Welcome back to our drug calculation series. In the first two installments we reviewed two common calculation methods - the
universal formula and
dimensional analysis (DA). In Part 3, we used DA to
calculate continuous intravenous (IV) drips, beginning with units per hour (u/hr). In this blog, we will now use the DA method to calculate continuous IV drips in micrograms per minute (mcg/min). Don’t forget, every nurse should be comfortable with basic metric conversions. You can find a handy conversion chart in our
Nursing Pocket Card: Common Calculations.
Intravenous Drips: Convert mL/hour to mcg/min
Example: You receive shift report that your patient is on a nitroglycerin drip for blood pressure control. You check the pump and it is running at 6 mL/hour. The label on the bottle reads 50 mg in 500 mL 0.9% sodium chloride solution. How many mcg/min is the patient receiving?
Step 1: What label is needed? Nitroglycerin is delivered in a continuous drip dosed at mcg/min. This is placed on the left side of the equation.
Step 2: Next we need to convert the concentration from mg/mL to mcg/mL in order to get the same label in the numerator. There are 50 mg in 500 mL. Convert the mg to mcg by multiplying by 1000.
The concentration is
100 mcg/mL.
Step 3: Place the same label in numerator on the right side of the equation then alternate labels in the numerator and denominator so the labels cancel out.
Step 4: Multiply numerators, multiply denominators, then divide numerator by denominator.
Answer: 10 mcg/min
Intravenous Drips: Convert mcg/min to mL/hr
Now let’s reverse the problem and convert mcg/min to find the pump rate mL/hr.
Example: The provider places an order for a nitroglycerin drip at 10 mcg/minute.The pharmacy delivers the infusion bag of nitroglycerin and the label on the bottle reads 50 mg in 500 mL 0.9% sodium chloride solution. At what rate should you set the IV pump?
Step 1: What label is needed? You want to set the IV pump in mL/hr. This is placed on the left side of the equation.
Steps 2: Next we need to convert the concentration from mg/mL to mcg/mL. There are 50 mg in 500 mL. Convert the mg to mcg by multiplying by 1000.
The concentration is
100 mcg/mL.
Step 3: Place the same label in numerator on the right side of the equation. Flip the concentration so that mL is in the numerator and 100 mcg is in the denominator. Then alternate labels in the numerator and denominator so the labels cancel out.
Step 4: Multiply numerators, multiply denominators, then divide numerator by denominator.
Answer: 6 mL/hour
Remember These Tips:
- Check that your answer makes sense clinically.
- Double check your work.
- Have a colleague or pharmacist check your work.
- Know general therapeutic drug doses for commonly administered medications
I hope this review has been helpful. Next month, we will review continuous IV infusions for weight-based drugs (mcg/kg/min) using dimensional analysis. Be sure to check back then!
References:
Cookson, K.L. (2013). Dimensional analysis: Calculate dosages the easy way. Nursing2013, 43(6), 57-62.
Koharchik, L.S. & Hardy, E.C. (2013). As easy as 1, 2, 3! Dosage calculations. Nursing Made Incredibly Easy!, 11(1), 25 – 29.
Wilson, K.M. (2013). The nurse’s quick guide to I.V. drug calculations. Nursing Made Incredibly Easy! 11(2), 1 – 2.
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