2+2 = ? CRIs and Math!
Megan Brashear, BS, CVT, VTS (ECC)
VCA Northwest Veterinary Specialists Clackamas Oregon
Understanding how to perform basic and advanced math calculations is an important aspect of veterinary nursing. Not only should the nursing team be comfortable performing calculations, but they should also serve as a double check for math performed by the veterinarian.
All drugs are measured both in weight and in volume, and both pieces of information are necessary to perform calculations. Weight is commonly measured in grams, milligrams, micrograms, milliequvalents, or millimoles and represents how much of the drug is present. Volume is commonly measured in milliliters or liters and describes the amount of liquid the weight of drug is suspended in. When both the weight and volume are given, that is the drug concentration. For example, a drug concentration of 20mg/ml means that in each 1ml of liquid is dissolved 20 milligrams of drug.
It is important while working in the hospital to agree between nurses and veterinarians that drug orders are always listed by weight and not volume. Many drugs come in multiple strengths which can drastically change the dose. 0.5ml of a drug that is 1mg/ml is 0.5mg, but 0.5ml of that same drug with a concentration of 10mg/ml is 5mg. By keeping the orders in weight, the appropriate volume will always be calculated by the nursing team.
The first step to calculating medical math is determining the volume needed when administering a drug. Start with the order:
Administer 22mg/kg to a dog weighing 10kg; drug concentration is 100mg/ml
22mg x 10kg = 220mg (this is your dose)
Dose (220mg) / Concentration (100mg per ml)
220mg / 100mg per ml = 2.2ml
Remember dose divided by concentration as this is how to calculate all drug volumes. The concentration of drug will always be listed on the bottle, and the dose will be given as an order, or the weight per kilogram of body weight will be given and need to be calculated. Another example:
Administer 0.5mg/kg diazepam (5mg/ml) to a 7kg dog
0.5mg x 7kg = 3.5mg (dose)
3.5mg (dose) / 5mg per ml (concentration) = 0.7ml
Some drugs are defined as percentages. Furosemide is labeled as 5%, mannitol as 20%, and enrofloxacin as 2.27%. In order to determine the concentration of these drugs, move the decimal point one place to the right. 5% is the same as 5.0%; move the decimal one place to the right and a drug that is 5.0% has a concentration of 50mg/ml. Mannitol that is 20% (the same is 20.0%) has a concentration of 200mg/ml. Enrofloxacin at 2.27% has a concentration of 22.7mg/ml.
The order is to administer 2.5mg/kg of furosemide (5%) to a cat weighing 6.2kg
2.5mg x 6.2kg = 15.5mg (dose)
15.5mg (dose) / 50mg per ml (concentration) = 0.31ml
The order is to administer 2mg/kg of lidocaine (2%) to a dog weighing 32kg
2mg x 32kg = 64mg (dose)
64mg (dose) / 20mg per ml (concentration) = 3.2ml
When given a drug with a concentration in micrograms, milliequivalents, or millimoles, it is important to ensure that the units match (the dose and the concentration both match) prior to calculating the dose. For example: Administer 3mcg/kg of fentanyl (50mcg/ml) to a 22kg dog
3mcg x 22kg = 66mcg (dose)
66mcg / 50mcg per ml = 1.32ml (both the dose and concentration are in mcg)
If the units do not match, that step must be done prior to calculating the volume. For example, the order reads: Administer 4.5grams of mannitol (20%) to a dog
4.5 grams (dose) / 200mg per ml (concentration) units don’t match!
4.5grams x 1000mg per gram = 4,500mg (dose)
4,500mg (dose) / 200mg per ml = 22.5ml
Calculating fluid additives is done in a similar manner, there is an ordered dose and a drug concentration, but these drugs are added to an amount of IV fluids. Most fluid additives are ordered per liter; for example KCl 20mEq/L. This means that 20mEq of potassium chloride (2mEq/ml) are added to 1 liter of crystalloid fluids.
