Haemodialysis UF Quantity and UF Charge are NOT the Similar

Haemodialysis UF Volume and UF Rate are NOT the Same
October 13, 2020 0 Comments

I’ve detected some confusion between ultrafiltration (UF) quantity and
UF price. These are NOT synonymous—they imply totally various things.

Whereas the UF quantity has been with us for so long as there was
dialysis, current curiosity within the UF price seems to have launched some
uncertainties about precisely what the distinction is.

I’ve been instructed (accurately or incorrectly) that Fresenius North America
has not too long ago launched a suggestion that the UF price must be
stored beneath 13 ml/kg/hour. Sadly, this half-step in the fitting
course might have merely added to this confusion.

Dialysis removes two important issues:

  1. Solutes – dissolved electrolytes (salts) and molecules of waste
    (toxins) made by the physique’s every day functioning
  2. Fluid (water). Convective constructive or destructive pressures that
    are utilized to both aspect of the dialysis membrane largely decide
    the quantity of water that’s eliminated. By adjusting/altering these
    pressures, extra (or much less) water might be eliminated.

The aim of this weblog is to set the ledger straight! So, right here
goes…

The UF quantity is the quantity of water that have to be eliminated in a
single therapy to return a affected person to his or her goal (or base)
weight. The ultrafiltration quantity is usually expressed when it comes to
weight (the place 1 litre = 1 kilogram).

The UF price is the pace at which that quantity is eliminated. (Right here is
a helpful, free calculator
we constructed that can assist you calculate the UF price).

However, wait a second: how do we all know how a lot water to “ultrafilter”?

As all who’re concerned in dialysis know, most sufferers quickly achieve
weight (learn water; bear in mind 1 kilogram = 1 litre) between the top of
one dialysis and the beginning of the subsequent. In these fortunate few who retain a
urine output because of residual renal operate, this achieve might be
minimal. Nevertheless, in time most sufferers will lose all native kidney
operate and move little or no urine. Because of this, all fluid consumed
between remedies —the inter-dialytic interval—will have to be eliminated at
the subsequent therapy to return the affected person to their base (NB: I personally
choose the time period ‘goal’) weight.

Whereas we’d like this goal end-dialysis weight to be what’s
typically described as “dry weight,” in actuality, it not often is
. True dry
weight is what a physique would weigh IF the quantity of water in every of
the physique’s three fluid major compartments (mobile, extracellular,
and intravascular) had been to be perfect. Sadly, the precise
measurement of dry weight stays a holy grail. Though there are all
types of strategies to attempt to decide it, dry weight is a notional objective
relatively than an precise, definable worth.

So, we do our greatest—and, most of the time, our greatest is a really unhealthy
greatest —to guess at what we predict dry weight must be… and because it actually
isn’t dry weight, we name it “base” or “goal weight,” as our greatest
approximation of true dry weight. Possibly, sooner or later, bioimpedance,
biochemistry or different organic options to the “dry weight” conundrum
can be discovered, however for now, the aimed-for put up dialysis weight (the
“goal weight”) stays only a greatest guess.

So, why all that discuss weight?

Properly, as each dialysis affected person is aware of, they are going to be weighed firstly
of dialysis. The distinction between this weight and the put up dialysis
goal weight will then be calculated. Mostly, the predialysis
weight is larger than the goal weight. This distinction (in kilograms)
equals the quantity (in litres) that have to be eliminated through the dialysis
run by the method known as ultrafiltration.

The quantity to be eliminated [gain 2 kg = remove 2 litres; gain 3 kg
= remove 3 litres; ‘gain’ 4 kg = remove 4 litres … etc.]
is the UF
quantity
. Though small changes could also be made to this quantity to
account for saline flushed again on the finish of dialysis, or fluid
consumed throughout dialysis (e.g. a cup of tea), usually the UF quantity
equates the litres (or kg) gained within the interdialytic interval.

However…and right here’s the crux of this weblog…the UF quantity is NOT the identical as
the UF price! The UF price is a pace, not a quantity, and refers back to the
quantity of water that have to be eliminated in any given time!

