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?
STEP 1: Based on clinical practice guidelines, figure out your goal prescribed spKt/V.
This is NOT the correct goal spKt/V. Try another choice.
Correct! This should be your goal prescribed spKt/V.
STEP 2: Calculate your V
This is NOT the correct calculation for V. Try another choice.
Correct! This is the best calculation for V. Move on to STEP 3.
STEP 3: Review your known variables for Kt/V:
Kt/42,000 = 1.4
STEP 4: Calculate Kt = 1.4 x 42,000 = 58,800ml
STEP 5: Assume K is known and is 250ml/min (based on choice of dialyzer (KoA)) and Qb is 400ml/min to calculate t
t(treatment duration) = 58,800ml / 250 ml/min = 235 minutes or 3.9 hours
Therefore, this patient will need to be dialyzed for 4 hours to achieve goal single pool Kt/V of 1.4.
- The delivered Kt/V often tends to be lower than the prescribed Kt/V which is why we target a spKt/V of 1.4 to achieve a minimum goal spKt/v of 1.2.
- When calculating treatment time using manufacturer’s provided in vitro K and KoA values (as we did in this case), it is important to remember that these values tend to be about 20% higher than actual in vivo values. This is one of the reasons why delivered Kt/V may be lower than prescribed Kt/V.
- In smaller patients, determining treatment time using goal spKt/V of 1.4 may lead you to under dose dialysis since a low ‘V’, mathematically increases the ratio spKt/V, gives a false sense of reassurance and may prompt you to decrease dialysis time. Smaller patients have higher post dialysis urea rebound, so their eKt/V may be significantly lower than spKt/V. In addition, short dialysis treatments may not be adequate to remove middle molecules and other uremic solutes as well as may result in higher ultrafiltration rates and intra-dialytic hypotension. Therefore before reducing dialysis time, other variables such as BP, phosphorus and potassium control, ultrafiltration goals etc. need to be considered. US KDOQI 2006 recommends a minimum session length of 3 hours irrespective of Kt/V.