A New, Dialysis Fluid Restriction Calculator

This blog post was made by Dr. John Agar on July 12th, 2018.
A New, Dialysis Fluid Restriction Calculator

Over the past few years, the good dialysis pendulum has finally—if a little belatedly—swung away from a singular focus on solute clearance (likely fixed on the wrong solute) and towards understanding volume and the ways to achieve optimum volume management.

I have written a number of blogs like this one (https://www.homedialysis.org/news-and-research/blog/113-how-much-can-i-drink-doc) for KidneyViews on the central importance of volume in both the intravascular and extravascular spaces, and the absolute dialytic imperative to modify it gently, and slowly. Several litres of fluid simply cannot be removed in a hurry. Short, sharp, high UF dialysis lays a rapid path to circulatory instability and organ stun: the heart, the brain, the gut, the muscles, the residual kidneys…all and any are threatened by the rapid fall in perfusion (blood flow) and tissue oxygenation that accompanies the sudden reduction in blood volume and it’s surrogate measure, blood pressure, that brutal dialysis brings.

In the dialysis hours—and they should be long, not short—the dialysis prescription holds the key to slower volume change. This is primarily achieved through a combination of longer session duration and higher session frequency. Both matter. Each compliments the other. Each needs the other.

But there is more than one way to skin a cat! What is done inside the hours of dialysis is largely ours to control, but outside dialysis sessions, the patient is, to all intents and purposes, on his/her own. In the 12 - 15 hours of conventional centre-based care, the machine and the prescription have control, but in the 143-146 hours/week where the patient is away from the dialysis unit, the patient is in sole control. The treating team has little or no control over that vital period beyond giving the usual post-dialysis exhortations to limit fluid intake, limit sodium, limit “wet” foods (like watermelon), etc. But, when it comes to the encouragement to “limit fluid intake,” it begs the question: “By how much?” Here, we flounder …

It is clear that the rate of fluid removal—the ultrafiltration rate (UFR)—matters, and matters deeply, though there remains disagreement over what that rate should be:

  • Charles Chazot (Tassin, France) has data to show that perfusion injury begins to occur as the UFR rises beyond 6-7ml/kg/hr.1

  • In our own service in Geelong, Australia, we now aim to keep, and we largely achieve, a cross-service UFR of <8ml/kg/hr, although I confess to suggesting a UFR maximum of 10ml/kg/hr only 2-3 years ago.2

  • In 2010, the Centers for Medicare and Medicaid Services (CMS) proposed a UFR of <15 ml/kg/hr as the fluid removal rate target,3, though this was revised downwards to a recommendation of <13 ml/kg/hr in 2016.4

  • In the US, the currently recommended target UFR is <13ml/kg/hr—despite that Jennifer Flythe showed compelling but imperfect data in 2011 from a re-working of the HEMO trial that suggested a rapid ramping of all-cause mortality and cardiovascular risk occurred as the UFR rose above 10ml/kg/hr.5

As yet there is no international agreement on which rate (or which “recommendation”) is correct, but Chris McIntyre’s group has clearly demonstrated that the faster fluid is removed, the greater the risk of tissue misadventure through hypo-perfusion and oxygen deprivation.6 Though clearly more work is needed in this space, I will let the reader reach his/her own conclusion about where upper limit for the rate of ultrafiltration should likely lie. One thing is certain: the duration of the dialysis session is the only rational way to control the rate of fluid removal, and slower means longer.

Just as clearly, the volume of fluid that must be removed, estimated by its surrogate, interdialytic weight (IDWG), is also critical. Drink more, gain more, and there is more to remove! In turn, this can only translate into faster removal, or a longer dialysis session. Without exception, the latter is the better option, though sadly and all too often, it is the removal rate that is ramped up within a strangely immutable short session time rather an obvious—but avoided—session extension.

Mel Hodge wrote an erudite blog on the “broken” US for-profit dialysis business model (https://www.homedialysis.org/news-and-research/blog/117-it-s-past-time-to-destroy-the-dialysis-business-model) suggesting that if dialysis were reimbursed by the hour, longer dialysis sessions would immediately result—to the benefit of the patient, rather than the provider. Though clearly the best solution I have ever seen for the unusually short dialysis practices of US providers, as yet there seems little appetite for change.

The “methods” taken to limit interdialytic weight gain are as limited as is their success. Salt restriction in all, and sugar control in diabetics, are our cornerstone instructions and chief weapons against excessive inter-dialytic fluid gains, and a high subsequent UFR. In sequence, then, a high UFR leads to on-dialysis cramps, nausea, vomiting, crashing BPs and “flats,” post-dialysis torpor, and a raging post-dialysis thirst. The dried-to-a-crisp patient is left to limp home after dialysis to hoe into pickles and salt—regardless of anything we might advise to the contrary. While we plead: “drink less fluid,” there is no science, and no formula, to guide a reasoned level of fluid intake.

