Flow Fraction question

[willowtreewren]

I asked at our clinic (granted some time back) how the flow fraction was determined and got a cryptic answer that there was a formula for it.

Do any of you experts know what the flow fraction means and how it is determined?

Thanks,
Aleta

[petey]

Aleta,
I looked back through all of my paperwork to see if I could find the "expert" answer to your question (as one of my IHD friends told me recently that I think I'm an expert on everything!  ), but I couldn't find the answer.

So, I'll tell you what I know (though you have to understand that this is not the official answer -- just our experience).  I, too, asked when we first started home hemo training about how the FF (flow fraction) was determined.  Like you, I got some crazy-ass, round-about, didn't-make-sense answer (Marvin's home hemo nurse is an EXPERT at this type of answer).  Originally, the doctor set Marvin's FF at 30, and we went with that for the first year and a half on home hemo.

About six months ago at Marvin's monthly check-up, the nurse said that Marvin's labs were so good the doctor would up Marvin's FF to 35.  He told us to use that, and we did for a couple of weeks.  It cut Marvin's time on the machine by anywhere from 10-20 minutes (depending on how much we set the machine to take off -- UF) per treatment.  But, Marvin said he didn't think he was getting a "clean" enough run at the 35 FF; he said he could tell the difference.  So, we notified the nurse that we would be going back to a 30 FF (which, by the way, the nurse totally didn't understand our reasoning for dropping back  ).  Finally, Marvin just said, "I've been having hemo treatments for 14 years, and I can tell when I'm not getting a clean enough run.  The higher FF is decreasing the quality of my treatments."  Since we went back to 30 FF, Marvin said he has been getting "clean" runs.

I think  (NOTICE the "think" here) that the flow fraction controls how quickly the dialysate passes the blood in the filter.  The higher the flow fraction, the faster they pass each other and the less toxins the dialysate pulls off the blood.  Of course, the two fluids never mix, but they do pass each other in the hair-like filters (Marvin cut open a filter to see what it looks like on the inside -- millions and millions of super-thin strings!  it's really quite wild to see and examine the inside of one).  Anyway, I think (NOTICE I said "think" again) that the doctors start at a "standard" FF (maybe for body size?) and then see how that works.  If it appears that clearances are not good enough, they drop the FF.  If things look super good in the labs, they increase the FF.

What's Carl's FF?

Marsha

[Meinuk]

No one should be intimidated by this - even the staff don't really understand it (they either go by a patient's weight or a chart in a book and then set the machine). 

The formula is this:

The Flow Fraction (FF) is defined as sum of dialysate flow (Qd) and ultrafiltration
rate (UFR) divided by blood flow (Qb):

Flow Fraction (FF) = Qd + UFR

Qb
Target FF = 35% (range 25% - 40%)

These numbers - even KT/v  - are just numbers.

KT/v is flawed, and a terrible indicator of dialysis efficiency.  Bill has written a lot about it, he is good at math (I am not) http://www.billpeckham.com/from_the_sharp_end_of_the/2008/01/optimal-dialysi.html. 

The FF will be calculated on your size, and your prescription (how long your runs will be/dialysate volume and frequency). Dialysis staff are trained to do the initial calculations and set the machine.  If you are not getting good results (not feeling better than in center) the it is time to explore what can be changed.  More time on the machine?  Or changing your FF. 

I hope that Peter or Bill will post about this.  They have both done calculations to get optimal dialysis from NxStage.  I did a year at FF32, and I am 5'10, 220 (yeah, I know, linebacker material).

The most important take away is: Longer and slower is better..  and doing it at home is BEST.

[willowtreewren]

Thanks both of you. On the NxStage users forum I got a really good answer AND a link to a good write up. (Petey, I'll send it to you).

Carl's FF is 30. According to this NxStage paper, the higher the FF the faster the dialysate and blood pass each other. That is exactly why Marvin didn't feel like he was getting enough treatment.

I just really like to understand everything. Anna, your description is just what the paper said.

Knowledge is power. 

Aleta

[petey]

I thank you, Meinuk, for the official explanation (see, dw, there are people on here smarter than I am          and I can admit it).

