RRT Calculator
This page will guide you step by step through prescribing renal replacement in the Macclesfield DGH ICU. It should not be used to prescribe renal replacement in any other ICU and should only be used with the PrismaFlex machines that we have in CVVHDF mode.
Enter values or modify defaults below:
Explanation:
Renal replacement in the ICU at Macclesfield is relatively infrequent, so we have tried to keep it as simple as possible while still delivering an evidence based dose and avoiding filter clotting problems. Specifically:
- We use continuous, rather than intermittent therapy.
- We deliver the dose as 50% haemodialysis and 50% pre-diluted haemofiltration.
- Anticoagulation is with a LMWH infusion. Where there are contraindications to anticoagulation, we use none.
The idea of this calculator is to assure the delivery of a prescribed dose. Currently there is no ICU evidence for convective over diffusive therapies or intermittent vs continuous RRT, so we ask that doctors concern themselves mostly with dose rather than mode of delivery.
1. Get a form
Find a renal replacement prescription form. They are kept in a filing cabinet in the office. Put a patient label on it. Tick the appropriate boxes in the top three sections:
filter
We use the Hospal M150 AN69 filter set. It has a surface area of one and a half square meters, (M150) and uses a surface treated (ST) version of the AN69 acrylonitrile / methallyl sulphonate biocompatible membrane. The surface treated version is slightly more expensive but does two things. It makes it safe to use with ACE inhibitors and it binds heparin. This means it is less likely to clot than a non-ST filter if used with no heparin infusion.
prime
The filter set has a volume, including all tubes of 189ml. It needs priming with a litre of saline 0.9% containing 5000iu of unfractionated heparin while all the bubbles are removed. Always prime the filter with heparinised saline, even if the patient has contraindications to anticoagulation.
Washback, the fluid used to flush the blood back into the patient when the treatment is finished, is always saline 0.9% and we always lock the catheter with enoxaparin 5mg/ml, which is the same strength as in the infusion.
dialysis / replacement solution
We always use Prismasol 4 for both dialysis and replacement solutions.
2. Now go to the patient’s bedside and collect some information
Determine the patient’s actual body weight and height. You need the height because although anticoagulation is prescribed by actual body weight, renal replacement dose is prescribed by ideal body weight. (IBW). Write them on the chart in the top right hand corner.
Get hold of yesterday’s renal replacement observation chart. From this, determine:
Blood pump rate
This should normally be about 250 mls/min. The higher the blood flow, the lower the dialysate and replace rates will have to be for the same dose. Also, the filter is less likely to clot at high blood flow rates because the filtration fraction will be less. It may not have been possible to get the blood flow up to 250ml/min. The line may not be good enough or the patient may be too unstable. If this has been the case, write down the average rate that has been possible so far on the form.
Downtime
The dose of renal replacement delivered is affected by the “downtime” – the time for which it is interrupted for filter changes, trips to CT etc. Write on the form the time in hours during the next 24 hours for which the filter is likely to be down. 4 hours per 24 is a good start if you have nothing else to go on. Add one or two hours if a scan or surgery is planned.
3. Decide how much fluid to remove
You will have to take into account fluid balance, presence of pulmonary or peripheral oedema, cardiovascular stability and any extra infusions, eg blood. Removing more fluid increases the ultrafiltration rate and therefore the clearance. This will reduce the required dialysate and replacement flow rates for the same dose. In an emergency it may be necessary to remove a lot of fluid quickly at the start of a treatment. However, a secondary analysis of the RENAL trial in 2019 suggested that a net ultrafiltration rate of more than 1.75ml/Kg/hr (actual body weight) was associated with a lower 90 day survival. The calculator will warn you if you excede this.
4. Decide on a dose
Prescribing the right “dose” of renal replacement is important. By dose, we mean the clearance per unit time that the machine is doing and for how long. This is expressed as ml/kg/hour in continuous therapies.
