Can You Calculate A Drip Rate?

We rarely need to calculate a drip rate due to our fluid pumps doing all the work for us. But what if your fluid pump gives out, or your pumps are all being used by other patients and the doctor tells you a fluid rate to start a patient on with no pump?

Do you know how to do this? Let's walk through this medical calculation together by doing some word problems:

Problem 1

You are admitting a patient to the hospital. Charley is a 10-year old female spayed Labrador with gastroenteritis. The hospital is very busy and all of the fluid pumps are currently being used by other patients. You are asked to start fluids at 180 mL/hr for Charley after giving her a 400 mL bolus. Calculate the drip rate for this patient after her bolus is given. Her body weight is 80 pounds. The drop size for your administration set is 10 drops/mL.

The body weight given in this question is irrelevant. You already know what rate you need because the doctor gave it to you (180 mL/hr). You would only need the body weight if you were calculating her actual fluid rate (the drip rate is the drops per minute).

Formulas for drop sizes per mL are as follows (you may wish to memorize these):
10 drops/mL: (mL/hr) / 6 = drops/minute
15 drops/mL: (mL/hr) / 4 = drops/minute
20 drops/mL: (mL/hr) / 3 = drops/minute
60 drops/mL: (mL/hr)/ 1 = drop/minute

So to calculate using these simple formulas:
180 mL/6 = 30 drops per minute

If you did not know this formula, or if you forget, just convert it using what you know. First convert hours into minutes:
180 mL per hour divided by 60 minutes in an hour: 3 mL per minute
You administration set will give 10 drops per 1 mL
So, multiply 3 mL/min X 10 drops/1mL (the mLs cancel out) =  30 drops/minute

Now check your math:
30 drops per minute X 60 minutes in a hour (minutes cancel out)= 1800 drops per hour
Your set gives 10 drops in 1 mL. 1800 drops/10 drops/mL (drops cancel out) = 180 mL per hour (correct)

Now let's try a different problem solving 2 ways, with and without the above formulas:

Problem 2

Your infusion pump is broken and a patient needs a chemotherapy cocktail of 500 mL to be delivered over 5 hours. Your set delivers 20 drops/mL. What is the drip rate?

Solution 1:

500 mL/5 hours = 100 mL/hr

100 mL/hr X 20 drops/mL = (mLs cancel out) = 2000 drops/hr

2000 drops/hr / 1 hr/60 minutes = (hr cancel out) = 33.3 drops/minute

Solution 2 (using the above formulas):

The first step is the same. 500 mL/5 hours = 100 mL/hr

You know 20 drops/mL gives 3 drops per minute because you remembered the formulas.

100 mL/3 = 33.3 drops/minute

(So you would give the infusion at 33 drops per minute since you can't give 0.3 drops) over approximately 5 hours as instructed.