The most important take away is: Longer and slower is better..  and doing it at home is BEST.

I agree!  I agree!  I agree! -- wait a minute, we are still talking about dialysis, aren't we?             

I did a year at FF32, and I am 5'10, 220 (yeah, I know, linebacker material).

Move over on the line, Meinuk!  Marvin's 6 feet tall and weighs 265 pounds.

[petey]

Please, please send me the link, Aleta!  I want to know everything, and I love to read!

[willowtreewren]

Hi, Petey,
It is already on its way to you!

 

[Meinuk]

Petey, don't you DARE! I may not post a lot, but your relationship with Marvin (and the dogs) has been a GREAT influence on me.  You both have self educated about home hemo, you keep sane, healthy and productive... (I'll never forget Marvin trying to volunteer at a unit).

You both are an amazing example of how to make it work.

And yeah, I was getting dirty with the longer and slower...  what can I say, I miss Susie (Goofynina) she made everythig dirty and I loved her for it!!

[petey]

Aleta,
Thanks for the link!  I got it and read it and then re-read it.  I didn't have that piece of information anywhere in our stacks and stacks of paperwork about NxStage.  It really explains a lot, and it gave answers to several questions I've had (but couldn't get a straight answer from Marvin's HHD nurse -- maybe I should forward it on to him?).  Thanks for sharing it with me!

Meinuk -- Don't worry about me.  I was teasing one of my IHD friends a little in this thread, too.  I know I'm smart (and my friend does, too  ), but I really do realize that I don't know everything (contrary to popular belief).  But, when you put us all together, we probably do know it alll (this includes my other friend)!  I wish it weren't so, but most of what I know comes from experience (i.e., I wish I didn't have to experience it to know it).

Longer, and slower, and at home ... I think that will be my new motto (and you'll have to GUESS if I'm really talking about dialysis  ).

[Hemodoc]

Great discussion on FF.  I have taken a look at this issue from another perspective.  That of getting the NxStage machine to its maximuum per session utility.  I started out in training with 20L at 35% FF and then went home on 30L and 35% FF for 5 four hour sessions/week.  I truly have not felt that this was enough dialysis.

So yesterday, I had my monthly meeting and asked for permission to max out this little machine.  Thus, last night, I did a 40L run with F of 45% to keep my time to 4 hours like I did with the 30L.  It is the first time that I have felt like I got a good cleaning on the NxStage. 

I understood before I started the NxStage that I would need more than usual since I still have a fair amount of the muscle mass I had when I was a little bit of an athlete.  My clearance on 20L was 0.5, and on 30L 0.77.  I am hoping that the labs I do in a couple of days will show a clearance over 1.0.  That is my goal for NxStage at home.

The issue of optimal FF should be predicated on the theory of how low flow dialysate works.  The theory is that there is 100% capture of all dialysate clearance below a certain threshold.  I must confess that the NxStage does appear to obtain this effeciency althouth no system works at 100%.  Where is that threshold precisely is really what this thread is discussing.  I took a look at the flow curves and hypothesized that we could push the dialysate flow rate to about 175ml/min without loosing that efficiency.  Take a look at a post I did on this issue on DSEN.

http://www.billpeckham.com/from_the_sharp_end_of_the/2009/01/nxstage-dialysate-flow-rates.html

For my experiment last night, with 40L at 4 hours with 45%FF, the dialysate flow rate works out to about 167 ml/min.  I understand that this is counter to the usual discussions on NxStage FF, but I feel the best I have since starting NxStage.  Can't wait to see what happens over the rest of the week.  So, there are different ways of looking at this.  Stepping back and looking at it by calculating the individual dialysate flow rate for your prescription makes the most sense to me in comparing the FF differences.  In my opinion, the max FF is one that will take you up to the 175 ml/min rate and not exceed it.  Rates higher than 175 ml/min will start to bring down the efficiency of the dialysate.

So, is the total amount of dialysate the most important factor or the rate at which it is given? My opinion is that totol dialysate volume is the most important factor and getting that volume in at a rate that does not exceed 175 ml/min is how I am approaching this issue for my own treatments.  Thankfully, I have an excellent nephrologist who is going along with my requests for an unusual NxStage prescription.  I think that I have found my thresholds after a two month experiment.

