Does Fluid Intake During Dialysis “Count” as Fluid Weight Gain?

This blog post was made by Dr. John Agar on April 16, 2020.
Does Fluid Intake During Dialysis “Count” as Fluid Weight Gain?

I was recently asked the following [edited] question:

“Do you include rinse-back and an on-dialysis drink in working out the amount of fluid to take off? Let’s say I gained 1 litre and thus have 1 litre to remove to reach target weight. But, I then (1) have a drink during dialysis and (2) include the fluid I use for rinse-back. Do I need to take off 1.5 litres? Or, just the 1 litre?”

This is a very good question, and there is no absolutely right answer to it. There are several variables that are hard to be certain about, but let me try to break it down without confusing the situation even more!

In the example, a 1 litre weight gain is pretty minimal for an adult. The heavier someone is, the less a 1kg (1 litre) weight gain is, relative to body weight. As most adults are somewhere between 55-100kg in weight, 1 litre is at most about 2% of body weight. Assuming that a dialysis session will need to be at least 2 hours long (for solute removal, regardless of fluid status), removal of 0.5 litres/hour (i.e., no more than 1% of body weight/hour) would not challenge the blood volume at all. Nor, for that matter, would removal of 1.5 litre over 2 hours (or more).

But, most likely gain more than 1kg (1 litre), and the more fluid there is to remove, the faster it must be removed in any given dialysis treatment. When fluid to remove is a volume greater than about 7-8 ml per kilogram of body weight per hour of dialysis, trouble starts. This is when the vexed issues of (1) fluid consumed during dialysis and (2) rinse-back fluid start to come into play.

Imagine that the amount to remove is 4 litres, and the treatment is a planned 4 hours. This requires removal of 1 litre per hour. This might still be a safe rate if the patient is heavy—but may already be unsafe if the patient is light. Ideally, fluid removal rates should be less than 7-8 ml for every kg of body weight in each hour of dialysis.

See how BODY WEIGHT influences ultrafiltration (UF) RATE. The first example table* uses the same amount to remove and same dialysis duration, but differing target weights.

*Note that ALL of the examples below are made up purely to illustrate points.

Target Weight

Fluid Gain

Time (Hours)

UF Rate

Outcome

Example 1

150 Kg

4 Kg (= 4 liters = 4000 mL)

4

(4000 ÷ 150) ÷ 4 =6.6 ml/Kg/hr

Safe

Example 2

100 Kg

4 Kg

4

(4000 ÷ 100) ÷ 4 = 10 ml/Kg/hr

Getting Dangerous

Example 3

50 Kg

4 Kg

4

(4000 ÷ 50) ÷ 4 = 20 ml/Kg/hr

Downright Lethal

See how TIME influences the UF rate. The second example table uses the same amount to remove and target weight, but differing dialysis durations.

Target Weight

Fluid Gain

Time (Hours)

UF Rate

Outcome

Example 1

100 Kg

4 Kg

6  

(4000 ÷ 100) ÷ 66.6 ml/Kg/hr

Safe

Example 2

100 Kg

4 Kg

4  

(4000 ÷ 100) ÷ 410 ml/Kg/hr

Getting Dangerous

Example 3

100 Kg

4 Kg 

2  

(4000 ÷ 100) ÷ 220 ml/Kg/hr

Downright Lethal

See how the AMOUNT TO REMOVE influences the UF rate. The last example table uses the same target weight and treatment time, but a differing fluid gain to remove.

Target Weight

Fluid Gain

Time (Hours)

UF Rate

Outcome

Example 1

100 Kg

2 Kg

4  

(2000 ÷ 100) ÷ 4 = 

5 ml/Kg/hr

Very Safe

Example 2

100 Kg

4 Kg

4  

(4000 ÷ 100) ÷ 4 = 10 ml/Kg/hr

Getting Dangerous

Example 3

100 Kg

6 Kg 

4  

(6000 ÷ 100) ÷ 4 = 15 ml/Kg/hr

Very Unsafe

So, should fluids taken in during dialysis and rinse-back fluid given afterward “count” when you calculate how much to remove in a given session? Let’s look at both.

  1. Fluids you drink during dialysis

    To a degree, it depends when you drink this fluid.  Early on in a treatment, your gut will likely absorb the fluid, so it should technically count. But, we (and many other dialysis services) suggest that you drink no more than a standard cup or tea (or the like) during a treatment, or about 180-200 ml. This is a reasonably small amount, and in the range of normal weighing error. Thus, we don’t count it as extra fluid to remove. If anything, it will add to the fluid gain for the next treatment. Certainly this is the case if you drink fluid at or near the end of a treatment. Then, it is clearly best to regard this as part of the fluid gained between treatments to remove next time.

  2. Post dialysis rinse-back

    This is clearly post-dialysis fluid, and should strictly be counted as the start of the next fluid gain.

In general, then, you would not want to remove the “extra” volume at a treatment. Doing so runs the risk of removing water that is not yet in your bloodstream. Pulling water that is not in your blood can cause your blood pressure to drop—which is not likely to be a good ploy. You would need to add water or even saline (water + salt) to correct the error, or you may have all of the symptoms and effects of volume depletion: 

  • Nausea
  • Vomiting
  • Dizziness
  • Clouded mental state
  • “Flats”
  • Organ stunning

The key variable to get right is your predialysis target weight. If target weight is correct, removing more water beyond that can reduce your blood volume. Of course, the real problem is that we really don’t know how to accurately measure a true and ideal target weight (in real time). For all that has been written about bioimpedance techniques, ultrasound to assess the vena cava or lung fluid status, hormone analysis, etc...none are practical at the clinic chair/bedside. And, they are certainly utterly impractical for home patients. Target weight is, in truth, still a clinical guess, though a wise physician and/or a well-trained home patient can get pretty close to it, most of the time.

I don’t think over-drying (i.e. removal of extra volume) is wise. It is better to try to get as close as possible to the assessed target weight and count any post-dialysis fluid return as the first gain for the next treatment. At the end of the day, do we know? Well, no! But, this is the best I can do to explain the inexplicable and advise the unadvisable.

Comments

  • Sandy Lawrence

    May 13, 2020 8:12 PM

    Thank you for this simple pictorial easy explanation - Just Perfect to give to my patients [and colleagues], who have difficulty understanding &/or retaining verbal information/discussions. A wonderful resource to add to my repertoire - Kudos/acknowledgment to Dr Agar of course
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  • David Leong

    Apr 17, 2020 6:50 AM

    Thanks for the helpful information!
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