"How much can I drink, Doc?"

Can we truthfully answer this simple question?

I am not sure if I yet have all my ducks in a row, so bear with me, but I have been doing something dangerous…I have been thinking!

In HD, we talk of wanting to preserve residual renal function (RRF), or, rather we bemoan the loss of it, and we draw comparisons with the “relative” preservation of RRF that is reported in PD.

But are RRF-HD apples truly the same as RRF-PD apples? Well…no!

In HD, solute clearance estimated by the dreaded Kt/V generally takes care of itself. But, we are so trained to think declining eGFR, declining solute clearance, ergo impending dialysis, that when HD actually starts, our solute-driven thinking persists. We neglect to make the switch from easy-to-solve solute, to hard-as-hell volume. While that may seem an oversimplification, I believe the core premise is true.

IF:

• The vascular access is sound
• The membrane surface area (m²) has been correctly prescribed
• The blood flow rate is appropriate
• The duration of dialysis (‘t’) is ample

Then solute clearance (at least as measured by Kt/V, and as most know it) is catered for. Though there is far more to solute clearance than urea clearance,¹ for the purposes of this discussion, I will leave solute clearance at that; it is not the dragon that needs to be slain here.

The bête noir of HD is volume, and when it comes to what is or is not left of residual function, it is not RRF_solute that matters, but RRF_{urine output} Let me call it RUO.

The fundamental difference between RRF_solute—as we routinely think of it in PD—and RUO—as we should be thinking of it more in HD—and the reason for this blog post emerged from an email exchange with Allan Collins (of USRDS fame).

Allan cited several example calculations of how much an anuric patient can drink within any interdialytic period, based on anuric interdialytic weight gain. Anuric weight gain from one dialysis to the next is simple to calculate: It is the maximum permissible weight gain (read fluid intake) that will ensure that the maximum ultrafiltration rate (UFR_max) during the next treatment is less than the rate that risks myocardial stun².

Myocardial stun is risked if the dialytic UFR exceeds 10 ml/kg/hour. As we know the intended interdialytic duration, the intended duration of the next dialysis, and the weight of the patient, a maximum allowable interdialytic fluid intake plan can be calculated for and given to each patient at the end of each treatment. Indeed, this maximum intake has always been known—it is just that we rarely if ever use it.

How often have I heard the question ‘how much fluid can I drink, Doc?” Well, we know—exactly—how much this should be, individualized for each patient. And it differs, from patient to patient. Yet, too often, patients of very different body habitus are given the same blanket intake advice.

Next, if any RUO is present, the calculated anuric interdialytic fluid restriction volume can be eased by an amount equal to the interdialytic urine output + a small additional volume to recognize other insensible losses, though it would be to the patient’s advantage to ignore insensible loss and just concentrate on the predicted daily volume allowance + the mean daily interdialysis urine volume.

So, in HD, what clearly matters is NOT the preservation of RRF but the preservation of a “volume out” urine output. In PD, what matters most is solute RRF (at least at the beginning), while in HD, volume RUO matters most.

Now, while we all know those simple principles, they are often not coherently translated to clinical care. If we use the wrong terms, we confuse the intent. We should be talking less about RRF in HD, and more about RUO. And, RUO is much easier to measure. Buy a bucket!

A nice PD study done here in Australia a couple of years ago, the BalANZ Study, showed that while clearance didn’t alter significantly between a group treated with standard dialysate and a group treated with a more physiological PD fluid, the urine output was better sustained in the study group compared with the controls. So, even in PD, we may have been barking up the wrong preservation tree—the RRF tree instead of the RUO tree—though in PD, solute clearance is more subject to inadequacy than it is in HD, and thus both RRF and RUO remain important.

As patients progress through CKD, we slavishly follow the eGFR. It is habitual. It mesmerizes us. We even teach the patients to care, and ask, “what’s my percentage kidney function, Doc?” even though it is not, in fact a percentage at all, but just looks like one. We have, as a professional craft group, developed a solute-driven mind-set. This solute mind-set has led us to fail our patients on two basic volume counts:

1. We tend to cut and paste a uniform intake volume onto our patients:
   1. Without much logical thought for base body weight, age and frame size, whether weight, surface area, BMI or any combination of these.
   2. Without any logical thought for the impact of this uniformity on the individuality of UFR required during the next dialysis, and individuality that will ensure that the upcoming UFR is kept below 10 ml/kg/hr).

   A simple nomogram example is provided here to predict the fluid intake of an anuric patient that is required for any given body weight to ensure that a subsequent 4 hour dialysis UFR does not exceed a myocardial stun risk limit of 10 ml/kg/hr.

   Note:

   • This nomogram would need to be redrawn to apply for different dialysis durations
   • It does not include allowances for insensible loss
   

2. Do we ask our patients to wield a simple measuring cup, now and again? No, we don’t—or most of us don’t. “Volume out” is ignored as we dote upon blood result after blood result.

   We don’t develop a volume mind-set to match our solute one. When the patient transitions into dialysis, we and our patients fixate on the blood!

   We haven’t been trained to think about, to measure, and to review, that is which is the most simple thing to measure and review of all: the volume of urine passed. We ought to: (1) know how much urine is passed in serial interdialytic periods at, say, monthly intervals and (2) understand and modify the interdialytic fluid intake advice we give to patients accordingly.

While lip service is paid to what patients can drink: “Your daily intake should be, at a maximum, though any amount less is preferable, ‘x’ plus your urine output,” do we all actually measure this simple metric and adjust our intake recommendations, progressively, as the dialysis months pass? Well, no, not that I am aware of.

But we should.

And, we should be comparing incident differences between the many time, frequency and site models that now make up the dialysis spectrum to assess the impact of these variables on preserving the thing that, to our patients matters almost more than anything: “How much can I drink, Doc?”