Thursday, December 11, 2008

Plasma Exchange Nuts & Bolts

There are many indications for plasma exchange in nephrology:  some common ones include antibody-mediated rejection of a kidney transplant, Goodpasture's, ANCA-associated vasculitis, and TTP--all of which are commonly associated with loss of renal fellows' sleep--that is, you may very well be called upon to start plasma exchange in the middle of the night for one of these conditions.

There are two basic ways to achieve plasma exchange:  the Blood Bank Way (in which plasma is separated from blood cells using a centrifuge) and the Nephrology Way (in which a membrane plasma separator cartridge is installed within a dialysis machine circuit).  They both work, and which one gets the call appears to be highly center-specific.

Here's a quick-and-dirty guide to writing a plasma exchange order, realizing that specific circumstances may well alter what the final order will be:

1.  Estimate plasma volume (=.065 x weight in kg) x (1-Hct).
2.  Decide how many plasma volumes to remove (we typically use 1.5 plasma volumes, which ends up being between 40-60 cc/kg).
3.  Specify what the replacement solution will be:  you need to replace the plasma volume amount removed in a 1:1 fashion; a good default is using 2/3 volume of 5% albumin and 1/3 volume normal saline.  For example, if you remove 4 liters total, you could replace with 2.66 liters of albumin & 1.33 liters of normal saline.  You may have to use some FFP as well (for example, if the coags are elevated or you are doing plasma exchange for TTP).
4.  Specify calcium replacement:  usually 4-6 grams.
5.  Specify anticoagulation:  you can certainly try going it heparin-free, though in my limited experience clotting occurs more frequently than in dialysis.  A 2000 unit heparin bolus, if tolerated, is not a bad idea.

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