The take home message is as in all medical issues, you must fit the treatment to each patient and find with titration of dosage what works the best for that individual.  Thus, any FF that keeps dialysate flow rates below 175 ml/min will keep you in the theoretical 100% dialysate clearance that NxStage is based upon. Thus, I don't believe that there is going to be just one ideal FF.  It will be a range with different patient preferences with in that range.

[willowtreewren]

Thanks for your input, Peter.

We do 20 liters/tx, but in the next couple of weeks I'll be coming up with 2 days of treatment and 20 liters left over from the SAK. Being one who hates waste, I was looking for options in how to use that extra dialysate without having really, really long sessions. I don't want to raise the BFR.

This gives me good information to consider.

Aleta

[silverhead]

You can run out the Dialysate in the Pureflow, and finish the session with a couple of bags, just watch the D use and then switch over to the pre-hung bags, only takes a few seconds. Have done it several times.....

[willowtreewren]

Great tip, Silverhead.

But we will have two days of dialysis from the 60 liter SAK. That will leave 20 liters that won't last until our next treatment. We could, of course use bags those two days, but I was thinking of upping the treatment to 25 liters those two days.

That way I'll only have 10 liters left over.

Aleta

[silverhead]

We run 25 all the time, so have to deal with pumping out 10 every other day, most times I pump it, if in a hurry, I just pull the bag out and put it in the bathtub and cut a bottom corner and empty it, do not understand why they do not make a 50 liter sak for type 2 dialysate.....
Tom

[alrightstill]

Willow - I know I don't post a lot.. but i read a lot.

I just had to laugh a little because that was the same exact answer my neph gave me back in 2004 when I started NxStage.. "theres a formula"..

I've been at a FF of 28% since then.. my labs have been great. 

[tyefly]

So yesterday, I had my monthly meeting and asked for permission to max out this little machine.  Thus, last night, I did a 40L run with F of 45% to keep my time to 4 hours like I did with the 30L.  It is the first time that I have felt like I got a good cleaning on the NxStage. 



For my experiment last night, with 40L at 4 hours with 45%FF, the dialysate flow rate works out to about 167 ml/min.  I understand that this is counter to the usual discussions on NxStage FF, but I feel the best I have since starting NxStage.  Can't wait to see what happens over the rest of the week.  So, there are different ways of looking at this.  Stepping back and looking at it by calculating the individual dialysate flow rate for your prescription makes the most sense to me in comparing the FF differences.  In my opinion, the max FF is one that will take you up to the 175 ml/min rate and not exceed it.  Rates higher than 175 ml/min will start to bring down the efficiency of the dialysate.

I wanted to follow up with Peter    aka  Hemodoc  and find out how things have progressed using this new prescription.....  do you have new lab numbers.....   Clearence......    let us know......

[Hemodoc]

I will update next week after I do the labs on Monday hopefully.  I feel quite well with the 40L.  Hopefully we will get around 1.0 clearance, up from 0.77 on 30L but being Monday after a day off, those days are a little lower usually with the clearance.  Take home message is that I did not feel like I was getting good dialysis at 30L and I do at 40L.  In my opinion, the Achiles heel of NxStage is the low clearance.  It looks like increasing the dosage is a viable option to consider with your health care team.  As a national rep from NxStage told me, they don't recommend the 45% FF.  So, we will see what the numbers say, but the patient says this was a great change for the better.  In medicine, the numbers are important, but seeing how the patient reacts is always the most important factor.

[tyefly]

Peter.....  its been too long  and  we all want to know how you are doing with your clearances with the nxstage....   Have you made any other changes.....   keep us up to date.....  we are depending on your expertise........

           Kathy

[Hemodoc]

I am still on 40L at FF of 45% for a 4 hour or there about run.  My machine got air in the dialysate line at 4 minutes to go, so I couldn't do my post.  They are sending me a new set of tubes to do it in a couple of days.  My last clearance was 0.88 on 40L.  I still have about 20 pounds to get back to my personal ideal weight which may give me just a little bit more of an edge if I can get back on the bandwagon again with my diet and exercise.