Which means:

  • If there are 2 litres of water to take away (UF quantity) and the
    dialysis run is 2 hours, the pace of elimination—UF price—can be 1
    litre per hour
    .
  • If there are 4 litres of water to take away (UF quantity) and the
    dialysis run is 2 hours, the pace of elimination (UF price) can be
    2 litres per hour.

The UF price is ruled by two elements:

  1. The amount that have to be eliminated (the UF quantity).
  2. The time (or sessional period) allowed for that elimination.

So, the UF quantity = litres, however the UF price = litres per hour.

So as to add one piece of complexity to this easy distinction, sturdy knowledge
has proven that if water is eliminated too quick and the circulating blood
quantity is contacted too rapidly, organ perfusion pressures drop. In
flip, this dangers organ ischaemia and compromises organ oxygenation.

This has led to the idea, superior by Jennifer Flythe et al,
that there’s a most price at which water might be eliminated. If
this price is exceeded, organ “stun” and cardiovascular morbidity and
mortality
are at heightened threat.

The UF price relies on a 3rd issue: it isn’t simply the quantity
that needs to be eliminated and the time allowed for its elimination, however the
measurement of the individual being water-depleted… i.e. the individuals’ physique
weight…or, from knowledge that Emily See from our service has generated and
reported at numerous conferences within the final yr or so, physique floor
space
.

Thus…the UF price is healthier expressed in mL/Kg/hour.

Word that the “Kg” on this equation is the affected person’s goal put up
dialysis weight
. Ideally it must be the affected person’s “dry weight,” however
dry weight is a notional quantity, whereas the goal weight is actual.

I’ve argued—particularly with my American colleagues—over what a protected UF
price is perhaps. I argue {that a} most UF price must be no higher
than 10 ml/kg/hr
. I observe that Fresenius (USA) has not too long ago suggested a
most price of 13 ml/okay/hr.

I disagree! Simply check out Flythe’s graph—knowledge drawn from the HEMO
examine knowledge. What level on that graph says “protected”? I relaxation my case. I
know, if I had been a affected person, the ultrafiltration price that I might need!

Associations between UFR and CV and all-cause mortality

As one single instance of how this works, think about a affected person whose
goal weight is 100 kg and who has gained 5 kilograms
. S/he should lose
5 litres to return to focus on weight:

  • A 3-hour dialysis would imply eradicating 5 litres (= 5000 mL) in 3
    hours = 1,666 ml/hour = 1666 ÷ 100 kg or 16.6 mL/Kg/hr. That
    would do irreparable injury to the center!
  • If the identical affected person had 4 hours of dialysis: 5000 mL to take away ÷
    4 hrs ÷ 100 kg goal weight —> 12.5 mL/Kg/hr.
  • Do a 5-hour dialysis and the ultrafiltration price drops to 5000
    ÷ 5 ÷100 = 10 mL/Kg/hr (and solely simply “protected”).
  • Higher could be 6 hours with an ultrafiltration price of 8.3
    mL/Kg/hr
    .

For comparability, our Geelong knowledge calculates our imply unit UF price throughout
150 centre-based sufferers to be 7.95 ± 3.11 mL/Kg/hr, whereas our
median unit-wide UF price is 7.73 ml/kg/hr. NB: these knowledge exclude our
50 sufferers at dwelling on prolonged hour and excessive frequency nocturnal
dialysis. What, for curiosity, is your unit’s imply and median UF price?

So, lastly, we come to “what to do”

There are solely two methods to change (i.e. decrease) the UF price!

  1. Have much less water to take away in any given time
  2. Take extra time to take away the identical quantity of water

All of us now how exhausting it’s for dialysis sufferers to limit their water
consumption. We encourage them, cajole them, some even bully them, however, at
the top of the day, limiting water consumption is simply not doable for some
(or most) sufferers. Earlier than you scold them, strive it your self! Attempt to
restrict, day after day after day, to the kinds of limits you search to
impose in your sufferers.

So, if water consumption limitation has its limits, just one course
stays:
Dialysis sessional period should be longer. And, that
durational extension have to be enough to make sure that the UF price is not any
higher than (for my part) 10 ml/kg/hour.

The problem is the right way to obtain an ultrafiltration price ≤ 10 ml/kg/hr?

In Geelong, for nearly all sufferers, we do. How? Properly, we do longer
dialysis.

Finish of story.

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