20mEq (dose) / 2mEq per ml (concentration) = 10ml added to 1 liter
The order reads: Make IV fluids 15mg/L metoclopramide (5mg/ml)
15mg (dose) / 5mg per ml (concentration) = 3ml added to 1 liter
On some occasions, the liter of fluids will be partially empty and the medication needs to be calculated for the remaining fluids. To do this calculation, it is necessary to first calculate the remaining factor by taking the amount of fluids left in the bag and dividing it by the amount of fluids in a full bag. For example:
Make fluids 15mEq/L KCl (2mEq/ml); 600ml remaining in liter
600ml / 1000ml = 0.6 (remaining factor)
15mEq (dose) x 0.6 (remaining factor) = 9mEq (adjusted dose)
9mEq (adjusted dose) / 2mEq per ml = 4.5ml KCl added to 600ml; makes 15mEq/L
Make fluids 24mg/L metoclopramide (5mg/ml); 750ml left in liter
750ml / 1000ml = 0.75 (remaining factor)
24mg (dose) x 0.75 (remaining) = 18mg (adjusted dose)
18mg (adjusted dose) / 5mg per ml (concentration) = 3.6ml added to 750ml; makes 24mg/L
Dextrose is a medication commonly added to IV fluids and the order is to create a dextrose solution. For example, make the IV fluids 2.5% dextrose or 5% dextrose. In order to calculate this with any amount of IV fluids, the following formula should be used:
(amount of fluids in bag x % dextrose desired)
% of dextrose being added
Most hospitals are working with a stock solution of 50% dextrose. For example: Using 50% dextrose, make the remaining 400ml of fluid a 5% dextrose solution.
(400ml remaining in bag x 5% desired)
50% adding
400 x 5 = 2,000
2000 / 50 = 40ml of 50% dextrose added to 400ml of fluids will make a 5% dextrose solution
In order to be exact with math, an equal amount of fluid should be removed from the bag of fluids prior to adding medications. With small volumes (such as 3.4ml of metoclopramide) this is not necessary, but when creating 1 liter of a 5% dextrose solution, 100ml of 50% dextrose needs to be added. In this case of a large volume of additive is required and 100ml of fluid should be removed from the liter of fluids prior to adding the dextrose.
The importance of nurses understanding fluid additives is highlighted when fluid rates change and drugs have been added to the fluids. A 5.7kg cat is on IV fluids with 40mEq/L KCl (2mEq/ml) running at 18ml/hr. The orders are to increase the cat to 30ml/hr to replace his losses. With the increase in fluid rate he is going to receive a higher rate of KCl, but how much higher? In order to calculate this, the first step is to determine how many mEq of KCl are in each ml of fluid.
There are 40mEq in 1000ml
40mEq/1000ml = 0.04mEq/ml
The cat is receiving 30ml/hr
30ml per hour x 0.04mEq per ml = 1.2mEq the cat is receiving per hour
1.2mEq per hour / 5.7kg body weight = 0.21mEq/kg/hr
This value can be reported to the veterinarian and decisions made regarding fluid rates and additives. One of the great benefits of a nursing team that is comfortable with medical math means that extra double checks are available and hospitals can provide even better patient care.
While appropriate medical calculations are vital to patient care, appropriate labeling is also necessary when adding medications to IV fluids. Each infusion must be labeled with the drug name, the drug dose per milliliter or liter, the time and date the drug was added, an expiration date (if necessary), and the initials of the person who added the medications. Fluids with dextrose added require extra care as dextrose creates a hospitable environment for bacteria. Fluid bags and fluid lines containing dextrose should be discarded and changed out every 24 hours and special notice taken to ensure the IV catheter and IV line connection points are kept clean at all times. When multiple medications are added or ordered for infusion a drug compatibility chart must be consulted to ensure patient safety.
