Medical math: practical use of constant rate infusions (CRIs) in practice (article and videos).
Originally published on Veterinary ECC Small Talk: https://www.veteccsmalltalk.com/vetemccsmalltalk/2017/8/8/practical-use-of-constant-rate-infusions-criselliot-kneba
What are constant rate infusions?
Constant rate infusions (CRIs) are a simple, flexible, and inexpensive way to administer intravenous medications to hospitalised and surgical patients. They are administered using standard fluid pumps or with syringe drivers (either electronic or spring loaded), and many medications can be added to a patient's pre-existing intravenous fluids. Several different medications can be administered as constant rate infusions, including anaesthetic, analgesic, sedative, antiarrhythmic, and anticonvulsant agents.
This article will aim to give examples of patients that may benefit from a CRI and discuss the mathematics and logistics behind administering them. Detailed explanation of the use, effects, and interactions of specific drugs are beyond the scope of this article, but may be found in formularies such as the BSAVA Formulary or Plumb’s Veterinary Drugs Handbook, as well as various other texts.
When can a CRI be useful?
1) Painful elective procedures such as orthopaedic or thoracic surgery.
2) Moderate to severe post-operative pain.
3) Trauma such as bite wounds, road traffic accidents, or fractures.
4) Patients with co-morbidities such as cardiac or respiratory compromise who would benefit from the anaesthetic gas sparing effects of CRIs.
5) Patients with hepatic or renal disease that may not be good candidates for anti-inflammatories or
6) Treating seizures or arrhythmias refractory to boluses of medication.
7) Treating electrolyte or glucose imbalances.
8) Metabolic conditions such as diabetic ketoacidosis.
How to setup a CRI using a fluid bag
1) Decide what medication(s) you wish to administer and ensure they are appropriate for your patient. If using multiple drugs, ensure there are no interactions between them. Websites such as www.drugs.com have good information on drug interactions, but if you have any questions then check the Data Sheet, your formulary, or contact the manufacturer.
2) Decide at what rate you wish to administer the CRI. For example, if you are adding metoclopramide to a patient’s current intravenous fluids then you will use their fluid rate as a starting point. If you are making a separate bag for the CRI or using a syringe driver, then it is easiest to use even flow rates to make titrations easier (usually 2-10 mL/hour).
3) Decide your dose and create your mixture. Metoclopramide is typically given at 1-2 mg/kg/day.
a. Example 1: You have a 10 kg canine patient in with a suspected ileus. You have ruled out mechanical obstruction, so you want to trial a metoclopramide CRI to see if it will make any difference to their comfort and appetite. You do not have many fluid pumps, so you want to add the medication to the patient’s maintenance fluids. You decide on a starting dose of 1.5 mg/kg/day.
b. You calculate this patient needs 15 mg/day (1.5 mg/kg/day x 10 kg), or 0.63 mg/hr (15 mg/day x 1 day/24 hours).
c. The concentration of metoclopramide you have in stock is 1 mg/mL, so you calculate that 0.63 mg/hr is equal to 0.63 mL/hr (0.63 mg/hr x 1 mg/1mL).
d. The current fluid rate for your patient is 30 mL/hr (3 mL/kg/hr) and the size of the fluid bag is 1,000 mL. You calculate that this bag of fluids will last the patient approximately 33.3 hours.
e. Since the patient needs 0.63 mL/hr, and the bag will last them 33.3 hours, you calculate that you will need to add 21 mL of 1 mg/mL metoclopramide to the bag.
f. You decide this is too expensive for the client, so you opt to use a 500 mL bag instead. You now add 10.5 mL of metoclopramide to this bag and start the mixture at 30 mL/hr.
4) Monitor patient response and titrate as necessary. To adjust your dose, divide your desired dose by your current dose and multiply it by the current rate of administration.
a. (1 mg/kg/day / 1.5 mg/kg/day) x 30 mL/hr = 20 mL/hr
b. (2 mg/kg/day / 1.5 mg/kg/day) x 30 mL/hr = 40 mL/hr
5) How to setup a CRI using a syringe pump
1) The concept is like using a fluid bag, except that you may choose to deliver the drug undiluted if you wish. You will also be using a lower rate of administration, so using a dedicated IV catheter, or a port very close to the existing IV is important to prevent under/over dosing the patient. Be sure to have your syringe drivers calibrated as recommended by the manufacturer and do not ignore warning messages.
2) Decide on the starting dose for the CRI. Doses are generally listed as either dose/kg/hour or dose/kg/minute. For example, Fentanyl can be administered at rates from 0.02 to 0.08 micrograms/kg/minute, which is 0.0012 – 0.0048 milligrams/kg/hour. Be sure not to confuse micrograms (generally written as mcg or µg) with kilograms as the difference is a factor of 1000.
a. Example 2: A 10kg dog presented with severe pancreatitis. You feel that despite methadone administration your patient is still uncomfortable. You decide to run a fentanyl CRI at a dose of 0.03 µg/kg/minute at a rate of 3 mL/hr.
i. By making your flow rate the same number as your dose, you can easily titrate up or down. If you wanted to administer 0.02 µg/kg/min, you change your fluid rate to 2 mL/hr, etc.
b. You calculate this patient needs 0.3 µg/min, which is equal to 0.018 mg/hr.
i. 10 kg x 0.03 µg/kg/min = 0.3 mcg/min.
ii. 0.3 µg/min x 60 minutes per hour = 18 µg/hr.
iii. 18 µg/hr / 1000 mcg per mg = 0.018 mg/hr.
c. You decide to use a 60 mL syringe to maximise the duration of the CRI. If you are administering the CRI at 3 mL/hr, then your mixture will last 20 hours (60 mL / 3 mL/hr)
d. You know that your patient needs 0.018 mg/hr, so to last 20 hours, the 60 mL syringe will need to contain 0.36 mg of fentanyl (0.018 mg/hr x 20 hr). The standard concentration of fentanyl is 50 µg/mL (0.05 mg/mL), so you calculate how many mL you need
i. 0.36 mg / 0.05 mg/mL = 7.2mL of fentanyl added to the 60 mL syringe.
e. You pull up 52.8 mL of 0.9% NaCl into your syringe, then add the 7.2 mL of fentanyl, label the syringe, and start administering it at 3 mL/hr.
3) Give a loading dose if necessary. Loading doses are typically given for fentanyl, lidocaine, and ketamine, although a loading dose of fentanyl may not be necessary if the patient already has analgesia on board.
4) Titrate your CRI. This may be may involve increasing or decreasing the rate depending on the patient’s response and changing needs. Drugs such as fentanyl accumulate over time, especially in patients with hepatic disease, so be sure to re-evaluate patients frequently.
5) Alternatively, if you did not want to create a dilution or were not concerned with the rate of administration, you would stop after Step B in the example above and calculate the mL per hour. In the above example, our patient with need 0.36 mL/hr of pure fentanyl. Most syringe drivers will only administer in 0.1 mL increments, so you would need to round up or down, or decide on a different dose to be accurate.
A note on CRI calculators: The only person accountable for the accuracy of the medication administered is you. Do not assume any calculator is accurate – always verify them before use. Some CRI calculators assume a standard drug concentration. Always double check the concentration assumed by the calculator against your own stock, as they may differ.
A short video about how to calculate a fentanyl CRI for veterinary practices.
