CRRT question

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Just started doing CRRT at my hospital and I had a question that has been nagging at me. We mostly do CVVH on out patients and I know when we do that we can either add in the replacement fluid pre filter or post filter. What I do not understand is that if we add replacement fluid post filter then how can the solutes be removed by convection? Correct me if I'm wrong (which I probably am), but I was under the impression that as the replacement fluid is added it drags the solutes out of the filter by convection. Now this makes sense if it is added pre filter, but if the replacement fluid is added post filter then it cannot drag anything out of the filter at all since it is added only after the blood has already passed through the filter. Any help would be greatly appreciated. Thanks.

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Specializes in Critical Care, ED, Cath lab, CTPAC,Trauma.

Convection is the movement of solutes through a membrane by the force of water. Convection is sometimes called “solvent drag”. Think about a coffee maker. When the water drips through that filter it doesn’t come out the other side as pure water, does it? It carries along with it flavor molecules, caffeine and other substances that make it coffee... those molecules get to the other side of the filter by convection. The water pulls the molecules along with it as it flows through the membrane. Convection is able to move very large molecules if the flow of water through the membrane is fast enough.

In CRRT this property is maximized by using replacement fluids.

Replacement fluids are crystalloid fluids administered at a fast rate just before or just after the blood enters the filter. The increased fluid flow rate across the filter allows more molecules to be carried through to the other side. To better understand this phenomenon, think of a quiet stream as compared to a raging river. The stream could never shift a boulder, but the powerful raging river could easily drag a boulder downstream. This is what is convection; the faster the flow through the membrane, the larger the molecules that can be transported replacement fluids are used to increase the amount of convective solute removal in CRRT. remember... that despite their name, replacement fluids do not replace anything.

Many professionals new to CRRT mistakenly believe that if replacement fluids are added to the therapy, fluid removal rates are decreased or eliminated. This is not the case....fluid removal rates are calculated independently of replacement fluid rates. The decision to infuse replacement fluids before or after the filter is made by the physician. Replacement fluids administered pre-filter reduce filter clotting and can be administered at faster rates (driving higher convection) than fluids administered post-filter.

The downside of pre-filter replacement fluids is that they invalidate post-filter lab draws; the lab results will show the composition of the replacement fluid rather than that of the efficiency/function of the filter..

Some physicians use labs drawn in this way to gauge the efficiency of the filter, and thus prefer post-filter administration of replacement fluids.

So your thought about convection is essentially right. Pre-filter more convection post filter less convection.

A great ICU resource...icufaq.org

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Thanks Esme. So when replacement fluid is added postfilter, it is essentially the blood flow rate through the filter that is driving convection?

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Specializes in Critical Care, ED, Cath lab, CTPAC,Trauma.

Yes....:)

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