And so to the point of this blog:

…I would love a buck for every time I have heard a dialysis patient ask, “So, how much can I drink, John?

We should know the answer, but, oddly, we still do not seem to have ever thought through a sensible answer to this simple question. Too often a response is avoided, or prompts a recommendation like, “500 ml + urine output.” The literature is full of this poorly advised advice; an oddly thoughtless instruction that pays no heed to patient size, age, gender, nutritional status, or dialysis prescription. 500 ml for a 45 kg elderly lady is not the same as 500 ml in a 120 kg teen, irrespective of urine output!

But, all of the necessary parameters to predict and advise a safe, individualized fluid restriction, and a safe ultrafiltration rate during the next dialysis are known! Work though this simple exercise to establish an individualized fluid restriction (FR):

  • IDWG – interdialytic weight gain can be measured. It is body weight prior to the upcoming dialysis minus the weight at the end of the last dialysis.

  • UO – the average daily urine output can be measured.

  • DI – the dialytic interval is known: it is the duration from the end of one dialysis to the intended start of the next (commonly 1-3 days).

  • SD – the session duration (SD) is known. It is the intended, prescribed duration of the upcoming dialysis treatment (most commonly between 3 and 5 hours).

  • TW – the target weight is known (or should be) … it is the weight the nephrologist has advised should be the target to achieve by the end of dialysis.

Using these simple measures, an equally simple formula can be devised that individually advises the required fluid restriction, treatment by treatment, that will ensure that the next dialysis session will achieve any desired ultrafiltration rate, knowing the interval to the next treatment and its’ intended duration.

For those interested in the mathematics, here is the formula:

Fact 1
IDWG in Kg = [FR in mL – UO in m:] x DI in days.
IDWG = [FR - UO] x DI
Fact 2
As IDWG in Kg = the desired upcoming UFR mL/Kg/Hr x SD in hours x TW in Kg,
then, IDWG = UFR x SD x TW
Fact 3
As each formula equates to IDWG, one must equal the other.
[FR - UO] x DI = UFR x SD x TW
Final Formula
FR = [(UFR x SD x TW) ÷ DI] + UO.

This formula can easily be converted into a simple calculator to advise interdialytic fluid restriction, using the following data set by the physician:

  • Target UFR

  • Session duration

  • Target weight (“dry weight”)

In addition:

  • Dialysis interval is commonly determined by the dialysis roster.

  • Interdialytic urine output is a volume that should probably be re-measured once every couple of months, as it tends to slowly decline in most patients over time. This is the only required patient input.

This formula then informs an answer to the question, “How much can I drink, Doc?” backed by some simple science and allowing patient-centred individuality. This seems a better choice than either guessing, or even avoiding the issue completely. Thus, it finally becomes possible to achieve the key outcome: a desired, safe, and predictable UFR for every dialysis.

While FR = (UFR x SD x TD) ÷ DI + UO has not yet been validated or tested against large populations, a requisite step to gain wide acceptance, individual dialysis services are encouraged to play with it in parallel with their other fluid restriction practices and see if it works and helps.

CAVEAT: Until the formula is validated and any populations where it may not be reliable have been identified, it should only be used as a parallel aid, not an infallible predictor. It may not be reliable in children, in low weight adults, or in those who are very heavy. The limitations of the formula—and there will undoubtedly be some—need to be fully understood. But, trial use in our own patient population suggests that for most, it works well.

The Medical Education Institute (MEI) through its Home Dialysis Central site has built the formula into an open access Dialysis Fluid Restriction Calculator.

Use it here https://connect.calcapp.net/?app=2adoxr#/ or install it to your homescreen on your mobile device to use offline!

MEI already published the Barwon Health UFR calculator we developed in Geelong, Australia in 2014 [see: https://www.homedialysis.org/ufr-calculator], a resource that many patients, especially those at home, find useful. Let us know what you think of our Dialysis Fluid Restriction Calculator. You may even like to suggest a better name for it!

Summary

To date, only two options exist to control the UFR maximum during any one treatment:

(1) Extension of session time to restrict the UF rate

(2) Limitation of IDWG to reduce the need for an excessive UF rate

While longer dialysis sessions remain the best and optimum solution for excessive interdialytic weight gains, the introduction of variable extensions of session time would be, at least for most services, logistically complex. As a result, a simple formula that individualizes the interdialytic fluid intake for each interdialytic period is an attractive other choice.

A simple calculator developed at Barwon Health, Geelong, Australia, and translated into an Internet resource by MEI, Madison, Wisconsin, may help answer the age-old dialysis patient question, “How much can I drink, Doc?”

