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Mechanical Circulatory Support Series: Sweep

Sweep? What is Sweep? You may have heard this term in reference to medicine, or you may just be familiar with what your parents had you doing to help around the house when you were younger.

To oversimplify the concept, sweep, or sweep gas, is the term used to describe the gas flow across an oxygenator membrane in extracorporeal oxygenation devices such as ECMO. If you are familiar with this concept, I hope to detail some concepts and perhaps convey some new information. If you are unfamiliar with sweep, fear not!



This being my first post with FOAMFrat, let me briefly introduce myself:

I'm Brian, unfortunately no relation to the legendary recently-appointed CEO of FOAMfrat Brian King (AKA "Tyler's Boss")

I've been a Paramedic for over ten years and had flown for about six years before attending the Texas Heart Institute in Houston, TX. I'm currently a board-certified cardiovascular perfusionist and maintain my paramedic certifications, working part-time at a local EMS agency.

My primary interest in these blog posts is to write about the unique transport considerations of Mechanical Circulatory Support Devices (MCS) and to help educate the readers on the physiological concepts of MCS devices and how they work. I hope you enjoy the content, and please reach out with any questions or if there are any topics you'd like to see in the future. My contact information is at the bottom of the blog!


So I mentioned earlier that sweep was how we impart gas exchange in extracorporeal oxygenation, but why is it called sweep? "Minute ventilation," "tidal volume," and other respiratory terms seem more intuitive than "SwEeP."


Truthfully, ask ten people, and you'll get ten different answers, but let's dive into how gas exchange occurs in an oxygenator, and maybe you'll come up with your own interpretation! There are three important concepts to understand in relation to gas exchange within the body and an ECMO circuit:

  • Fick's Law of Diffusion

  • Partial Pressure

  • Flow

Fick's Law Fick's Law of Diffusion states that gas will move from an area of higher partial pressure to an area of lower partial pressure toward equilibrium.

Okay, easy enough. So what about partial pressure?


Partial Pressure

Partial Pressure is a measured force (usually mmHG or "torr")


This force is caused by the collection of the same molecules within a given container or Atmosphere

This is the same concept used during pre-intubation nitrogen washout. This also explains why, at higher altitudes (where there is more "container"), the percentages of gases remain the same, but hypoxia is induced due to a lower partial pressure.


So higher partial pressure = higher or wider concentration gradients = higher rate of gas exchange. Great, so how do we change partial pressure? In short, there are two ways to change partial pressure.

  1. Change the concentration of a gas within the container. -This is done by manipulating your blended gas, most commonly referred to as FiO2.

  2. Change the container size. -Altitude is the biggest determinant, but thermodynamics also plays a factor here. This must be considered during flight operations, especially in fixed-wing flights or transports over mountainous regions in unpressurized aircraft. As altitude from sea level increases, partial pressures will decrease. -An easy way to determine FiO2 requirements at altitude is to use the following formula:


Flow

We've already seen how increasing the percentage of a gas will increase the partial pressure within a given container, thus creating a gradient for gas exchange to occur via Fick's Law. But now we have to consider that we are working with two containers.

  1. The Oxygenator

  2. The Body

Just as ventilating someone in cardiac arrest, without a pulse and no CPR, will not allow distal gas exchange, the same can be said with ECMO. You must have Cardiac Output or Flow in order to oxygenate and, equally important, remove CO2 from the patient. That is to say; the tissues must receive a constant supply of blood that is rich in oxygen and poor in CO2 in order to exchange gas at the cellular level. Stagnant blood quickly equalizes the O2/CO2 equilibrium, and gas exchange no longer occurs. The really cool thing about ECMO... we can usually make the Cardiac output whatever we want it to be :D

Essentially, the greater the flow moving through the oxygenator, the better the oxygenator is able to offload CO2 and load O2 onto the red cells.

Of course, just as with mechanical ventilation, it is possible to send the patient into a respiratory alkalosis or acidosis by over or underflowing the patient or by over or under-sweeping the patient. This is why we usually monitor inline gas measurements on cardiopulmonary bypass and why ABGs should be frequently assessed in ECMO patients, especially freshly cannulated patients.

Coming up in another blog, I will discuss the balance between hemodynamics and acid-base balance while on ECMO. You can't change one without affecting the other! TLDR:

  • - Sweep is the term for the blended gas moving across the oxygenator in extracorporeal membrane oxygenation

  • - Fick's Law is the physical principle that allows gas exchange to occur by means of simple diffusion

  • - Partial Pressure is the driving force behind Ficks Law. The greater the pressure gradient between two solutions, the greater rate of gas exchange

  • - Flow significantly affects how efficiently gas exchange can occur, as partial pressure is a passive process; blood must constantly be circulating in order for the partial pressure gradient to exist.

So, why do you think the term is named "sweep"? To me, I imagine it as gas moving across, or "sweeping across," a membrane pushing gases in both directions as they attempt to reach equilibrium. Let me know what you think in the comments!

Thank you for making it to the end of the blog, and I hope you were able to take something away from this reading. Of course, I welcome any feedback, questions, comments, or suggestions for future blog posts. You can reach me at bcress@perfusiondynamics.org

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










References:

Brogan T.V., Roberto L., MacLaren G., et al. In: 5th edition. T.V.B., editor. Extracorporeal Life Support Organization; Ann Arbor (MI): 2017. Extracorporeal life support: the ELSO red Book; p. 868. (Extracorporeal life support: the ELSO red book). [Google Scholar]


Wrisinger WC, Thompson SL. Basics of Extracorporeal Membrane Oxygenation. Surg Clin North Am. 2022 Feb;102(1):23-35. doi: 10.1016/j.suc.2021.09.001. PMID: 34800387; PMCID: PMC8598290.


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