The Mathematics of Dialysis vs. Two Normal Kidneys

This blog post was made by Dr. John Agar on December 1st, 2016.
The Mathematics of Dialysis vs. Two Normal Kidneys

Dialysis is a study of contradictions:

  • On the one hand, how extraordinary; yet on the other, how meager.

  • On the one hand, how bright; on the other, how much a shadow.

  • On one hand, the normally working human kidneys; on the other, our attempts to replace their function.

So, how close does dialysis get to replacing normal kidney function? Let's work through some simple mathematics!

  1. The human heart beats, on average, 70 times a minute. Each beat (or contraction) of the heart ejects, on average, 70 mL of blood. Thus, every minute, 4,900 mL of blood is circulated by the cardiac pump, which makes the cardiac output ~5 litres/minute.

  2. Physiological studies show that the two human kidneys receive (as renal blood flow) between 20% and 25% of cardiac output. Reports vary a little, but are always within that range. Even if we use the lower estimate, i.e. 20%, the two kidneys receive (and filter) a total of ~1 litre of blood/minute.

  3. Therefore, 1 litre x 60 minutes each hour x 24 hours each day x 7 days each week means the kidneys filter just over 10,000 litres of blood/week. Not a bad workload—and don't even try to think how many litres that equals in a normal human lifetime!

How, then, does haemodialysis, the most common and most efficient non-transplant option in renal replacement therapy (RRT), stack up?

To begin, some basic dialysis data are needed before we can understand how dialysis compares to normal kidney function. For these data, I will contrast two regions that, in most respects, are very similar—Australia and New Zealand (ANZ) and the United States (US)—yet where dialysis practice differs quite significantly.

  • In ANZ, most centre-based dialysis services provide 3 treatments a week, the duration of each being about 5 hours.

  • In the US, though the same number of treatments are given/week [3], the duration of each treatment is significantly shorter; commonly ~3.5 hours.

  • In ANZ, the pump that drives blood through the artificial kidney commonly runs at a speed of between 300-350 ml/minute (say 325 ml/minute).

  • In the US, to compensate for the shorter treatment duration, blood pumps are often run at speeds of 400 ml/min, or significantly more.

I will not debate here the pro's and con's of pump speed practice. I have addressed this in a previous blog. Suffice it to say that in my view, a high pump speed does not make for a happy vascular access!

Now, let's do the comparative mathematics.

Take ANZ first:

  • Based on these data, the blood presented weekly to an average dialysis system in centre-based care in ANZ = 325 ml/minute (i.e. 0.325 L) x 60 minutes x 5 hours x 3 treatments/week = 292.5 litres.

Scroll back for a moment and compare: two normal kidneys receive 10,000 litres of blood/week but a dialyser (the artificial kidney) in ANZ gets a measly 300 litres to play with, if it's lucky.

Yet, while that is clearly a stark difference, the comparison in the US is even starker!

  • The average calculation for a centre-based patient in the US is: 400 ml/minute (i.e. 0.4 L) x 60 minutes x 3.5 hours x 3 treatments/week = 252 litres; a low volume by ANZ standards, even after (my view) the higher access-damaging pump speeds used in the US have been factored in.

To compare these differences in another way, in ANZ only 292/10,000 = 2.93% of the blood presented in a week to two normal kidneys is presented for dialysis. In the US, it is even less: a paltry 2.52% (= 252/10,000).

What about home-based dialysis?

  • As a comparison, many home patients, especially in ANZ, perform up to 8-9 hours (mean = 8.5 hours) of overnight, while-asleep dialysis for an average of 5 nights/week. Most who undertake these schedules also run access-friendlier pump speeds of 225-250 ml/minute (mean = 0.2375 L). This provides 0.2375 x 60 x 8.5 x 5 = 605 litres/week for dialysis.

