Ep0026: Med Math Equations

Dosage calculation is something that every nursing student either loves or dreads! We all have to pass math exams every semester. It’s either easy points or makes you a nervous wreck! If you’re the second type, I hope this little explanation helps simplify it a bit for you. Because after all, we believe here at NRSNG that nursing school shouldn’t be that hard.  There are four main types of questions we get. 1 is drip rate or drops per minute. 2 is total infusion time, 3 is dosage, and 4 is IV pump rate – always in mililiters per hour. Let’s talk through the basics of how to do each of these.

Calculating drip rate, while not used very often anymore, is very important to know in case pumps fail or the power goes out. The information you need for this formula is the total volume to be infused, total time in minutes, and the drip factor. The drip factor can be found on the packaging for the tubing and will likely be given to you in the problem in nursing school. So let’s say you need to administer 200 mL over 2 hours or 120 minutes. The drip factor is 15 drops per mL.  You multiply the total volume in mL by the drip factor (200 x 15) to get a total number of drops. Divide that by the time in minutes and you’ll be left with drops per minute. In this cast 200 x 15 / 120 = 25 drops per minute. Always round to the nearest whole drop. To titrate that on a roller clamp, you’d think that’s approximately 1 drop every 2 seconds, then time it for a whole minute for accuracy.

To find total infusion time, you will divide the total volume to be infused by the rate. For example “How long would it take to infuse 400 mL at 75 mL per hour?”.  400 divided by 75 = 5.333 or 5 ⅓ hours, which is 5 hrs and 30 minutes. Or how about this. The nurse is infusing a bolus of normal saline. She starts the bolus at 2pm and will run 500 mL at 200 mL per hour.  At what time will the bolus be completed? It’s not a trick question, just has an extra step. We’ll still divide total volume to be infused over the rate in mL per hour. So 500 / 200 = 2 ½. So just find out what time 2 ½ hours later is. In this case, 4:30pm.

3rd, to determine a dose or volume to be administered, you need to know the desired dose, the available dose, and the quantity that dose is in – also known as the concentration. For example, the nurse needs to administer 20 mEq of Potassium Chloride. The available dose is 40 mEq of Potassium Chloride in 30 mL of liquid.  So, desired dose is 20, divided by available dose which is 40, times the quantity which is 30. 20 / 40 x 30 = 15 mL of liquid.

Finally, to find the rate to set on the pump, you need to find the total volume to be infused in a certain amount of time and divide volume by hours. This one may require extra steps to find the total volume or dose – which is the problem we just did!  Once you’ve calculated the volume required, simply divide that over total time in hours to get mL per hour. Here’s an example: The nurse is to administer 1g of Vancomycin over 2 hours. The bag contains 2g of Vancomycin in 500mL. First – how much total volume is required? If you have 2g in 500mL, then 1g is in 250mL. So now we know that we need a total of 250 mL to infuse over a total time of 2 hours. 250 / 2 = 125 mL/hr.

I hope this has helped to make med math a bit simpler. Make sure convert all units before starting these formulas, know your rounding rules and weight-based calculations for pediatric doses, and always double and triple check your answers if you aren’t sure.

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