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 is 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: Plug in the known variables for Kt/V
[K x 210min] / 42,000ml = 1.4
K = [42,000ml x 1.4] / 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: Refer to any normogram or calculators easily available online to decide what your dialyzer’s KoA should be in order to achieve a spKt/V of 1.4
STEP 7: Check the normogram to make sure the in vitro KoA of the dialyzer should at least be 766ml/min.
- As mentioned in the earlier case, in vitro KoA values (provided by manufacturer packet insert) are about 20% higher than in vivo values. Therefore when using in vitro K and KoA values to calculate dialysis prescription, it is important to confirm delivered spKt/V and adjust treatment as needed to meet goal.