[Zach]

I am still on 40L at FF of 45% for a 4 hour or there about run.

Any idea how the NxStage filter compares with the Optifux 200 (or 250) when it comes to middle molecule clearances?

 

[Hemodoc]

For anyone that has looked into NxStage for information, it is quite difficult to obtain.  Interestingly, the only place that I have seen a comparison to the 200 is from the NxStage dialysis cartridges box.  Don't have any right now, but I will get back later.  In their handout, the NxStage gets about 2/3s the clearance of the 200 but they are comparing the 200 at a lower level of blood flow than usual treatments.

For myself, at 20L, my clearance was 0.5, 30L - 0.77, 40L - 0.88 with NxStage.  I am about 70% of the clearance of my incenter FMC H with 200s.  My goal is to see if I can get to 80 or 85% clearances.

[Zach]

Interesting.

Just relating to Kt/V _urea my clearances from the 180 is 2.0 per treatment.
At some point I'd like my center to conduct a pre and post middle molecule clearance of say p-cresol and beta2-microglobulin.

Note that in the documentation, Fresenius uses Lysozyme (MW 14,30 Da) as a surrogate for MM.

 

[tyefly]

Interesting.

Just relating to Kt/V _urea my clearances from the 180 is 2.0 per treatment.
At some point I'd like my center to conduct a pre and post middle molecule clearance of say p-cresol and beta2-microglobulin.

Note that in the documentation, Fresenius uses Lysozyme (MW 14,30 Da) as a surrogate for MM.

 

Zach......   need clarification for us dummies......   what is lysozyme (MW 14,30,Da) as a surrogate for MM..... what is that statement...   and what does this mean......    and good for you with those clearances......  How are your labs for phos and K.....  do you feel that dialysis is controlling those levels or is it mainly diet for you with the middle molecules......??

Peter  thx for letting us know how you are doing...  I am still a little concerned about the nxstage ablilty to get really good clearances.... how are your other labs.....  phos, K, and others....  are your middle molecules being removed with your treatment...

[Zach]

Zach......   need clarification for us dummies......   what is lysozyme (MW 14,30,Da) as a surrogate for MM..... what is that statement...   and what does this mean......    and good for you with those clearances......  How are your labs for phos and K.....  do you feel that dialysis is controlling those levels or is it mainly diet for you with the middle molecules......??

My clearances for phosphorus (PO₄) and potassium (K) are pretty good:  4.1 and 5.1 respectively.
It's due to a combination of what I eat, always taking my phosphate binders, never skipping a treatment or cutting my time short.

As far as the middle molecules (MM), it's all about the treatment time on hemodialysis ... longer is better.  In fact, I'm trying to convince my dialysis unit to start an in-center nocturnal dialysis program (8 hours x 3 nights/week) ... specifically to increase the middle molecule solute removal.

As far as my knowledge of Lysozyme, it is limited to what wikipedia has written:
http://en.wikipedia.org/wiki/Lysozyme

Fresenius uses Lysozyme because it is a middle molecule (MM) solute similar in size to beta2-microglobulin (12,000 daltons) and other nasty MM.

Middle molecule solutes are removed at different rates depending on the type of dialysis filter.  Generally, Low Flux filters remove beta2-microglobulin less than 10ml/min, whereas High Flux filters remove beta2-microglobulin at a rate greater than 20 ml/min.

Middle molecule solutes are also time dependent in their removal, so the longer the the treatment time, the better clearance you have.  The monthly labs do not measure middle molecule clearances.  And Kt/V is only a measurement of urea, a very small molecule which is quickly removed.  Perhaps we should have a Kt/V that measures beta2-microglobulin or another middle molecule solute.

Potassium is another small molecule solute which clears quite quickly.  Phosphorus, while also a small molecule solute, acts like a middle molecule in that it's clearance is time dependent.

Here is some information on the topic:

Fluid and solute removal: How and why—parts one and two
By John Agar, MD
Barwon Health, Geelong, Australia

http://www.homedialysis.org/resources/tom/200711/
http://www.homedialysis.org/resources/tom/200712/