Critical Care Calculations

After reviewing this chapter, you should be able to:

1. Calculate dosages in mcg/min, mcg/hr, and mg/min

2. Calculate dosages in mg/kg/hr, mg/kg/min, and mcg/kg/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.

Tips for Clinical Practice

Electronic infusion devices are routinely used to administer medications that are potent and require close monitoring. If an electronic infusion device, such as a volumetric pump or syringe pump, is not available, a microdrip set calibrated at 60 gtt/mL can be used.

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

100 mg:100 mL=25 mg:x mL100x=100 × 25100x100=2,500100x=25 mL/hror

Solution Using Formula Method

25 mg100 mg × 100 mL=x mL/hrx=25 mL/hr

To infuse 25 mg/hr, set the IV rate at 25 mL/hr.

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.

Equation will be as follows:

Example 2:

A solution of Isuprel 2 mg in 250 mL D5W is to infuse at a rate of 5 mcg/min.

Step 1:

Calculate the dose per hour.

Step 2:

Convert 300 mcg to mg to match the available strength.

1,000 mcg=1 mg300 mcg=0.3 mg

Step 3:

Calculate the rate in mL/hr.

2 mg:250 mL=0.3 mg:x mL2x=250 × 0.32x2=752x=37.5=38 mL/hr

To infuse 5 mcg/min, set the IV rate at 38 mL/hr.

NOTE

Conversions may be made from solution strength to dosage ordered, or the opposite.

Solution Using Dimensional Analysis

Note: In this equation you will need two additional conversion factors, 60 min = 1 hr and 1,000 mcg = 1 mg.

Remember

Ratio and proportion can be set up by using several formats.

Now that you have seen examples of calculating the rate in mL/hr, let’s look at some other calculations.

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 Dimensional Analysis

To determine mg/min, 24 mg ÷ 60 min/hr = 0.4 mg = 400 mcg/min

CAUTION

Accurate math is essential because these medications are extremely potent.

DRUGS ORDERED IN MILLIGRAMS PER MINUTE

Drugs such as lidocaine and Pronestyl are ordered in mg/min.

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 1:

Convert the client’s weight in pounds to kilograms.

2.2 lb = 1 kg

To convert the client’s weight, divide 150 lb by 2.2.

150 lb÷2.2=68.18 kg=68.2 kg

Note that the conversion could also be done using ratio and proportion or dimensional analysis.

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).

1. Convert mcg/min to mcg/hr.

2. Convert mcg/hr to mg/hr.

3. Determine IV flow rate.

Solution Using Ratio and Proportion

400 mg:250 mL=8.2 mg:x mL400x=250 × 8.2400x400=2,050400x=5.1=5 mL/hr

Solution Using Dimensional Analysis

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Feb 11, 2017 | Posted by admin in PHARMACY | Comments Off

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CALCULATING RATE IN mL/hr

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CALCULATING CRITICAL CARE DOSAGES PER HOUR OR PER MINUTE

DRUGS ORDERED IN MILLIGRAMS PER MINUTE

CALCULATING DOSAGES BASED ON mcg/kg/min

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