation to cannulae sizing:

Graft mm x 3 = size in Fr. For example, a 10 mm dacron graft is approximately 30F.

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Want Hands-On ECMO Training?

Understanding cannula selection and flow dynamics is one piece of building strong ECMO practice. We offer hands-on training courses throughout the year for physicians, nurses, respiratory therapists, and other healthcare professionals looking to build or sharpen their ECMO skills, cannulation included. See our current courses for available programs and dates.