While there used to be evidence for better outcomes with higher doses, two large multi-center randomized controlled trials: the Veterans Affairs/National Institutes of Health Acute Renal Failure Trial Network (ATN) study and the Randomized Evaluation of Normal versus Augmented Level (RENAL) Replacement Therapy Study, have provided level 1 evidence that clearences above 25ml/kg/hr do not improve outcomes in patients in the ICU.
The calculator defaults to 35ml/Kg/hr because 25 seems to correct things unacceptably slowly. Clinicians still have the freedom to prescribe whatever dose they like.
5. Go and sit down next to a computer, or use your phone
Enter all the numbers you have found into the top half of the calculator at the top of the screen and press “calculate”. This will give you the “Pre Blood Pump”, “Dialysate” and “Replacement” rates and the rate to run the enoxaparin at. Please note that “Pre or Post” should always be set to “Post”. This calculator gets around the following problems:
- Renal replacement dose should be by ideal body weight. This is the weight that should be put into the machine. Dialysing short, fat people by actual body weight may cause problems with line flow and considerably overdose them.
- Evidence for dose is in ml/Kg/hr, but what you are being asked to prescribe is rates on pumps. You could just use the total effluent rate (dialysate + replace + fluid removed) as the clearance, but actually it is also affected by blood flow rate and predilution inefficiency. The PrismaFlex allows you to dial in rates converted to ml/Kg/hr but this is effluent rate, not clearence.
- Even having prescribed an adequate dose, it usually doesn’t get delivered because of downtime. The calculator gives pump rates that will deliver the prescribed dose taking account of all the inefficiencies, in the time for which it is likely to be possible.
ANTICOAGULATION
If there are no contraindications to anticoagulation we use exoxaparin 5mg/ml for infusion and line lock. This requires no anticoagulation monitoring. For a few months we recorded anti-Xa levels, but no longer do so. The levels that we did were on the low side, but no worse than those recorded in studies of critical care patients on 40mg of enoxaparin od. So anticoagulating the filter with LMWH is providing adequate systemic thromboprophyllaxis.
If there are contraindications to anticoagulation, such as:
- recent CVA
- platelet count < 20
- recent surgery
- high risk of haemorrhage
then no infusion is used, although the filter and set are still primed with saline containing unfractionated heparin.
If the filter is anticoagulated, don’t anticoagulate the patient.
The enoxaparin infusion dose has been calculated to provide adequate systemic thromboprophyllaxis as well as filter anticoagulation, so do not give normal thromboprophyllaxis (eg sc enoxaparin) while on an enoxaparin infusion. However, it must not be relied on to fully anticoagulate the patient.
If the patient is fully anticoagulated, don’t anticoagulate the filter.
eg: INR > 2 or APTTR > 2, patient fully anticoagulated with a treatment (not prophylactic) dose of LMWH, eg tinzaparin 175iu/Kg or enoxaparin 1mg/Kg.
NOTES:
1. The “Pre Blood Pump” is pre-dilution, which is less efficient than post-dilution. Our calculator allows for this inefficiency and it will never be an issue with current evidence-based doses. The pre/post split capability of the PrismaFlex exists mainly to support the use of citrate anticoagulation, which we do not use. However, if you use NO post-dilution, (which you set on the “Replacement” pump) there will be a blood / air (rather than fluid / air) interface in a chamber of the machine, increasing the risk of clotting. For that reason, we always set the “Replacement” pump at 200mls/hr. This will use a single 5 litre bag every 24 hours. The calculator allows for this addition to the UFR, so you may notice that the pre blood pump and dialysate rates are slightly less than they were when we used the Prisma, for a given prescription.
2. The “red numbers” that you see on the right hand side of the PrismaFlex screen when you are setting the flow rates will not exactly add up to the dose prescription for the following reasons:
- MaccGas has increased the dose to compensate for estimated downtime.
- The red numbers do not allow for pre dilution inefficiency.
- The red numbers do not take into account either fluid removal or blood pump rate, both of which affect the dose.
- There is no single display of “total clearance”, you have to add the HF and HD rates.