Forget Kt/V. Dialysis is about replacing kidney function. That means fluid and toxin removal.
Here are a 2 facts: High flux dialyzer membranes allow all toxins to pass from the blood to the dialysate and back without restriction. Modern dialyzers are designed to clear all the toxins in the blood with BUN = 100 mg/dL, at 500 mL/min blood flow in one pass, assuming normal pathologies and dialysate flow = 30 L/H.
If I’ve got either wrong I’d really like to know.
Basicly, the dialyzer keeps the large molecules in the blood but allows all the smaller molecules to pass back and forth without restriction. It’s like mixing pink koolaid and clear water. The time factor is measured in microseconds.
The first mL of blood entering the dialyzer has lost half of all it’s toxins within the first half inch of flow at 500 mL/min and is completely clean by the time it reaches the other end. The second mL encounters dialysate already contaminated by the first but it doesn’t matter because in a minute the dialyzer is full of blood and the gradient of toxin concentration in the blood goes from max to zero over the length of the dialyzer. The dialysate has an equal and opposite toxin concentration gradient.
Now, what happens if the blood flow is reduced to 200 mL/min without changing the dialysate flow? Each mL of blood gets to spend more time in the dialyzer. Each mL gets completely cleaned sooner. Reduce the dialysate flow to 200 and it again takes the whole dialyzer to get the same result.
If the pre BUN is 50 instead of 100 the dialysate flow can be cut in half to achieve the exact same result. 50 BUN, 200 blood flow and 100 mL/min (6 L/H) dialysate is just as clean after dialysis as 100 BUN at 500 and 500.
There is nothing magic here. It’s 8th grade science and math. Hopefully I’ve got this right and we can move on to rebound and total toxin clearance in a 2 hour cycle.