Tuesday, March 29, 2011

CONSULT ROUNDS: A primer on CVVHDF orders

How does one calculate the prescribed dose on CVVHDF?
Here is an order set:
BFR 150cc/min
DFR  1500cc/hr
Pre pump flow 700cc/hr
Post pump flow 400cc/hr
Ultrafiltration goal 100cc/hr

Our formula for estimated GFR is uv/p ( v=volume, u= urine crt and p= plasma crt). In this case, would be dialysate crt and plasma crt.  We have to make one assumption:- since the rate is so slow, the equilibration of dialysate crt and plasma crt would cancel out and your GFR = Volume.

So then, your diffusive clearance would be your DFR, which in this case would be 1500/60 or 25cc/min
your convective clearance would be your pre +post + UF you ordered= 1200/60 or 20cc/min
Hence, you are getting this person 45cc/min of prescribed GFR. Doesn't mean that is the DELIVERED dose.
One more aspect that might be missing is the residual urine output which might be good and might only add to the GFR.

Just an interesting exercise that we should do all time we do CVVHDF orders.
Also, this helps pharmacy guide your medication prescriptions

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