Blood-Gas Exchange Unit

Air-Oxygen Blender

Figure 10 : Example of an Air-oxygen blender

The air-oxygen blender is a device that determines the desired fraction of oxygen being delivered to the oxygenator. This is called FdO2 (fraction of delivered oxygen percentage) and varies between 21%, O2/air mix, and 100% O2.

Oxygenator

Fig. 11. Example of Oxygenator

The blood gas-exchange unit is made of a membrane oxygenator. Modern oxygenators are composed of hollow fibers that separate the blood and gas phase and allow gas to diffuse across it. The fibers are usually made of silicone rubber, polypropylene (microporous), or polymethyl pentene.

The two main functions of the oxygenator are to oxygenate the patient's blood and remove carbon dioxide. Blood from the drainage side is pumped in on one side of the membrane, and gas from the blender is pumped into the other side. The rate at which gas is delivered is called the sweep speed and can be set anywhere between 0-15 L/min. The fresh gas flows countercurrent to the blood maintaining a favorable gradient for oxygenation and decarboxylation. Thus allowing oxygen to diffuse across the membrane into the blood and carbon dioxide to diffuse out. This process is known as counter diffusion.

Figure 12: ECMO circuit

Oxygenation depends on:

The flow, higher flows allow more blood to be oxygenated.
The FiO2: increasing FiO2 achieves a higher partial pressure of oxygen.
The membrane integrity: if there is something present on the membrane (e.g. blood clots) that impairs diffusion, oxygenation will decrease.

Carbon dioxide removal depends on:

The sweep: higher sweep speeds result in higher CO2 removal.
The flow: if the flow is increased without increasing the sweep, then CO2 removal can be impeded.
The presence of water vapour in the membrane: this can impede CO2 removal.

Heat-Exchanger

Fig. 13: Example of a Heat Exchanger

The oxygenator is connected to a heat exchanger which allows the blood to be warmed as it passes through the oxygenator.

The blood is warmed to approximately 37 degrees Celsius before leaving the oxygenator and enters the return cannula where it is delivered back to the patient. Blood in the circuit is exposed to the ambient temperature so it must be warmed to avoid heat loss.

References

[1] Basics of ECMO: Part 1 : Virtual Library. (2024, May 27). WFSA Resource Library. https://resources.wfsahq.org/atotw/basics-of-ecmo-part-1/[2] icuECMO: The ECMO Circuit. (n.d.). http://icuecmo.ca/icuECMO_content/icuECMO_ECMO_circuit.html

Chan Ching Wen Hazel

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