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Clinical Application

Case 2: Constrained T

Not a minute more than 3.5 hours!

67 year old African American male with history of diabetes, hypertension and advanced chronic kidney disease has been initiated on dialysis in the hospital. He has received 6 treatments so far and has done well without significant issues with hypotension or cramping. He will now be transitioning to your outpatient dialysis unit. The patient weighs 70kg and has a well functioning AV fistula. You have done your smart calculations (Case 1) and know that he needs to be dialyzed for 3.9 hours for a goal sp Kt/V of 1.4. However, the patient is adamant that he will not stay for more than 3.5 hours because that’s what his friend does. How would you calculate the initial dialysis prescription now?

STEP 1: Start with T since it it a non-negotiable for the patient
T = 3.5 hours or 210 min

STEP 2: Calculate your V
V = 60% of 70kg = 42L or 42,000 ml

STEP 3: Remember your goal spKt/V
spKt/V = 1.4

STEP 4: Do the math
1.4 x 42,000ml / 210min = 280ml/min

STEP 5: Assume that you would like to prescribe a Qb of 400ml/min and a Qd of 500ml/min

STEP 6: Select different KoA options to see what range your KoA should be to reach at goal spKt/V of 1.4

Clinical Application

Case 1: Dialysis Prescription

Calculate an initial dialysis prescription (Rx), based on goal spKt/V

67 year old African American male with history of diabetes, hypertension, and advanced chronic kidney disease has been initiated on dialysis in the hospital. He has received 6 treatments so far and has done well without significant issues with hypotension or cramping. He will now be transitioning to your outpatient dialysis unit. The patient weighs 70kg and has a well functioning AV fistula.
How would you calculate the initial dialysis prescription based on goal spKt/V?

Clinical Application

Playground Debrief

K INCREASES WITH INCREASE IN Qb

CLINICAL PEARLS:

  • 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…

CLINICAL PEARL:

  • 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

CLINICAL PEARLS:

  • 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

Basics

Transport Across Membrane


Transport of ‘toxins’, electrolytes, and water occurs from blood to dialysate and of electrolytes from dialysate to blood. This occurs via two methods: Diffusion and Convection. In this module, we will focus here only on transport of solutes from blood to dialysate.

Diffusion

Convection
KT/V

Limitations of Kt/V

  • Kt/V overestimates dialysis adequacy in thin, malnourished patients and elderly females due to sarcopenia and a low ‘V’. This may lead to under-dosing of dialysis. Kt corrected for body surface area may be a more accurate measure of dialysis dose in these patients.
  • There is a high index of error related to inaccurate post-dialysis BUN measurement.
  • Single treatment may not represent other treatments ( missed or shortened)
  • spKt/V or eKt/V is not applicable to daily or nocturnal dialysis. It is useful only for conventional 3 times a week intermittent hemodialysis.
  • Kt/V is a marker only of urea clearance and not of other toxic/uremic solutes that exist.
  • Other uremic toxins such as phosphate, β2 microglobulin, guanidino compounds etc. do not follow the same hemodialysis kinetics as urea and therefore their clearances are not accurately reflected by Kt/Vurea.1
  • A high Kt/V(>1.4) has not been shown to predict better survival in any randomized controlled studies (e.g. HEMO study2).This implies that there are other determinants of poor survival in hemodialysis patients besides urea clearance and that despite a high dialysis urea clearance (Kt/V), patients may retain other toxic solutes that ultimately are equally or more important than urea in influencing prognosis on dialysis.3
  • Despite these controversies surrounding KT/Vurea, it remains the most frequently used parameter for determining dialysis adequacy and its routine monitoring may help to identify problems of dialysis delivery such as access recirculation etc.

Sources:

  1. Comparing the urea reduction ratio and the urea product as outcome-based measures of hemodialysis dose. AU Li Z, Lew NL, Lazarus JM, Lowrie EG SOAm J Kidney Dis. 2000;35(4):598.
  2. Eknoyan G, Beck, GJ Cheung AK, et al. Effect of dialysis dose and membrane flux in maintenance hemodialysis. N Engl J Med 2002; 347:2010.
  3. Meyer TW,Sirich TL,Fong KD et al. Kt/Vurea and non urea small solute levels in the hemodialysis study. JASN 2016;27:3469
KT/V

Why Care About Kt/V?

  • Urea is considered a surrogate to other small molecular weight uremic toxins and is easy to measure in the blood pre and post hemodialysis.
  • Although its use has limitations, it is an easily measured and useful marker of hemodialysis adequacy.
  • Kt/Vurea has helped standardize dialysis dosage based on urea clearance. It provides a tool to avoid grossly inadequate dialysis.
  • Its routine monitoring may help to identify problems of dialysis delivery such as access recirculation. 
  • Urea reduction ratio (URR= 1-postdialysis BUN/predialysis BUN) is a less optimal measure of dialysis adequacy as it does not account for ultrafiltration during dialysis and assumes that V is constant during dialysis.