A Primer on Haemodialysis “Weight”
Weight! Is there a more emotive, discussed, feared, misunderstood word in all language? Like many of us, I dejectedly peer each morning at my “mirror weight”, and pretend I don’t see what I see. And, like many of us, I make New Year’s resolutions—year on year—and order slabs of Lite N Easy™, vain (and in vain), making pacts with myself—or with the devil within...yet, none of it works.
To add insult to injury, and as if to taunt me more, I am assailed as TV ads market impossibly thin and smiling waifs who trumpet endless unsustainable diets.
But, for a dialysis patient, weight puts on a new and scary dress. For weight, to a dialysis patient means fluid, and fluid can mean death. So:
- What is weight, as it matters to a dialysis patient?
- What is the difference between body and fluid weight?
- Can we really tell what weight is “right” at any given point within the tumultuous cycle that is dialysis?
- What is “dry weight” or, as I think it should be referred to, “target weight”? Are they the same thing, or are they different? And, why does it all seem to matter so much?
- Finally, how do we (or can we) assess what a correct target weight should be? Are there signs? Or, is it just all hocus-pocus and guess work?
- What fluid inputs and outputs matter, or may need to be compensated for, either during or after dialysis?
- And ... what target weight is the right target?
As “weight”, in a dialysis patient, changes up, and down almost by the minute, is it any wonder that it seems so hard to ever really know what weight to call, or how much weight (i.e., fluid) to remove? Let me now try to plot a path through the “weight minefield” of dialysis.
Weight is Volume and Volume is Fluid
Body weight is a mix of “solid” structures and fluid (water and salts). Our “solid” structures, bones, skin, and major organs, may seem solid. But, much of our organ weight comprises the weight of the fluid within them. This fluid dwells in three spaces, or compartments:
- Intracellular - the fluid within our cells, which makes up the largest fluid volume (and fluid weight) in our bodies.
- Extracellular or interstitial - the fluid surrounding our cells. This fluid wets and bathes all cells. It provides pathways though which cell nutrients come in, and cell products and wastes move out. The “in-between” spaces—partly a supportive weave and partly a kind of glutinous jelly—are awash with tissue fluid. This “interstitial space” is a web of “connective tissues” that bind and link our major structures and organs together. Tissue fluid expands and wets the connective tissue that fills the void between and around our cells and organs. It serves as a link or transport system for cell nutrients, products and wastes to move from and return to the main highway network that serves our bodies—the bloodstream, or vascular system.
- Intravascular - inside our blood vessels. This vascular superhighway is a transport network that connects all the distant corners of our bodies. The fluid in blood is partly intracellular (within red cells, white cells, and platelets that make up the “solid” components of blood). And, it is partly extracellular (the mix of water, electrolytes (salts), and proteins that we call plasma). The cellular and plasma components of blood exist in a rough 50:50 ratio.
So, if there is fluid everywhere—inside all cells, around all cells, and forming the system that transports throughout the body—how much fluid is there, in total? Remembering that 1 litre (in volume) = 1 kilogram (in weight), water makes up about 42 litres in volume (60% in weight) of a 70 kg “standard” adult male”. A “standard” adult female body contains a little less water (55% by weight. The lower water percentage is due to a higher fat content, since fat contains less water.)
The following schema from Anaesthesia UK shows where the fluid is within the main “compartments” of the body:
Again, it is critical to understand that every nook and cranny of the body is wet, soggy, and awash with fluid. Just as each body part has a weight, so does the fluid that lies within and around, including the fluid that makes up our blood.
Normal kidneys are really good at fine-tuning our fluid to ensure that all spaces are internally in balance, and in balance with each other. While fluid can flow from one space to another, lots of things can bar rapid and immediate fluid balance (equilibration), like cell walls, concentration gradients, and proteins. Normally, this equilibration is gentle, seamless, and in the background, and we are unaware of it. But, take the kidneys out, and our regulator is suddenly out of action. The release valve—our urine volume—is compromised. True, some patients will pass urine for a while, even when waste build up demands that dialysis start. But, sooner or later, the luxury of a urine output for fluid control will be taken away. Now, fluid taken in by mouth can no longer get out again. Without dialysis to remove it, a patient rapidly becomes waterlogged: blood pressure rises, lungs fill with fluid, breathing is hard, and the heart labours.
