Constant rate infusions (CRIs) are used to manage a wide variety of conditions in veterinary medicine. Their use is especially beneficial in drugs with a short half-life.

By removing the need for frequent redosing, a CRI makes drug administration easier and less prone to error.

Even drugs with long half-lives, however, may be delivered via CRI to maximize clinical benefits while minimizing the risk of side effects.

Drugs that are frequently delivered by CRI include the following:

• Analgesics, such as morphine, fentanyl, ketamine, lidocaine, and dexmedetomidine
• Anesthetics, such as narcotics, propofol, and etomidate
• Anti-seizure meds, such as phenobarbital, diazepam, and midazolam
• Cardiovascular drugs, such as dopamine, dobutamine, epinephrine, procainamide, diltiazem, and nitroprusside

In addition, a variety of other medications, such as insulin, glucagon, mannitol, and furosemide, may also be delivered by CRI. Even some antibiotics may be administered via CRI, as long as they function via time-dependent killing and not concentration-dependent killing. 

CRI Calculations: Tricky but Not Impossible!  

If you don’t administer CRIs on a regular basis, you may not be comfortable with calculating dosing information off the top of your head. That’s okay! There are many resources available to walk you through these calculations, both online and in textbooks. Smartphone calculators can be helpful, but it’s important to understand the math and logic behind the calculations so that you can check your work and decrease the likelihood of errors. 

When you look up a CRI dose, it will be expressed as an amount of drug per unit of patient body weight per unit of time. For example, you may see a CRI dose written as 30 ug/kg/min or 1.8 mg/kg/hr. Your goal is to use this written dose to determine a practical way to deliver the medication, either directly (by syringe pump) or by adding the drug to a carrier (such as IV fluids). 

Calculating CRI Doses 

Imagine that you have a 10 kg dog in need of a 10 ug/kg/min CRI of lidocaine. The concentration of lidocaine is 20 mg/ml. How can you use this information to treat your patient?

First, multiply the dose by the dog’s weight, like you would do if calculating any other drug dosage. This will allow you to determine how much medication the dog needs per minute:

10 kg x 10 ug/kg/min = 100 ug/min

The ug/min dose can then be converted to mg/min, in order to make further calculations more simple: 

100 ug/min x 0.001 mg/ug = 0.1 mg/min

Next, divide the dose by the concentration of the drug, to determine how many milliliters of drug are required for each minute of therapy:

(0.1 mg/min) / (20 mg/ml) = 0.005 ml/min

You can also convert the dose from ml/min to ml/hr, in order to match the format that is typically required for syringe pump administration:

0.005 ml/min x 60 min/hr = 0.3 ml/hr

If these initial calculations leave you with a practical, measurable drug volume, you may be able to deliver your CRI via a syringe pump. A dose of 0.3 ml/hr as calculated in this example, however, will be challenging to deliver with a syringe pump. 

While syringe pumps are often capable of delivering small doses, it is difficult to ensure accuracy and you run the risk of significantly interfering with delivery every time you flush an IV catheter, change a fluid line, etc. Therefore, many CRIs are delivered using a carrier, such as IV fluids. 

Delivering a CRI via IV fluids

Before adding any medication to a pet’s IV fluids, it’s important to check labels to ensure that the drug is compatible with the fluids (and fluid additives) the pet is receiving. As long as no incompatibility is noted, this is often the preferred method for delivering a CRI. 

Now, let’s assume that the 10 kg dog from the above example is receiving maintenance fluids at a rate of 25 ml/hr. How much lidocaine (20 mg/ml) should you add to a 1 L bag of fluids to deliver a lidocaine CRI at a dose of 10 ug/kg/min?

You have already calculated that the dog needs 0.3 ml/hr of lidocaine, but how can you use this knowledge to determine how much lidocaine to add to the fluid bag?

First, determine how many hours the bag of fluids will last the patient. You can do this by dividing the volume of the bag by the fluid rate: 

(1000 ml) / (25 ml/hr) =  40 hours 

Next, determine the drug volume that must be added to provide 40 hours of continuous therapy for the patient: 

40 hrs x 0.3 ml/hr = 12 ml 

Therefore, you must add 12 ml of lidocaine to the 1 L fluid bag in order to provide a CRI of 10 ug/kg/min lidocaine for the next 40 hours. Before adding your lidocaine, however, it is important to remove 12 ml of fluids from the bag. (This ensures that your final volume is 1000 ml and not 1012 ml!) 

Additional Resources 

There are a number of CRI calculators available online and via smartphone apps. 

The Veterinary Anesthesia and Analgesia Support group, in particular, maintains a number of CRI calculators and other resources designed to assist with the practical usage of CRIs in veterinary medicine.

Regardless of the calculator you’re using, however, it’s important to understand the basic math behind these calculations. This will allow you to minimize errors and confirm any calculator results that don’t make sense.