Gassy

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When
putting a patient on CRRT, the choice of buffer these days is largely dependent
on whether or not you want to use citrate for anticoagulation. Most of our
patients get either citrate or bicarbonate. However, not so long ago, the main
buffer was lactate. When bicarbonate was first being used as a buffer for CRRT,
it had to be added separately to each bag and, in our institution at least, it
came in a glass bottle next to the CVVH fluid.
More
recently, the bicarbonate comes in a separate compartment of the same
replacement fluid bag and just prior to use, a valve is broken and the bicarbonate-rich
fluid is mixed with the rest. I had previously assumed that this was because
you do not want to mix bicarbonate with calcium because of the risk of
precipitation. However, in our institution, we currently use calcium-free
replacement fluid and so there is no risk of precipitation (the calcium is
given intravenously to the patient based on a sliding scale).
It
turns out that the reason for the separate bicarbonate bags is much more
interesting. Most i.v. fluid bags are gas permeable. Therefore, if you leave bicarbonate
in the bag for a prolonged period of time, CO2 will leach out of the bags. By a
passive process, the bicarbonate in the fluid will then be converted to CO2
which will come out of solution and will in turn leach. You will, in the end be
left with very little bicarbonate in the bags. To get around this, the
manufacturers of dialysate fluids used put the bicarbonate in a separate glass
bottle, This is expensive and cumbersome and is prone to errors if somebody forgets
to add it to the solution. Instead, now the bicarbonate-containing dialysate
fluids are double-bagged. The inner bag contains the solution and is
gas-permeable as before. The outer bag is constructed of a thicker,
non-permeable plastic that keeps the CO2 inside. Also, to reduce diffusion
further, the air between the two bags has a relatively high CO2 concentration. 

1 comment

  1. What machines are being used at your institution? At ours we use NxStage where the bags come premixed with 22 or 32 Meq/L in 5 L bags and we never had this issue presumably due the manufacturers using CO2 impermeable bags! I did hear something about multi-compartment bags in PD fluid though but never in CRRT solutions. Thanks

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