The assessment of fluid gained in this way is performed by one simple piece of equipment: a scale! This, then, is the basis of fluid weight gain and loss, a concept that so haunts all dialysis patients. It should now be clear that, try as we may, we can never know the weight (or volume) of any one part of a living and breathing human. The ratio of water to “solid” in any one organ or person will vary greatly between individuals, and in any one person at varying times. No two of us are exactly the same, except by chance. And, no one of us is always the same over time.
Herein lies the conundrum for the dialysis physician: how to guess—and it is a guess, albeit supported by some background science—the correct fluid weight of any one patient, since that weight is constantly on the move!
In a haemodialysis (HD) patient, and I will now focus on HD, fluid in the body will slowly expand in rough cross-compartmental equilibrium during the course of a 1, 2 or 3 day inter-dialytic break. Then, it rapidly shrinks as fluid is plundered, mainly from the blood vessels, during a treatment. Cross-compartmental disequilibrium results. The other two compartments try to play “catch-up”, as best as they can. But, the faster fluid is pulled out of the blood vessels, the greater is the disturbance to fluid equilibrium. And, there are those pesky rate-governors that slow free flow between blood and interstitium, and between the interstitium and cells. For a more detailed description, see my two partner blogs on fluid removal ratesand the dialysis waterfall.
When assessing weight, a dialysis doc tries as best as s/he can to fix a weight that, considering all of the variables, is a likely optimum body weight. There are several “weight” terms used in dialysis, and they all mean slightly different things. The key to understanding them is to clearly grasp the following principle:
- Weight change from food intake, i.e., the “weight gain” so battled over in the media is body weight gain, a change that occurs gradually over time.
- The rapid weight loss after an HD treatment is ALWAYS and ONLY fluid weight change. (This change reverses as weight is regained between the end of one treatment and the start of the next.)
That said, there are a number of fluid-dependent weights that apply to dialysis. These include:
- Pre-dialysis weight
- Ideal weight … a non-dialysis weight range (BMI-guided)
- Post-dialysis Weight
- Dry Weight
- Target Weight
- Achieved Weight
- Interdialytic Weight Gain (IDWG)
- Weight to lose (remove) during dialysis
This is a scale-measured weight. It is commonly checked upon arrival at a clinic or prior to HD at home. Ideally, this should be naked-weight. Indeed, all weights are only fully reliable if measured naked. As this is clearly not practical for clinic patients:
- Take off as much heavy clothing as possible.
- Record the clothing type and approximate weight. In this way, treatment by treatment, roughly the same weight can be “allowed” for clothing
- Wear the same clothes for the pre- and post-treatment weights, so clothing weight can be ignored as a variable in any intra-dialytic weight change.
- If you drink during dialysis, or have to disconnect to pass urine, factor the weight of this fluid, in or out, in to any change between pre and post dialysis weights.
Ideal weight is a notional value. It suggests what a healthy body should weigh for a person of a given height, gender, ethnicity, and age. This is not specific for an HD patient. Rather, it is a range (not too fat, not too thin) that health experts believe all should to aspire to. Many formulae have been devised from a range of differing assumptions. Ideal weight has been more recently been incorporated into the broader concept of Body Mass Index (BMI). The World Health Organisation (WHO) states the range for a healthy BMI = 18.5 - 25 for both men and women. Here is a BMI calculator.
This is the scale weight of a patient at the end of HD. It is not “dry” weight, nor—unless the dialysis has achieved it—target weight. It DOES mark the starting point for the next inter-dialytic weight gain (IDWG), or, as occasionally it may be, loss. IDWG is the difference between weight at the end of one treatment and the start of the next. Clothing and footwear matter, as patients typically are weighed clothed. Match, as best as is practical, clothing type and weight, from one treatment to the next.