  • In the US, the most commonly used home profile seems to be a 400 ml/minute x 2 hour/treatment x 6 treatments/week program; this is the one that is most often delivered by the NxStage system, a low-flow system that, by its design, offers less efficient solute clearance compared to the ANZ-preferred single-pass systems. Thus, US home patients under this model present 0.4 L x 60 minutes x 2 hours x 6 treatments/week = 288 litres/week to a less efficient system. While this “short daily” home model clearly presents more blood for dialysis per week than does US centre-based practice, it is still less than the average ANZ centre-based volumes and compares even less favourably with the 600+ litres/week that are presented to the more efficient single-pass systems used by many/most ANZ home dialysis patients.

All this goes to show that:

  1. Current dialysis systems are—even at best and wherever they are used—a poor substitute for the “real deal.”

  2. To talk about centre-based dialysis practices as if they are comparable, nation-to-nation, is clearly incorrect.

  3. Similarly, home dialysis practices, country-to-country, are also quite dissimilar, and should not be expected to necessarily have similar efficacy or outcome.

  4. All patients should understand the effect that blood flow rate (pump speed) in litres/minute x minutes/treatment x the number of treatments/week may have on them. By doing their own calculation, patients can begin to explore the macro-influence of time and frequency on their program. Perhaps, then, they may also better see how little their dialysis is affected by pump speed.

  5. This may help explain why my narrative and mantra has always been longer and more frequent, but slower, too.

Now, let's take this a step further, for it is gob-smacking to realise how meager is any/all dialysis, no matter how it might be improved (or diminished) by adjustments to duration, frequency or speed, when it is compared with the amazing power provided by two normal ordinary little kidneys.

Normal kidneys do far, far more than simply filter the vastly greater volume of blood presented to them than that which can presented to a dialysis system - even with the very best combinations of time and frequency. Normal kidneys go much further. They then adjust the filtrate of blood by additional excretion and/or reabsorption within the tubular structures of the kidney... and no dialysis systems beyond those still in the experimental stage yet offer any mimic of tubular function.

And, like the ads on late night TV…”there's more.”

Normal kidneys do other nifty stuff, like making erythropoietin, and converting inactive into active vitamin D; stuff that is far beyond the capability of any current, or any (at least medium-term) future dialysis system; stuff that, in our rudimentary efforts to mimic kidney function, is currently only possible by “giving back” or supplementing in the form of medicines.

Normal kidneys also:

  • Fine-tune sodium balance

  • Manufacture and release renin

  • Manufacture vasodilatory prostaglandins

  • Internally regulate and balance the vasoconstrictive effects of angiotensin

  • Use all these mechanisms (and more) to adjust blood volume and blood pressure…

So, when strolling the lakeside paths of Chicago after the 2016 ASN, I was put in mind of two Shakespearean phrases, both slightly “doctored” in their use here:

"What a piece of work is a man, how noble in reason, how infinite in faculty, in form and movement”...

...a phrase that makes us pause to reflect that, in our endeavours to mimic the human kidney through dialysis, our model, even in its most currently efficient form, is...

"...but a walking shadow; a poor player, that struts and frets its hour upon the stage, and then is heard no more...full of sound and fury, but signifying [little]"…

It behooves us to remember, when next we sit and talk with our patients on dialysis, that the treatment we are providing to “replace” their kidneys is really no replacement at all. “Renal replacement” is a misnomer, when it comes to dialysis techniques.

Certainly we should maximise what and where we can—those options being primarily duration and frequency—but we should also pause and take notice that, while we have taken some small first steps in these past 60+ years, there is oh, such a long way to go...

Comments

  • Gale Schulke

    Dec 9, 3:39 PM

    I never have my NxStage patients do less or the same dialysis as in-center pts. It defeats one of the purposes of Home Hemo. On the average we tend towards 3.5-4 hours 5 days per week which is 420-480 Liters. Those who are doing nocturnal are doing about 672 Liters (3.5 nights x 8 hours).
    I also don't recommend BFR greater than 350. The Nephrologists I work with tend to go with those formulas. I honestly think the short daily philosophy is pretty much gone now, fortunately. In our options teaching, we don't even use those terms. We discuss better quality of life with frequent dialysis, not shorter.

