NAPLEX Question of the Week: Drip Rate Calculations

A very common calculation performed in hospital pharmacy practice is the challenge for the week!

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CA is a 43 yo M presenting to your emergency department with no pulse. Using ACLS measures, the team was able to achieve return of spontaneous circulation (ROSC) and a norepinephrine drip was initiated after initial resuscitation fluids. The drip was titrated up to 40 mcg/min, and CA’s BP 5 minutes later was 90/54 mm Hg with a mean arterial pressure (MAP) of 66. The team decides to continue this rate to achieve a MAP goal > 65. Assuming a 16 mg/250 mL bag of norepinephrine is used and a steady administration rate of 40 mcg/min is infused, how long will the bag last? Provide answer in hours and minutes. 


Answer with rationale:

Two important competency statements for the NAPLEX are 4.3 (rates of administration) and 4.5 (drug concentrations, ratio strengths, osmolarity, osmolality, or extent of ionization) as these are areas in which pharmacists perform calculations in some practice areas on a daily basis.

We can set this problem up using a ratio since we know the drip rate of the norepinephrine and how much drug is contained in the bag. In this scenario, knowledge of the amount of volume is not necessary to solve the problem.  Since the rate is in mcg/min, we first need to convert this to mg/min since the bag is in mg to have consistent units. So 40 mcg/min = 0.04 mg/min. Now that we have the drip rate as the same units as the bag, we can find out how long it will last by using the following equation as there is 16mg in one 250mL bag:

0.04 mg/ 1 min = 16mg / X minutes

We can then cross multiply to get 400 minutes which is 6 hours and 40 minutes.

Christopher M. Bland

Clinical Professor, University of Georgia College of Pharmacy

Dr. Christopher M. Bland is a Clinical Professor at the University of Georgia College of Pharmacy at the Southeast GA campus in Savannah, GA. Dr. Bland has over 20 years of academic and clinical experience in a number of clinical areas. He is a Fellow of both the Infectious Diseases Society of America as well as the American College of Clinical Pharmacy. He is co-founder of the Southeastern Research Group Endeavor, SERGE-45, with over 80 practitioners across 14 states involved. Dr. Bland serves as Associate Editor for the NAPLEX Review Guide 4th edition as well as Editor-In-Chief for the Question of the Week. He has provided live, interactive reviews for more than 10 Colleges/Schools of Pharmacy over the course of his career.