Why would anyone want to use more dialysate than is actually needed

Why would anyone want to use more dialysate than is actually needed to clear a given amount of toxin? ie: if 7 Liters of dialysate is needed to clear a specific amount of urea over a specific amount of time, why would anyone want to use 40 Liters? or 15? In a home hemo process, with a NxStage cycler, specific patient and 2 hour cycle, If 7 Liters will clear the same number of grams of urea, why use more? Yes I know it won’t hurt to use more, but why would you want to?

Fornia, urea is the LEAST important of the toxins that are removed with dialysis. It just happens to be cheap and easy to measure–but it is also very tiny, and it freely moves in and out of cell walls. This means that you can remove urea very easily, without actually doing yourself all that much good (since it really isn’t especially toxic.) Even Dr. Frank Gotch, one of the developers of the (IMHO unfortunately) much-vaunted Kt/V (urea) formula for measuring dialysis “adequacy” admitted that it really wasn’t a good measure of other, more important toxins. So, one answer is that what you really need is more membrane contact TIME to remove not just urea but the OTHER, REALLY toxic toxins. We call these “middle molecule” wastes, and these are the ones that can build up and poison nerves, joints, and bones over time.

More dialysate in a standard machine means a greater “gradient” between levels of substances in the blood and levels in the dialysate–which then allows greater removal of things you don’t want. However, NxStage doesn’t work this way, it supersaturates the dialysate instead, and I’m not sure exactly how this affects the other toxins that really do matter. So, with the additional background that I’ve provided, you may want to ask NxStage this question directly–perhaps Dr. Krause–and let us know what you learn.

Thank you. Gradient works. Double the dialysate flow rate & gradient doubles. I would like to ask NxStage about this. Do you have info on how to contact them?

I would like to know how to relate the clearance rate of the midsize toxins to urea. I’m looking at a chart from NxStage of urea clearance (mg/min) vs dialysate flow (mL/min). The clearance rates for all toxins should be related.

It’s also related to blood flow. 5 L blood flows through the dialyzer in 20 minutes at 250 mL/min or 10 minutes at 500. Slower blood flow gives each toxin molecule more time per pass to get thru the membrane. Faster gives each mL of blood a better chance of meeting the membrane during a session (at 250 there are 6 passes in 2 hours vs 12 at 500 mL/min). In either case total blood flow is 5L over 2 hours (1.6 sec/mL).

Seems to me that slower is better as the cleaned blood has 10 or 20 chances of picking up more toxins from the extremities or from mixing with uncleaned blood before going back thru the dialyzer.

So, back to my first question, for a given blood flow rate, if the dialysate flow rate is sufficient to clear as much of the toxins as possible during the first pass, why would anyone want to run more dialysate than that? Other than it’s easier to manage.

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The thing I keep in mind is that the point of dialysis is to clear unwanted solutes from the fluid inside the cells from your nose to your fingertips. Dialysis’ job is to balance all the fluid in the body;to do this dialysis removes solutes from the blood - creating a solute deficit in the blood so that as it makes its way through the body it acts like a magnet and draws solutes out of the space between the cells and then it does get back to the start, rinse/repeat. But, it isn’t the same blood going through the second time and each time after that. If all dialysis had to do was to remove toxins from a static circuit of blood we could talk about multi passing the blood. But like a river the blood going through the fistula is never the same.

Thanks Bill, that was helpful. I think the answer is, “No one knows because the minimum amount varies from person to person and day to day so quit asking. More is better. QED.”

Unless you are the one tasked with making, delivering or handling the dialysate and then it’s “Just be glad you have a job.” Unless you aren’t being paid and then it’s “Just be glad you or your loved one doesn’t have to go to the center thrice per week and be treated as a carcas.”

For what it’s worth, considering that the only toxin that ever comes close to saturation in the dialysate is urea, and that it takes 40 to 60 minutes post dialysis for urea or creatinin to re-equilibirate, and that the rest of the toxins take up to 8 hours to re-equilibirate, I’m convinced that for most patients at home or in center you only need 10 to 12 liters of dialysate delivered over 2 hours to clear the max amount of toxins.

The dialyzer can only remove toxins that are in the blood. 10 liters per hour of dialysate delivered for 20 minutes at a blood flow of 300 mL/min will clear all of the pretreatment toxins in the blood for most patients. After that 4 liters per hour will clear all of the toxins that the cleaned blood pulls out of the extremities. (.33H x 10L/H + 1.67 x 4 = 10 liters)

I’d be happy to continue this conversation but my impression it won’t go anywhere,