Let's work through the math on calculating constant rate infusion word problem!

An 11-pound Yorkshire Terrier has been prescribed a 2 mg/kg/day constant rate infusion of metoclopramide.  The metoclopramide is to be added to the intravenous fluids.  If the concentration of metoclopramide is 5 mg/ml and the Yorkie’s fluid rate of Lactated Ringer's (LRS) is 20 ml/hr, how many milliliters of metoclopramide should be added to a 500 ml bag of intravenous fluids?

First convert pounds into kilograms by dividing pounds by 2.2 (there are 2.2 pounds per kilogram):

11 pounds/ 2.2 pounds per kg (pounds cancel out) = 5 kg

Now take the kg and multiply by the dose:

5kg X 2 mg/kg/day (kg cancel out) = 10 mg/day

Now divide by the number of hours in a day (24) to get the per hour rate:

10 mg/day / 24 hours/day (days cancel out) = 0.4 mg/hour

Next divide by the concentration of metoclopramide:

0.4 mg/hr / 5mg/mL (mg cancel out) = 0.08 mL metoclopramide/hour

Now find out how many hours this bag will last by taking the amount of fluids in the bag divided by this dog’s current fluid rate:

500 mL/bag/ 20 ml/hr = 25 hours of IV fluids/bag (at this current rate)

Lastly, take the number of hours this bag will last and multiply by the mL/hour of metoclopramide:

25 hours/bag X 0.08 mL metoclopramide/hour (hours cancel out) = 2 mL of metoclopramide / 500 mL bag

To keep it completely accurate, you would remove 2 mL of LRS fluids from the bag, and add back in your 2 mL of metoclopramide.

Want more practice? Check out this Link on Constant Rate Infusions via DVM360!