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Medication calculations
There are 4 basic steps to solving proportion problems:
- Set up a known ratio.
- Set up a proportion with known and desired units. Use x for the quantity that is desired or unknown.
- Cross multiply.
- Solve for x.
Example:
Dose Ordered: Medication B 15 mEq po every day. Available: Medication B 10 mEq/5ml. How many ml's will you give?
a) Set up proportion. (Units are matched therefore no need to convert- mEq to mEq and ml to ml)
b) Cross multiply and solve for x.
10๐๐ธ๐ 5๐๐
15๐๐ธ๐ ๐๐๐
10x=75 x= 75 10 15 mEq= 7.5 ml
Calculating IV Flow Rates:
To find hourly rate (stated in mls per hour)
Total volume of solution in mlโs = x mlโs Total amount of time in hours
Example: 1000 cc IV solution ordered to infuse over 8 hours.
1000 ml = 125 8 hrs. Answer: 125 ml/hour
Example: 1000 ml solution to infuse over 6 hours.
(Solve for X to determine the number of tablets, milligrams, milliliters, etc., you will administer to the patient by cross- multiplying)
1000 ml = 166.6 = 167 6 hrs.
Answer: 167 ml or ml/hr
(Remember: when answer does not come out evenly, round off to the nearest whole number. If 5 & greater round up, below 5, round down.)
To find flow rate stated in drops per minute:
Drop factor is the number of drops it takes to equal 1 ml with a specific type of IV tubing. The drop factor is stated on the tubing package.
ml/hr. x drop factor = gtts/min total amount of time in minutes
Example: IV order D51/2 NS at 120 ml/hr
The drop factor is a 15 gtts/ml and the flow rate is 120 ml/hr.
120ml/hr x 15 gtts/ml = 1800 = 30 gtts/min 60 mins/hr 60
IV drip calculations
Order is written as mg/hr. Example:
Order: Fentanyl 5 mg/hr. The bag is labeled 250 mg in 500 ml of solution. How fast will the IV need to be infused to give the correct dose?
- The IV rate will be as an hourly rate, so no conversion needs to be made for time. If the order was written with a different time, you would need to calculate the mg/hr. (use ratio and proportion)
- Put the problem in ratio and proportion.
5 mg = 250 mg 5 (500) = 250 x 2500 x = 10 ml/hr IV rate x ml 500 ml 250
