Monday, September 10, 2018

Consult rounds: CRRT calculations primer

Two must know concepts in CRRT is clearance and filtration fraction.

How does one calculate clearance in CRRT?
Let’s start with convection first- so mainly CVVH (like you are making coffee) 
Clearance = V (u/p) but here dialysate/plasma eventually equilibrates, so the Clearance =Volume. What is your volume in CVVH? – IT is your total effluent—so it is your Replacement fluid rate + UF rate
If you had post filter replacement- just add replacement fluid rate + UF
If you had pre filter replacement- you need to have a correction factor, because the blood that enters the filter pre filter has a different concentration and gets diluted. The correction factor is an equation that reduced the replacement fluid rate by a certain percentages that includes the blood flow rate.
The formula for the fudge factor is BFR/ BFR+ Replacement fluid rate. An amazing you tube video explaining this concept.

Now let’s do the diffusive clearance (like making tea with a tea bag) - you need to basically add all the rates- DFR+ UFR+ post filter rate. You tube video link for this calculation.
So the Clearance = Diffusive + convective if it’s CVVHD ( Remember to only add UFR once)

Here is an example of a patient getting CVVHDF with order set

BFR=100cc/min or 6000cc/hr
DFR= 1400cc/hr
Pre filter- 800cc/hr
Post filter- 200cc/hr

Let’s do diffusive first.  Clearance = DFR+ UFR = 1400+ 100= 1500cc/hour =25cc/min
Convective = pre filter + post filter +UFR( but we already counted UFR above) so can’t count again.
Also, pre filter needs a correction factor. 
The correction factor is Blood flow/ Blood flow +replacement fluid rate.  So that is 6000/6800= 0.88 for the pre filter part. So here the pre filter part will be 800 *0.88=705.

So the convective clearance = 705+ 200= 905cc/hr =15cc/min
So total prescribed dose gives you a clearance of 25+ 15= 40cc/min

So if this individual was 85kg
It would be 1500+ 905 cc/hr = 2405/85=28 cc/kg/hr
How does one calculate Filtration fraction?(FF)

FF===Effluent flow / blood flow with correction factor

Your effluent will include the DFR+UFR. You want the FF <20% to avoid clotting. So if you increase the numerator, your FF increases or if you decrease the denominator, FF increases. So, basically, you have to keep the blood flow high if you want to increase UFR or DFR to avoid the increased concentration of the plasma and increase clotting risk. Here is the FF YouTube video explanation.

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