Pre-filter versus post-filter replacement fluid in CVVH/CVVHDF

[DRCCRN]

Am I correct in my understanding that Pre-filter CRRT replacement fluid is used to prevent too much hemo-concentration that promotes blood clotting, also creating a pressure positive gradient, promoting ultrafiltration/convection; and post filter replacement fluid increases a negative fluid pressure gradient across the filter, promoting convection and solvent drag? Does someone have a better explanation why one uses both pre and post filter replacement fluid?

[Esme12, ASN, BSN, RN]

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

[Chisca, RN]

Replacement fluids would have to be accounted for (increased) in UF removal rates, if not where do you think that extra fluid is going to go? Also convection is enhanced by increasing the dialysate flow to 1.5x the blood flow. A dialysate flow that is faster than blood flow is going to drag solutes across the membrane when it pulls the water.

Small molecules move faster than larger molecules. Creatinine is a small molecule. If you have 100 creatinine molecules in a fixed space as they pass the dialysis membrane there is only so many times it will bounce across the surface of the artificial kidney to be removed. Now if we dilute those same creatinine molecules with pre-dilution saline and now we have 50 creatinine molecules for the same given surface area of the artificial kidney more creatinine will be removed because their will be a greater opportunity for these molecules to hit the surface of the dialyzer and leave the system.