Back With Part B of “Cir...
Highlights
- Back with part B of “Circuit and its pressures”.
Link for part A:
https://t.co/jCq78PcY7D
The oxygenator: Component 3
This is situated after the pump & before the return cannula. It oxygenates and decarboxylates the blood. Modern oxygenators are ‘hollow fiber oxygenators’
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- Fresh gas flows in a mesh of very small hollow fibers while blood flows in a counter-current manner around these fibers.
Hence, blood and gas phases are kept physically separate. Gas exchange occurs via very small pores in these hollow fibers.
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- In the older versions (polypropylene oxygenators), these pores were large enough to allow leakage of plasma proteins.
The current polymethtylpentene (PMP) oxygenators have much smaller pores so plasma leakage is minimized.
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- Now let’s talk about circuit pressures. From the point of view of pressures, the circuit can be broken down into two parts: pre-pump and post-pump.
Pre-pump pressures are negative: the pump “sucks” blood out the body
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- Post-pump pressures are positive: the pump “pushes” blood forward through the oxygenator and back into the body.
In general, higher the pump speed (more pump power), more extreme would be the pressures generated on either side.
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- An intuitive understanding of pre-pump (negative) and post-pump (positive) pressures helps understand the effects of circuit rupture.
If the rupture is pre-pump (negative) side, this may cause air embolism. A breakdown on the post-pump (positive) side would cause bleeding.
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- (i) Venous pressure (Pven or P1):
This is the negative pressure just proximal to the pump.
A sudden reduction suggests some obstruction on the pre-pump side: kink/clot in the pre-pump tubing/drainage cannula, “chatter” event etc.
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- (ii) Internal pressure (Pint or P2):
This is the post-pump but pre-oxygenator positive pressure. By itself, a sudden increase may occur in:
(a) Obstruction in the oxygenator (e.g. clots), or
(b) Obstruction in the post-oxygenator tubing/return cannula.
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- (iii) Arterial pressure (Part or P3):
This is the pressure in the tubing just distal to the oxygenator. A sudden increase suggests obstruction in the post-oxygenator tubing/return cannula
You may have noted that relative change in P2 & P3 may be used to detect oxygenator failure
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- Hence, if you're particularly interested in oxygenator health, this analysis can be refined by looking at P2 — P3.
(iv) Transmembrane pressure (ΔP) = P2 — P3
This is the pressure across the membrane lung (oxygenator). A sudden increase may suggest oxygenator failure from clots.
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- Hence, following pressure trends can provide a lot of information on all parts of the circuit.
Although some legacy systems (e.g. Rotaflow) don't have integrated pressure measurement capabilities, traditional pressure transducer systems can be used:
https://t.co/6mP5MqM6GF
/FIN (View Tweet)