Constant Rate Infusions (CRI) are commonly used in veterinary medicine for a variety of patient needs. Any drug that is beneficial administered as a continuous infusion as opposed to bolus dosing can be calculated to be administered concurrently with IV fluids or as a single infusion. Drugs with short half-lives must be administered continuously in this manner. Common CRIs include pain medication, anesthesia, electrolytes, blood pressure management, and insulin. Veterinary nurses are responsible for calculating these constant rate infusions, understanding the effects of the drugs given, and managing patients that may be on a variety of medications at once.
Management of CRIs starts with knowledge of the drug being delivered and any special needs of that drug. Some drugs are light sensitive; others may bind to plastic, others may not mix well with concurrent fluids or medications and require a dedicated IV catheter. Veterinary nurses are responsible for a thorough understanding of these drugs and their special considerations and also need to understand the desired effect of administered drugs and monitor patients for those effects. They also need to anticipate any adverse effects and how to manage those. Finally, nurses must have a knowledge of drug dosages for CRIs so that if fluid rates are changed, drug dosages are still administered appropriately. Proper dosing requires a good understanding of calculations and quick thinking.
Metoclopramide is a drug commonly added to already running IV fluids to enhance gastrointestinal motility. For example, a 23kg dog has fluids running at 110ml/hr. The veterinarian orders a metoclopramide (5mg/ml) CRI at a dose of 2mg/kg/day. How many milliliters of metoclopramide do you add to a liter of fluids that will run at 110ml/hr?
2mg/kg/day = 46mg/day (24 hours) of metoclopramide (23kg x 2mg/kg/day)
46mg/24hours = 1.91mg metoclopramide per hour (46mg per day / 24 hours in a day)
1000ml/110ml per hour = 9.1 hours (1 liter of fluids will last this dog 9.1 hours)
9.1 hours x 1.91 mg per hour = 17.4mg metoclopramide needed per 1 liter of fluids
17.4mg/5mg = 3.5ml of metoclopramide needed (drug dose/drug concentration)
If you add 3.5ml (17.4mg) of metoclopramide to 1 liter of fluids running at 110mlhr, a 23kg dog will receive 2mg/kg/day of metoclopramide
As the nurse monitoring this patient, the calculation should be taken one more step to determine how many milligrams of metoclopramide are in each milliliter of fluid so that if the fluid rate is changed, the dose of the drug is known. If this patient's fluid rate is increased to 140ml/hr, how much metoclopramide is the dog receiving?
17.4mg / 1000ml = 0.0174mg/ml (number of milligrams per liter divided by 1000ml)
0.0174mg x 140ml/hr = 2.43mg/hr (milligrams per milliliter multiplied by fluid rate per hour to get mg/hr delivered to the dog)
2.43mg x 24hrs = 58.46mg/day (milligrams per hour x hrs in a day)
58.46mg/23kg = 2.5mg/kg/day (milligrams per day divided by patient weight to get mg/kg/day)
This new dose can be compared to the therapeutic dose of metoclopramide and adjustments can be made as needed.
Pain management and blood pressure management medications are commonly administered as a constant rate infusion because in this format these drugs can easily be titrated according to patient needs. For example, a patient needing orthopedic surgery for a fracture repair may require a higher dose of drug pre and intra-operatively but as they recover the dose can be decreased, and the patient weaned off. Blood pressure support such as dopamine often requires fine tuning of titration up and down until the patient responds appropriately. In these situations, it is advantageous to create the CRI so that a fluid rate of 1ml/hr delivers a dose of 1mg/kg/hr to the patient (or, depending on the dosage, 1ml/hr provides 1mcg/kg/hr or 1mcg/kg/min to the patient). This way, when doses need to be changed quickly, it only involves a fluid rate change and not more math.
You will be monitoring anesthesia on a 12kg dog that sustained vehicular trauma and needs pain management. You are asked to create a fentanyl (50mcg/ml) CRI and start administering the dog 5mcg/kg/hr with the option to titrate up or down depending on patient response. You plan on making the CRI in a 100ml bag of 0.9%NaCl.
