The Dialysis Waterfall – Part 2
There is one last big mistake we make ...
Let us return, for a moment to the waterfall concept. While I described it in the context of fluid, it applies equally to solute clearance. Dialysis is not just a process to clean the blood, as all explanations of dialysis mistakenly say. Dialysis serves the primary and central role of cleaning the cells and the interstitium (the extravascular extracellular space). Think of this latter as the space that lies between all cells in the body, yet outside the blood vessels themselves. It is the 'middle' fluid compartment.
Fluid "waterfalls" from the cells → the interstitium → to the blood compartment, then, via the dialyser → to the drain. Solute wastes and excess electrolytes (like potassium) that accumulate during our moment-to-moment metabolism follow the same path. Blood is the only accessible compartment, but in most respects it is the least important: it is merely the conduit through which cellular and interstitial toxins are removed...down the waterfall!
That waterfall is not only rate-limited for fluid, it is rate limited for solutes, too, and different solutes move down that waterfall at different rates. Some zing down the gradients but others get stuck...like sticks on the rock face of a real waterfall...and take longer to be shaken loose.
So...our waterfall concept applies to both fluid and solute – yet we constantly make it seem that it is only the blood compartment that matters.
Finally...let us return to the best key performance indicator (KPI) for 'good dialysis'.
- Forget Kt/V...it is a useless KPI.
- A far better KPI for all dialysis units would be a target UFR that requires a fluid removal rate not >5 ml/kg/hr more than the plasma refill rate of ~5 ml/kg/hr. Interestingly—and I hope this hasn't escaped your notice—that equates to McIntyre's 10 ml/kg/hr myocardial stun cut-off!
But, best of all, remains any dialysis process or regimen that removes fluid at a rate that lies within the plasma refill rate (ie: a UFR of <5 ml/kg/hr). This is a dialysis regimen currently only fulfilled by frequent, extended hour dialysis programs.
Out with Kt/V as an indicator of good dialysis!
In with a fluid removal rate 'KPI' where, ideally, the target should be a removal rate <5 ml/kg/hr (= the approximate plasma refill rate), and certainly not >10 ml/kg/hr = a simple all-dialysis target KPI.
While the way to achieve this is through longer dialysis, it is not—and will never be—achieved through the angry abuse of patients as 'non-compliant' when it has not been their fault.