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    • John Agar

      Dec 12, 6:44 PM

      Gail ... we, like you, shoot for low flows (Qb) and longer time. Our centre-based patients (3 x 4.5-5.0 hrs) all run blood flows of 300-325 while our home nocturnals (4-5 x 8-9 hrs) all run blood flows of 225-250. If we used the NxStage (which we don't) I think we would adopt a daytime practice much like yours. With our overnight patients, we would likely shoot for the same duration, but 4+ a week ... though the 3.5 x week (ie: alternate night) regimen is a very helpful and satisfactory strategy too.

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  • David Rosenbloom

    Dec 5, 7:40 PM

    Thanks again John for another highly illuminating article on "the facts about dialysis." "Renal replacement" is an euphemism for "We failed but don't like to admit it." It belongs in the same class as "informed consent."

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  • Philip Varughese

    Dec 5, 8:25 AM

    A very interesting article for dialysis group..

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  • Mary DeNinno

    Dec 2, 1:23 PM

    This article needs to be part of every Treatment Options class as well as part of Orientation education for both patients and staff.

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  • Mel Hodge

    Dec 2, 11:45 AM

    The math is sobering… but it is not hopeless. The good Lord gave us two kidneys, but experience with kidney donors suggests that one simply serves as a back up. The first three of the five stages of chronic kidney disease are largely asymptomatic... to feel well, just stay out of stages 4 and 5. We don't need to wait for some scientific breakthrough to achieve this. We just need enough dialysis often enough - 24-48 hours per week spread over as many days as possible, certainly more than three days which now is the unfoutunate standard. Forget about getting there by speeding up the blood flow rate. The graph of clearance versus blood flow rate bends over significantly so relatively little is accomplished by high rates except to make you very sick for a number of hours because the high rates throw your body out of kilter.

    The only practical way to get the hours you need to feel well again is to do it while you sleep. Then the intrusion on your waking hours is minimal – much less than a thrice weekly visit to a dialysis center. It is a big step and it is not easy, but I believe that if you make up your mind that you want to feel well again you'll come to a point where you will wonder why you waited so long…

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    • John Agar

      Dec 2, 7:06 PM

      The logic in all of that, Mel, is inescapable.

      The more surprising imponderable is: ... why don't more nephrologists recognise your logic, and - for those dialysis patients who can - empower them to take up a better dialysis path.

      While clearly the functional gulf between even the very best dialysis possible (= nocturnal .. long, slow and gentle) and two normal kidneys remains deep and, for now, unbridgeable, the disconnect between that which is almost uniformly 'on offer' (= in-centre conventional dialysis), and that which is so clearly (1) better and (2) attainable - now - by 25-33% of all dialysis patients (= home-based nocturnal dialysis) is only sustained by unawareness, inanition, and timidity.

      While we may never fully be able to bridge the gap between even the best technology and the marvel that is evolutionary nature, we should and must keep striving to span the bridgeable gap between human fear and achievement .

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      • Mel Hodge

        Dec 5, 12:18 PM

        John,

        "The more surprising imponderable is: ... why don't more nephrologists recognise your logic, and - for those dialysis patients who can - empower them to take up a better dialysis path."

        I would suggest the following:
        1. Nearly all nephrologists report that they would avoid dialysis in a center and do dialysis at home. Therefore, the explanation cannot be ignorance of the benefits.
        2. Reportedly, dialysis training occupies only about two weeks of nephrology residency and it is acknowledged that this is too short. If this is so, then many nephrologists may be hesitant to prescribe something they don't understand.
        3. The prospect of fielding telephone calls from home dialysis patients with treatment issues may be disconcerting, particularly when coupled with lack of confidence in understanding the dialysis treatment process. Better to turn their patients' dialysis treatment over to in-center treatment.
        4. Nephrologists can bill for only a single monthly visit for a home patient, but multiple visits for an in-center patient.
        5. Dialysis in the United States is overwhelmingly a for-profit business. Payment for dialysis services by Medicare strongly biases dialysis companies to promote in-center treatment rather than frequent, slow treatment at home... and hence reinforces nephrologist avoidance (but just for their patients!).

