Friday, August 21, 2009
pre-dilutional versus post-dilutional CVVH
There are two general strategies for CVVH replacement solution entering into the blood circuit: pre-dilution (in which replacement solution is mixed with the blood prior to its entry into the filter), or post-dilution (in which replacement solution is mixed with the blood after it has passed through the filter). Each method has its own advantages and disadvantages. Where I have trained, we always use the pre-dilution method. The theoretical advantage here is that because the blood is diluted prior to entry into the filter, there is a lower risk of clotting the filter, allowing for longer filter life and reduced down-time for the CVVH machine. We often claim that CVVH is a continuous therapy, but due to technical issues it is not uncommon for the therapy to be halted for several hours out of a day; pre-dilution technique may help minimize this. Furthermore, pre-dilution allows for near-limitless ultrafiltration rates, essential when rapid volume removal is necessary. The main cited drawback of the pre-dilution method is that much of the fluid passing through the filter is actually replacement fluid--thereby leading to a reduced efficiency of about 10-15% in clearance when compared to tost post-dilutional method, in which all the fluid moving through the filter is the patient's. Generally this is not a problem given that this is a near-continuous therapy, but proponents of the post-dilution method claim improved efficiency as an advantage. It is important to specify pre- versus post- dilution methods in the interpretation of various publications in CVVH.