For centre-based patients, dialysis nurses will be able to work with each individual at the end of each treatment to advise a more structured fluid restriction plan, knowing the interval till the next, the duration of the next, the target weight for the next, and the usual interval urine volume before the next dialysis treatment. This should ensure a volume-safe dialysis the next time round, every time round, assuming a sensible UFR maximum has been set.

For home patients, self-entry/self-selection of the same data set into the calculator should permit a fluid intake plan to evolve.

If IDWG and fluid restriction can be better advised, it should be possible to prevent the frequent need for excessive UF rates during dialysis, and the risk(s) of organ hypo-perfusion and “stun” should thus be lessened.

References

  1. Chazot C et al. Even a Moderate Fluid Removal Rate during Individualised Haemodialysis Session Times Is Associated with Decreased Patient Survival. Blood Purification. 2017: 44. 89-97. See: https://www.karger.com/Article/Pdf/464346. -

  2. Agar JWM. Personal Viewpoint: Limiting maximum ultrafiltration rate as a potential new measure of dialysis adequacy. Hemodialysis International. 2016. 20(1). 15-21. -

  3. Arbor Research Collaborative for Health and University of Michigan Kidney Epidemiology and Cost Center: Clinical and Data Technical Expert Panel Meetings Synthesis Report, 2010.  https://www.cms.gov/Medicare/End-Stage-Renal-Disease/CPMProject/Downloads/ESRD2010TechnicalExpertPanelReport.pdf. -

  4. National Quality Forum #2700: Ultrafiltration rate greater than 13 ml/kg/hr.  http://www.qualityforum.org/ProjectTemplateDownload.aspx?SubmissionID=2700. Accessed May 31, 2016 -

  5. Flythe JE et al. Rapid fluid removal during dialysis is associated with cardiovascular morbidity and mortality. Kidney International 79(2). 250-257 -

  6. McIntyre CW. Recurrent circulatory stress: The dark side of dialysis. Seminars in Dialysis. 2010; 23:449–451 -

Comments

  • Pablo Molina

    Aug 18, 5:24 AM

    Excellent post Dr Agar. Congrats!

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    • John Agar

      Aug 20, 11:06 PM

      Thank you Pablo.
      I am glad it was useful.
      I just hope more read it.

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  • Matt

    Aug 15, 12:22 PM

    I'm new to dialysis (California) and only trying out the UF calculator (not this new one that you're talking about) to compute the rate the clinic I'm at is pulling off my fluid. It's always way over 13 ml/kg/hr. Why does your UF calculator use the weight before treatment instead of the "target weight" or dry weight? When I read the journal article (Flythe, et al.,), they use target weight to calculate UF not the weight before treatment.

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    • john agar

      Aug 15, 11:39 PM

      Dialysis 'pulls fluid' - to use a US shorthand term for ultrafiltration - from the body weight that you record when you stand on the scales at the start of a dialysis session ... in other words, your pre-dialysis weight.

      Your dialysis treatment will seek to achieve (reach) a desired and commonly physician-set 'target weight' at the end of the run ... the so-called 'dry weight'.

      But, if you read my blog that explains the various terms used for weight in dialysis... see: https://homedialysis.org/news-and-research/blog/201-A-Primer-on-Haemodialysis-%22Weight%22 ... you will soon realise that 'dry weight' is a notional state while target weight is an actual weight - it is reality versus notion.

      Ideally, your 'target weight' SHOULD be the same as 'dry weight', but in fact, it rarely is. We are very poor at getting target weight to accurately equate to dry weight! In addition, weight changes and considerable changes can occur in in body weight within the 2-3 month physician assessment at clinic that 'sets' the target weight. Indeed, we encourage our nurses to routinely reassess and adjust target weight between clinic visits!

      Your physician (or nurse) may try to determine a target that he/she THINKS is where your dry weight SHOULD be, but commonly this is a fair way off, and many maintenance dialysis patients are still some litres in excess of true dry weight. Indeed, almost by definition, any dialysis patient who is taking any anti-hypertensive drug is still overloaded, even if their nominated target weight can be reached. This simply acknowledges that the target weight that has been set for that patient is still too high!

      In short, and as yet, there is NO CLINICAL WAY to rapidly, simply, and accurately assess dry weight, and thus set a target weight that accurately reflects true dry weight.

      If you think about it, at the start of any upcoming dialysis treatment, the INTENTION is to return body weight back to target ('dry') weight by removing during the dialysis session the amount of weight (fluid) that has been gained since the last dialysis ... the inter-dialytic weight gain (IDWG). The aim is to achieve a weight at the END of the treatment that equates to the target weight ... though that target may still be an inaccurate reflection of true 'dry' weight ... see above!

      The difference between the weight at the start of dialysis, and the 'target' weight set by your dialysis physician is equal to the weight of fluid that needs to be removed. As I litre weight 1 kilogram, the difference between the pre-dialysis weight and the intended target weight at the end of dialysis must be the volume (in litres) that needs to be removed.

