A continuous rate infusion (CRI) is a dosing regimen used to deliver a constant amount of drug per unit time. The most common
CRIs are administered intravenously; however, other methods such as transdermal deliver mimic the characteristics of CRIs.
Intravenous CRIs are usually implemented when short-acting drugs are desired for long periods (e.g., propofol) or when consistent
plasma concentrations of drugs are desirable (e.g., fentanyl). While very commonly used for anesthesia and analgesia, CRIs
also are useful for administration of some antibiotics, antiemetics, and anticonvulsants. While the drugs used can vary,
the underlying principals are similar.
Most CRIs are of drugs with short duration of action. This technique facilitates rapid and accurate titration of dose to
meet individual patient requirements. Drugs with longer duration of action such as morphine are also given by CRI, but the
ability to rapidly change plasma drug concentration is blunted. Drugs with very long half lives are usually not given by
CRI because intermittent dosing is technically easier. An exception would be when a long acting drug is slowly titrated up
in effect to limit toxicity (e.g. phenobarbital).
Intravenous infusions are usually most accurately delivered with syringe pumps. Separating the drug from the intravenous
fluid also allows independent dosage adjustment when needed. However, adding drug to intravenous fluids is also a useful
method, especially when a relatively consistent dose is anticipated over a prolonged period. Specialized delivery devices
are also available for infusion of drugs over a period of time and can be utilized when desired.
Transdermal delivery systems are similar to intravenous CRIs in that a relatively consistent delivery of drug is achieved.
However, transdermal delivery has additional variables that require the practitioner to focus attention on the response of
the patient. For example, with transdermal fentanyl patches there is considerable individual variation in the patch contact
and delivery. Patients should be assessed for adequate therapeutic effect. Additionally, careful attention should be paid
to compounded transdermal medications that reportedly have sustained release. Transdermal delivery is a rather complex process
and not all medications available are well absorbed by this route.
The first step for calculation of a CRI is determining the amount of drug needed per unit time. This requires three things:
1. Patient weight
2. Desired dose
3. Duration of the infusion
An example is delivery of fentanyl to a canine patient during anesthesia. The dog weighs 10 kg, the desired dose is 5 mcg/kg/h
and the infusion is anticipated to last 3 h. The amount of drug needed in the 3 hour period is:
10 kg × 5 mcg/kg/h × 3 h = 150 mcg
The next step would be to determine the volume of drug you need in the time period. This requires that you know the concentration
of the drug. In the case of fentanyl, it is commonly available as 50 mcg/mL. Therefore:
150 mcg / 50 mcg/mL = 3 mL of fentanyl
It is worth noting that at this point some drugs may be loaded onto a syringe pump and delivered without dilution. Fentanyl
is a good example of this. However, some drugs are too concentrated to be accurately delivered without diluting them (e.g.
dopamine). The final volume is empirical and should be small enough for easy administration, yet large enough to allow accurate
delivery. With the example of fentanyl, I would typically dilute the 3 mL of 50 mcg/mL fentanyl into 15 or 30 ml of lactate
Ringers solution and deliever at 5 or 10 ml/h. This is easily achieved on a syringe pump. If a pump is not available, you
might add the fentanyl to a small bag of fluid and administer with an intravenous drip set. If a 250 ml bag is available you
could add the 3 mL of fentanyl then deliver at appx 83 ml/h (250 mL / 3 h = 93.3 mL/h). One word of caution, double check
that the volume or rate of fluid administration does not exceed the patient's capacity (e.g. 10 mL/kg/h during anesthesia
or 60 mL/kg/day at 1x maintenance).
Drugs with a relatively large therapeutic dose range (e.g. fentanyl) require less accuracy with administration than drugs
with narrow therapeutic ranges (e.g. dopamine). Syringe pumps are often preferred when accurate dosing is required.
A quick check of the math is to make sure the volume of drug seems reasonable. For example if the amount of drug required
is on the order of liters, you probably have a math error. Additionally, you can estimate the amount needed by taking the
amount given by intermittent injection divided by the dosing interval. For example if you usually would give the patient
0.5 ml of morphine every 6 hours you would anticipate using a similar amount in a 6 hour infusion.
Another common example would be adding morphine to intravenous fluids for overnight pain management following significant
orthopedic surgery. Again using the 10 kg dog:
1. Patient weight = 10 kg
2. Desired dose = 0.1 mg/kg/h
3. Duration of infusion = HERE IS THE TRICKY PART