Ideally, post dialysis weight should aim, as closely as possible, to be “dry weight”.
A long-held concept in dialysis, dry weight is what a person would weigh at the end of HD if all of the body compartments were in fluid equilibrium. That is, all body fluids are evenly and appropriately balanced. And, they are in ideal fluid volume ratios in the cells, in the interstitium, and in the bloodstream.
In almost all circumstances, “dry weight” is an unattainable concept: certainly with conventional HD. Dialysis can ONLY remove fluid from the blood. As blood is depleted of fluid during HD, a fluid drag effect is exerted on the extracellular fluid. This is the so-called reverse waterfall effect I have explained in my two previous blogs (see above). Briefly:
- Fluid is draaaawn and draaaaged from the interstitium into the bloodstream to replenish what has been removed.
- As this diminishes the fluid in the extracellular fluid, a further drag effect is created between the extracellular fluid and the intracellular fluid.
- In turn, fluid is draaaawn and draaaaged from within the cells to replenish the now reduced fluid volume of the extracellular space.
Fluid “transport” will never be fully in balance (in equilibrium) while HD is in progress. Fluid is constrained in its movement between the three compartments by:
- A range of osmotic forces
- Structural barriers like cell membranes and connective tissues
- Compartmental imbalances that dynamically occur during active HD
In most HD, equilibration can only occur in the hours after dialysis. This is the so-called “post-dialysis recovery phase.” Most dialysis patients know this recovery period only too well—and how long it can take.
Conventional HD always shrinks the fluid volume of the bloodstream more than the other two compartments. However, the slower the rate of ultrafiltration (water removal), the less the disequilibrium. At the end of almost all conventional HD treatments:
- The bloodstream volume will be contracted and fluid-lean.
- Excess fluid will remain “in transit” in and between the other two compartments, having missed removal during dialysis.
Without full equilibration, we cannot say (or know) if the weight reached at the end of HD is true “dry” weight. In most cases, it is not. “Dry weight” thus remains a clinical “notion”, but not a practical reality.
The only possible exception to this is long, frequent, slow (nocturnal) HD. In this case, the rate of contraction of the blood vessel volume is very gentle and very slow. Synchronous cross-compartmental volume (i.e., weight) equilibration can occur throughout all 3 compartments during a treatment. Only in this case can ideal weight be an attainable measure. But, for more practical value, there is “target weight”.
This is an easier and more clinically useful concept. A target weight sets a post-HD weight that can be attained in day-to-day practice. It offers a best guess for ideal weight. After using clinical clues, target weight is what a dialysis doctor concludes that the post-HD weight should be. While I may be self-delusionary, after 40 years of trial and error, I think I can now get this guesstimate right (or nearly right) most of the time.
Assessment of dry weight needs a practiced eye, and calm evaluation. To best decide target weight, the assessment should be timed to be after cross-compartmental balance is restored. Ideally, weight should be measured:
- Post-dialysis, and
- Post-equilibration, but
- Pre-resumption of fluid intake and further fluid weight gain
In practice, this rarely, or never occurs. It is impractical. By the time equilibration has occurred after dialysis:
- If you dialyze in-center, you have left the dialysis clinic to recover at home
- If you dialyze at home, you are home and not accessible to the clinic
So, in the end, target weight is a computational “best-guess” based on a set of clinical measures:
- Behavior of blood pressure during dialysis
- Other volume and cardiovascular markers
- Fluid-related symptoms
- The presence or absence of detectable excess fluid
- The next inter-dialytic weight gain (IDWG)
In clinical circumstance, however, what governs reality is “achieved weight”.
This is the post-dialysis scale weight. It would be great if it equalled target weight for each treatment. At the least, it should be close to target weight and, if this is the case, then relatively satisfactory fluid balance has resulted. (This assumes that the assessment of target weight has been (1) correct and (2) regularly reviewed and re-set.)
