K INCREASES WITH INCREASE IN Qb
- Dialyzer blood flow rate is a big determinant of K and ultimately Kt/V. Ideally Qb should be set between 400ml/min to 500ml/min as tolerated. Therefore when aiming for a goal Kt/V, ensure the delivered Qb is adequate.
- Dialysis catheters tend to allow lower Qb than AVF and AVG.
- K can NEVER exceed the Qb. If all the blood entering the dialyzer is cleared of urea in a minute, K is Qb.
K INCREASES WITH INCREASE IN Qd, but…
- Higher Qd increases dialyzer urea clearance (K) but as Qb approaches Qd, K starts to plateau. Therefore, Qd should typically be 1.5-2 times the Qb to maximize diffusive clearance.
- At a Qb of 400-500 ml/min, a Qd of UP TO 800ml/min makes sense.
- An increase of Qd from 500ml/min to 800ml/min increases K only by 8-12% when the Qb and KoA are not limiting i.e. a high efficiency dialyzer is used and the Qb >400ml/min (as there is more effective surface area for diffusion).
- Increasing Qd >800ml/min usually doesn’t add to the K or KT/V as the Qb becomes limiting (shown in graph). Dialystate is expensive, why waste it?
- Qd of 500-600ml/min is often adequate to achieve target urea clearance with a conventional Qb of 400ml/min
- An additional lesson is that with daily hemodialysis methodologies that have reduced Qds of 150 mL/min (for example, NxStage) or continuous veno-venous hemodialysis (CVVHD) techniques with Qds of 50–100 mL/min, there is no reason to employ higher Qbs or to use large dialyzers, as K will be limited by Qd.
K INCREASES WITH INCREASE IN KoA
- K increases with increasing dialyzer efficiency (KoA)
- However, the increase in K is most pronounced at adequate pump blood flow.
- At a Qb of <200ml/min, despite using a high efficiency dialyzer, K is low (the curves start to converge at a K of <170ml/min as shown in the graph).
- At a Qb of >200ml/min, the curves start to separate and K rises in proportion to the rise in KoA. Rise in K at this point is therefore ‘membrane limited’. Hence, if you want the most benefit out of your high efficiency dialyzer, you need adequate blood flow preferably >400ml/min
Feel free to go back to the playground case to understand how increases in Qb, Qd, and KoA impact K and Kt/V