Understanding the NxStage Filtration Fraction

[Hemodoc]

By Peter Laird, MD

Perhaps the single most commonly asked question by patients on home dialysis with the NxStage System One is "what is your FF?" The answers vary greatly without anyone in consensus over what is the best filtration fraction (FF). To answer that question, defining the terms is in order. The filtration fraction is actually a fixed ratio between the blood flow rate (BFR) and the dialysate flow rate (DFR) expressed in a fraction. For example if you have a BFR of 400 ml/min and you set your dialysate flow rate at 35% of that figure, the actual dialysate flow rate is 140 ml/min.  The filtration fraction is thus expressed in this example as 35%.

The NxStage System One is designed on a low dialysate flow rate to theoretically maximize the dialysate at a "100%" efficiency.  Anyone that studies physics understands readily that there is never a system that is 100% efficient. However, looking at dialysate flow curves and levels of urea removal, there are higher and lower levels of efficiency depending on how fast the dialysate flow rate is set.

Evaluating this well done slide by Wellbound, it is evident that at the right side of the curve, the efficiencies plateau so that increasing the dialysate flow rate does not increase urea clearances. Likewise, on the left side of the curve, as the flow rate is increased, there is a corresponding degree of increased urea clearance. The NxStage System One utilizes this increased efficiency of solute clearance for their low dialysate volumes compared to in-center machines. The line begins to diverge from highly efficient to lower efficiency around 200 ml/min dialysate flow rates. It is thus reasonable to set a maximum dialysate flow rate of 200 ml/min as the upper boundary.

I personally utilized this information in the design of my own NxStage short daily dialysis program where I started not with the FF or filtration fraction but with the dialysate flow rate directly.   Keeping the goal of less than 200 ml/min in agreement with the underlying goals of the NxStage System One, I took the FF to it's maximum parameters and kept my blood flow rate at a lower level. In many ways, time on the machine per week is the most important factor since phosphorus (PO4) and many of the middle molecules such as B-2 microglobulin are time dependent unlike urea which easily cleared quickly.

Fitting the NxStage System One into your daily life is in large part predicated on the amount of time per session and how many days a week it is completed. Many papers over the last two decades show conclusively that the longer per session a patient stays on the machine, the better the outcomes, short daily dialysis can be too short. Discussing all of the factors involved in deciding what your "best FF" will be is determined by your goals of therapy, time allowed to these treatments as well as the configuration of the NxStage System One. Finding the optimal FF is thus a very personal quest in conjunction with the recommendations of your medical team taking into consideration how the NxStage System One dialysate flow rates are designed.

http://www.hemodoc.com/2011/04/understanding-the-nxstage-filtration-fraction.html

[tyefly]

thx for the info...   Since time is the most important factor since phosphorus  and many of the middle molecules such as B-2 microglobulin are time dependent unlike urea which easily cleared quickly.. How much time does it take to reduce both of these... and also  why does urea come off so quickly while phosphorus takes longer... why does it take longer.. and how much longer..  is it better to do long dialysis on a daily basis or  shorter dialysis for more days of the week....  thx  again for the info Peter....

[Hemodoc]

thx for the info...   Since time is the most important factor since phosphorus  and many of the middle molecules such as B-2 microglobulin are time dependent unlike urea which easily cleared quickly.. How much time does it take to reduce both of these... and also  why does urea come off so quickly while phosphorus takes longer... why does it take longer.. and how much longer..  is it better to do long dialysis on a daily basis or  shorter dialysis for more days of the week....  thx  again for the info Peter....

It has to do with how large the molecules are, urea is a very small molecule relative to B-2 microglobulin.  PO4 acts like a middle molecule in its clearances.  In addition, these molecules reside in different "compartments." These compartments clear at different rates and the larger the molecule the longer it takes to clear.  That is why nocturnal dialysis on a daily basis is getting the best results.  Once again, the more dialysis you do, the better you clear these molecules.

[boswife]

This all interests me so much but i do have a hard time grasping it.  Hubby just got his tx changed and as far as how he feels and how his results came out (after 3 weeks on new tx) it looks great.  So, if blood results stay good even though it went from  [23 dialysate, 400 bfr, 35ff, and 2:59 hr],    to   [ 20 dialytate, 390bfr, 320 ff at 2:56 hour ]?  Im planning on slowing his bfr to 380 or less just to get him at least 3 hours, but was also wondering if that will empty that dialysate sak any sooner?