      But, what counts in dialysis is NOT just the total volume that needs to be removed, but the time there is in the prescribed dialysis schedule to do the removing! The REMOVAL RATE is all-important

      As an example, 'pulling' 2 litres out of the circulation in 2 hours versus 'pulling' 2 litres in 4 hours will mean that fluid will need to be removed in the shorter 2 hour dialysis option at TWICE the rate of the longer 4 hour option.

      The RATE of fluid removal matters ... it matters A LOT!

      Now ... back to your question ... it stands to reason that the pre-dialysis weight is crucial to understanding the STARTING POINT. The target weight is simply a best guess at the FINISHING POINT. The difference is the VOLUME that needs to be removed. The duration of the treatment determines the RATE OF REMOVAL. And, the slower the rate of removal, the safer the dialysis will be, and the less symptomatic it will be.

      There are only THREE options to ensure that the rate of fluid removal is kept as slow as possible.
      (1) have less to remove ... ie: limit the inter-dialytic weight gain through salt and fluid restriction = easy for a professional to say, but so difficult for a patient to achieve
      (2) lengthen the dialysis treatment so that there is longer on-dialysis time to remove whatever volume needs to be removed
      (3) dialyse more frequently ... though this latter option only works well if combined with (2) ... hence my preference to encourage, cajole, and educate to home dialysis - or better, home nocturnal dialysis.

      Motto: Drink less, or dialyse longer and more often.

      I hope that answers your question, Matt.

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  • John Agar

    Jul 14, 6:41 PM

    Regarding my reference to Mel Hodge in the text, Mel has just correctly reminded me of his rather more erudite, but similar-themed paper Re his brlliant disruptor of the status-quo idea ... a by-the-hour payment scheme!

    It is ... ‘From “Adequate” to Optimal Dialysis—By Adding a Single Word. Am J Kidney Disease. 2017:69(3):334-336’.

    But, until the unlikely time that such a bold solution were to be adopted, SOME way to help patients and staff put ‘design’ and ‘certainty’ into planning individualised inter-dialytic fluid limitation is needed. This Fluid Restriction Calculator is our contribution from Barwon Health. I can only hope some see it, test it, use it, critique it, improve it ... for it would be sad if it Is not - as we Australians say - ‘given a red hot go’.

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  • Mel Hodge

    Jul 14, 12:58 PM

    1. While there may be cross-border differences among nephrologists about the safe UFR limit, there are no cross-border differences in patient physiology. Since you are writing to many Americans it is dangerous to leave those patient-readers to resolve an issue that nephrologists haven’t resolved… and just plug in 13. Instead, I would tell them that they should not go above 5 ml/kg/hour without a written prescription from their nephrologist… and that if they were your patient you would set that limit at 8. -- as you pointed out that you do for your Australian patients. American readers should take your article to their nephrologist to get their prescription (and make their nephrologist read it and think). Discovering the safe UFR limit for you is like walking near the edge of a cliff in the dark -- you don't know where it is -- until you experience the consequences of falling off...

    2. As you noted, always keep in mind that most dialysis in the United States is a for-profit business and all providers are paid by the treatment, not by the hour, which creates an inescapable conflict of interest between their dialysis provider and them as a patient on certain issue -- especially the question of a safe maximum UFR... where for providers short=$$$

    3. I would add that many patients can largely stop worrying about their limit on fluid intake and drink whatever they want within reason by going to daily dialysis — certainly daily nocturnal dialysis (they should verify this for themselves with your formula).

    4. Similarly, enough dialysis should mostly eliminate the need for phosphate binders (I haven’t tried to figure out how the dialysis time required for a safe UF rate compares with the time required to get rid of enough phosphorus to eliminate binders compares… but in my view the longer of the two should control session length).

    5. For anyone teetering on the edge of a decision about whether to undertake home dialysis, these are more powerful reasons to go for it.

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  • Colin MackY

    Jul 13, 1:10 PM

    An excellent post as always, Dr Agar, I hope it gets the attention it deserves.

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  • Linda McKenney

    Jul 13, 9:32 AM

    I still have enough residual function that I don't really have a fluid restriction, but plugging my info into the calculator shows a very reasonable suggested restriction. I'll keep it in mind as my urine output decreases.

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    • John Agar

      Jul 13, 6:24 PM

      Remember the ENTIRE aim is to predict and achieve the next treatment UFR! Of course, I do not know what the values you have used are, but in my book, it would be UNreasonable if you were using a target UFR >13, or even >10. In my view, and as the blog reads, it ought to be ≤8. So, if anyone plugs in their numbers but are targeting a UFR that is too high, they have failed to grasp the critical nature of the UFR. Always, always, always strive lower .... always. Use the calculator to help you see how.

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