Target weight review is a core “must” in all dialysis patients! Body weight does change, up and down, slowly over time. Body weight loss is also common when there is an illness. If an illness is severe, so too will be the weight loss. And, post-illness recovery commonly sees body weight regained. Dialysis weight targets must thus be reviewed often and change, up and down, as body weight does. If this is not anticipated, reviewed, and adjusted for, imbalance between tissue weight and fluid weight can lead to:
- Fluid overload - if dialysis weight targets lag behind body weight loss
- Fluid depletion - if dialysis weight targets lag behind body weight gain
Often, this adjustment step does not occur. If you—the patient—are concerned that your body weight has changed, up or down, ask for an update of your target weight.
And so, we have come full circle:
- From the largely unattainable concept of “ideal weight”
- To the largely impractical and distant concept of post-dialysis post-equilibration “dry weight”
- To a best-guess but, with care and wisdom, close-to-the-mark “target weight”
- And, finally, to a measurable post-dialysis “achieved weight”
(NOTE: In the near future, we will be providing you with a new calculator that will allow you to work out exactly how much fluid you can safely drink to keep your ultrafiltration rate during the next dialysis session at a safe level. Watch this space!)
Inter-dialytic weight gain (IDWG)
This is the scale weight difference between the end of one dialysis and the start of the next. It is usually—but not always—a weight gain. But, it can be a loss, especially if there is a high urine output, or excess “other” fluid losses. For example, an ileostomy, colostomy, or another unusual fluid loss could lead to a loss between treatments. IDWG always and only represents fluid weight change, NOT body weight change.
Knowing the “target weight” for the end of the next treatment dictates how much fluid (weight) to remove (or, rarely, replace) during the next dialysis run.
It is important to control the IDWG so that the rate of ultrafiltration at the next dialysis to return to target weight is not too high. We can predict this with a formula that will be the subject of another blog-to-come.
Weight (fluid) to lose (or, rarely, to gain)
This is simply target weight minus measured weight at the start of a treatment.
Signs of fluid excess or a fluid deficit
It is important to be able to detect the difference between excess fluid vs. a fluid deficit —both relative to body weight. How can we tell?
If excess fluid is present, then as a guide, signs and symptoms may include:
- A rise in pre-dialysis systolic blood pressure (the upper number) to higher than 145-150 in the absence of blood pressure pills. If blood pressure pills are needed to control blood pressure in a dialysis patient, then by definition, salt and/or water are present in excess, even if the BP seems well controlled by the medication. Again, and while this may seem an overly didactic statement, I will make it anyway: no dialysis patient should ever need blood pressure pills for BP control! True, blood pressure medicines pills are sometimes used for reasons other than blood pressure control. For example, long-acting beta-blockers or ACEi/ARBs can be prescribed to help heart function. However, if salt and water balance is correctly adjusted and all dialysis weights are right, the blood pressure should be controllable by dialysis alone.
- Shortness of breath
- Trouble breathing if you lie flat
- Ankle swelling
- Neck vein enlargement (NOTE: There are other causes of shortness of breath, too, such as anaemia, lung disease, etc., and neck veins can be hard to interpret when there is a fistula, catheter, or a fleshy neck.)
- The dialysis staff may use other signs as well.
If you are salt and water (fluid) depleted, then as a guide, signs and symptoms may include:
- Dizziness – especially on standing from lying or sitting, or when you stand up after you have bent over.
- Your systolic blood pressure will likely be low, less than 100-105/-, and fall when standing (a postural drop).
- You may have a rapid heart rate (tachycardia) that increases on standing.
- You may be thirsty +/- nauseated. The membranes of your mouth and tongue may be dry.
There are other causes for these symptoms. The dialysis staff can help to distinguish fluid depletion from some of the other reasons.
I have dealt with the critical difference between ultrafiltration volume and rate in other blogs at this site, and now would be a good time to review these, in particular, this one.
So, I will leave you with this final thought. Though the fluid volume (i.e. weight) to remove matters—a lot—it is not the volume that is the most critical factor. What matters most of all is the rate at which that fluid is removed!