ordered 37.5 mg of a particular drug over 15 minutes. If the drug was available as 2.5 mg/mL of solution, how many mL would you need to give every 15 seconds?

Question

A physician has ordered 37.5 mg of a particular drug over 15 minutes. If the drug was available as 2.5 mg/mL of solution, how many mL would you need to give every 15 seconds?

Answer 1

This is easy to solve by focusing on the relationships and the units.  We know that the drug is 2.5 mg/mL in the solution, so we can say that 2.5 mg = 1 mL for the solution.  The doctor wants you to give 37.5 mg in 15 min, so we can say 37.5 mg = 15 min.  We are trying to find how many mL to give in 15 seconds. 

 

So our time is 15 seconds, and we need to relate the time to the other quantities.  Seconds and minutes are related by 60 sec = 1 min. 

 

At this point we have:

37.5 mg = 15 min

2.5 mg = 1 mL

60 sec = 1 min

 

Starting with seconds, we should be able to find minutes, right?  Then from minutes we can find mg of the drug.  From mg of the drug, we can find mL of the solution that needs to be delivered! 

 

Use these relationships as conversion factors, and cancel out the units you don't want while converting to the units you do want:

 

15 sec x (1 min/60 sec) x (37.5 mg/15 min) x (1 mL/2.5 mg) =  0.25 mL 

(notice all of your other units cancel!)

 

The "math" part on your calculator is 15 ÷ 60 x 37.5 ÷ 15 ÷ 2.5 = 0.25

 

I can teach you this method to use for any dosage/solution/IV drip rate calculation.  :)