        I believe that the financial incentive for dialysis providers could be reversed by changing the basis of payment for dialysis and that the resulting provider home dialysis incentive would be strong enough to get them to overcome the present nephrologist disincentives. I described this change (an idea I got based on a comment by Dori Schatell, although I'm told by journal reviewers that it has been around for a while) in the October Nephrology & Issues. It is also the basis for an editorial accepted by AJKD which should show up in the next several months.

        I hope that we can get to the tipping point where nephrologists -- reinforced by the revised self-interest of providers -- come to recognize that they can no longer responsibly avoid prescribing the same remedy for their patients for end stage kidney disease that they would prescribe for themselves...

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        • John Agar

          Dec 5, 6:28 PM

          Mel ... your concept of time-based remuneration is timely (pun not quite intended), valid, and would be - if it were to be adopted - revolutionary. To be honest, though your reviewers commented on it 'being around for a while' , if so, then it has neither been heard, nor widely promulgated. I think it is the best idea I have heard/seen for many a long time - and I will always remember it as yours, from the start, as I - for one - have not seen it previously discussed.

          Whether it will ever be brought to the starting gate, well - that is another matter. It should be, but then, the lobby to sustain the status quo is powerful indeed. It is an idea, though, that would be simple and straightforward to implement, and that - in my view - would be of inestimable benefit to patient outcomes as it would encourage longer hour and more frequent dialysis. I will certainly give it a run with the decision-makers over here, even if it doesn't get up on your side of the big pond.

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    • John Agar

      Dec 2, 7:04 PM

      The logic in all of that, Mel, is inescapable.

      The more surprising imponderable is: ... why don't more nephrologists recognise your logic, and - for those dialysis patients who can - empower them to take up a better dialysis path.

      While clearly the functional gulf between even the very best dialysis possible (= nocturnal .. long, slow and gentle) and two normal kidneys remains deep and, for now, unbridgeable, the disconnect between that which is almost uniformly 'on offer' (= in-centre conventional dialysis), and that which is so clearly (1) better and (2) attainable - now - by 25-33% of all dialysis patients (= home-based nocturnal dialysis) is only sustained by unawareness, inanition, and timidity.

      While we may never fully be able to bridge the gap between even the best technology and the marvel that is evolutionary nature, we should and must bridge keep striving to span the bridgeable gap between human fear and achievement .

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  • Ole Sørensen

    Dec 2, 12:44 AM

    Leaves you humble of the millions of years of evolution. Yes we humans are clever, but we can not in 60 years perfect, what nature has finetuned for eons.

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  • Jenn Trunk

    Dec 1, 4:48 PM

    Dr. Agar

    As a patient, I want to say thank you. I've been on dialysis before and will soon be there again, and this was the one question that no one could ever properly articulate an answer for me. So thank you....thank you for explaining it in a way that was easy to understand and very straightforward.

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  • Beth Witten

    Dec 1, 4:04 PM

    John, I sent a link to you blog to the social workers on the NKF's CNSW listserv. Here's what I said:
    OMG…This very informative blog for professionals explains with examples how dialysis works and doesn’t work as well as healthy kidneys. You might want to share it with your staff and use what you can to help you patients see why skipping or cutting treatments short is a bad idea. By calling dialysis “renal replacement,” patients get the idea that dialysis takes the place of healthy kidneys. This blog totally dispels that myth.

    I've already received a response from a social worker thanking me for sharing it. I want to thank you writing it!

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    • John Agar

      Dec 2, 1:56 AM

      Thanks Beth.

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