Critical Care Calculations
After reviewing this chapter, you should be able to:
1. Calculate dosages in mcg/min, mcg/hr, and mg/min
The content in this chapter may not be required as part of the nursing curriculum. It is included as a reference for nurses working in specialty areas.
This chapter will provide basic information on medicated IV drips and titration. Critically ill clients receive medications that are potent and require close monitoring. Because of the potency of the medications and their tendency to induce changes in blood pressure and heart rate, accurate calculation of dosages is essential. Medications in the critical care area can be ordered by milliliters per hour (mL/hr), drops per minute (gtt/min) (using a microdrop set), micrograms per kilogram per minute (mcg/kg/min), or milligrams per hour (mg/hr). Infusion pumps and volume control devices are usually used to administer these medications. Examples of medicated IV drips that require titration are Aramine, nitroprusside, Levophed, and epinephrine.
Titrated medications are added to a specific volume of fluid and then adjusted to infuse at the rate at which the desired effect is obtained. The drugs that are titrated are potent antiarrhythmic, vasopressor, and vasodilator medications; they must be monitored very carefully by the nurse. Because of the potency of medications used, minute changes in the infusion can cause an effect on the client. Infusion pumps are used for titration; when one is not available, a microdrip set calibrated at 60 gtt/mL must be used.
An example of an order that involves titration of medication is “Titrate sodium nitroprusside (Nipride) to maintain the client’s systolic blood pressure below 140 mm Hg.” The nurse may start, for example, at 3 mcg/kg/min and gradually increase the rate until the systolic blood pressure is maintained below 140 mm Hg. Each time there is a change in rate, the dosage of medication the client receives is changed; therefore it is essential that the dosage be recalculated each time the nurse changes the rate. Dosages are adjusted until the desired effect is achieved.
CALCULATING RATE IN mL/hr
Calculating the rate in mL/hr from a drug dosage ordered for IV administration is one of the most common calculations the nurse encounters. Let’s look at some examples illustrating this before getting into other calculations that may be done.
Example 1:
A solution of Trandate (labetalol) 100 mg in100 mL D5W is to infuse at a rate of 25 mg/hr. Calculate the rate in mL/hr.
Calculate the rate in mL/hr using the solution strength available.
Solution Using Ratio and Proportion
Solution Using Formula Method
Solution Using Dimensional Analysis
• As shown in previous chapters involving medications in solution, the dimensional analysis equation is set up by first isolating what is being calculated. In this example it is mL/hr; therefore mL/hr is placed to the left of the equation.
• The starting fraction will be the information in the problem containing mL. (mL is placed in the numerator.)
• Set up each fraction after the starting fraction to match the previous denominator.
• Cancel the units, reduce if possible, and perform the mathematical operations.
A solution of Isuprel 2 mg in 250 mL D5W is to infuse at a rate of 5 mcg/min.
Solution Using Dimensional Analysis
CALCULATING CRITICAL CARE DOSAGES PER HOUR OR PER MINUTE
Example:
Infuse dopamine 400 mg in 500 mL D5W at 30 mL/hr. Calculate the dosage in mcg/min and mcg/hr.
Solution Using Ratio and Proportion
The next step is to convert 24 mg to mcg, because the question asked for mcg/min and mcg/hr. Change mg to mcg by using the equivalent 1,000 mcg = 1 mg. To change mg to mcg, multiply by 1,000 or, because it is a metric measure, move the decimal point three places to the right.
Now that you have the mcg/hr, the next step is to change mcg/hr to mcg/min. This is done by dividing the number of mcg/hr by 60 (60 minutes = 1 hour).
(Note: This is in mcg/min; however, these meds are usually delivered in mL/hr by pump and you would need to take calculation further. You will see problems later in this chapter changing mcg/min to mL/hr.)
Solution Using Dimensional Analysis
Solution Using Dimensional Analysis
CALCULATING DOSAGES BASED ON mcg/kg/min
Drugs are also ordered for clients based on dosage per kilogram per minute. These drugs include Nipride, dopamine, and dobutamine. In these problems, the weight will be rounded to the nearest tenth for calculation.
Example:
Order: Dopamine 2 mcg/kg/min. The solution available is 400 mg in 250 mL D5W. The client weighs 150 lb.
Step 2:
Now that you have the client’s weight in kilograms, determine the dosage per minute.
Converting mcg/min to mL/hr then would be easy using this example (conversion factor 1,000 mcg = 1 mg).
Solution Using Ratio and Proportion
Solution Using Dimensional Analysis
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