1ml/hr needs to deliver 12mcg/hr
100ml bag = 100 hours of CRI (assuming 1ml/hr, since 1ml/hr = 1mcg/kg/hr)
12mcg x 100hrs = 1200mcg (dose per hour x hours of CRI)
1200mcg/50mcg per ml = 24ml of fentanyl needed (drug dose/drug concentration)
Run the CRI at 5ml/hr to deliver 5mcg/kg/hr to your 12kg patient
Because this calculation used 100ml as the final volume of the CRI, you must first remove an equal amount of 0.9%NaCl from the bag before adding your fentanyl. In this case, remove 24ml of 0.9%NaCl and discard it before adding 24ml of fentanyl. The label should read “Fentanyl 1200mcg QS 100ml NaCl”. The QS stands for quantity sufficient and means the total volume of the CRI is 100ml.
Look closely at drug doses as some CRIs are dosed as mcg/kg/min. Dopamine is a typical example of such a drug. This drug dose is used within a range and may need frequent titration until an appropriate response is reached. Creating the CRI so that 1ml/hr delivers 1mcg/kg/min will make these changes easier on the nursing team.
A 17kg dog has been battling hypotension, and you are asked to create a dopamine (40mg/ml) CRI and start administering 5mcg/kg/min to the dog. For ease of titrating, you will make the CRI so that 1ml/hr delivers 1mcg/kg/min. You will be making the CRI in a 250ml bag of NaCl.
1ml/hr needs to deliver 17mcg/min
17mcg x 60 minutes = 1020mcg/hr (because the fluid rate is in ml/hr, you need to convert the dose to mcg/hr)
1020mcg/1000 = 1.02mg/hr (because your drug concentration is in mg/ml you need to convert mcg to mg)
250ml bag = 250hrs (assuming a rate of 1ml/hr)
1.02mg x 250hrs = 255mg of dopamine needed for this CRI (the dose per hour x hours)
255mg/ 40mg per ml = 6.34ml of dopamine needed (drug dose / drug concentration)
For this CRI, you need to add 6.34ml of dopamine to 234.7ml of 0.9%NaCl. To deliver 5mcg/kg/min to a 17kg dog, run the CRI at 5ml/hr. If the dose needs to increase to 7mcg/kg/min, just increase the fluid rate to 7ml/hr. The label for this CRI should read "Dopamine 255mg QS 250ml NaCl”.
Common drugs used in constant rate infusions:
- Fentanyl: 100 times more potent than morphine, only lasts ~30 minutes after IV injection
- Loading dose of 2-5mcg/kg
- 2-5mcg/kg/hr for pain management; 10-45mcg/kg/hr for anesthetic effects (ventilate)
- Ketamine: Helps strengthen response to opioids, helps with wind-up pain
- Loading dose of 0.5mg/kg
- 10mcg/kg/min for surgical stimulation, then 2mcg/kg/min post-operatively
- Lidocaine: Supplement to anesthesia (lowers inhalant requirements)
- 25-50mcg/kg/min
- Dopamine: Increases cardiac output and causes vasoconstriction (dose-dependent)
- 2-10mcg/kg/min
- Dobutamine: Increases cardiac contractility without increasing heart rate
- 5-20mcg/kg/min in dogs; 2mcg/kg/min in cats
- Norepinephrine: Causes vasoconstriction, used for blood pressure management in sepsis
- 0.5-2mcg/kg/min
- Metoclopramide: Gastrointestinal motility, used in ileus and post-operative GI surgery
- 1-2mg/kg/day
- Potassium Chloride: Supplement in hypokalemia
- Do not exceed 0.5mEq/kg/hr
References:
Bryant, Susan. Anesthesia for Veterinary Technicians. Ames, IA: Wiley-Blackwell, 2010
McKelvey, Diane, K. Wayne. Hollingshead, and Diane McKelvey. Small Animal Anesthesia & Analgesia. St. Louis: Mosby, 2000
Seymour, Chris, and Tanya Duke-Novakovski. BSAVA Manual of Canine and Feline Anaesthesia and Analgesia. Gloucester, England: British Small Animal Veterinary Association, 2007