Chapter 5

Drug Dosages and Intravenous Calculations

Objectives Interpret drug orders and labels Calculate drug orders Identify and calculate medication volumes Calculate the volume of reconstituted

medication in powdered form Perform intravenous and intramuscular

calculations

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Objectives (cont’d.) Perform titration calculations Calculate drug dosages based on body

weight Calculate drug dosages based on BSA using

formulas and the West Nomogram chart

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Interpreting Drug Orders

Physicians use short-hand notations to prescribe drugs

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Interpreting Drug Labels

Labels give information such as:• Dosage strength• Brand name (or generic name)• Manufacturer• How to administer (orally, sublingually, injection,

or other)• Drug form (e.g., tablet, capsule, liquid) • Expiration date

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Interpreting Drug Labels (cont’d.)

What is the drug? What is the total volume? What is the strength?

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Dosage Calculation Methods

D = desired dosage H = on-hand dose V = vehicle A = amount to give Dosage formula:

• .

Proportions:• .

• Dimensional analysis7

Dosage Calculation Methods (cont’d.)

An order for 75 mg of phenytoin is given• Only 100 mg are available• How many tablets should be given?

Dosage formula:• .

Proportions:• .

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Dosage Calculation Methods (cont’d.)

Dimensional analysis:• .

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Parenteral Dosage Calculations

Administered through a non-gastrointestinal route (e.g. syringe)

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Figure 5.1 A 3-mL syringe

Figure 5.2 A 1-mL syringe

Parenteral Dosage Calculations (cont’d.)

0.25 mg of a medication by IM injection is ordered • In stock is a vial labeled 0.5 mg in 2 mL • What volume should be administered?

Dosage formula:• .

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Parenteral Dosage Calculations (cont’d.)

Proportions:• .

Dimensional Analysis:• .

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Reconstitution of Solutions

300 mg Zithromax IV is ordered • In stock is a vial of powdered Zithromax that

states:– “Constitute to 100 mg/mL with 4.8 mL of Sterile Water

for Injection”– Single-dose vial contains 500 mg

Dosage formula:• .

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Reconstitution of Solutions (cont’d.)

Proportions:• .

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Reconstitution of Solutions (cont’d.)

Dimensional analysis:• Express:

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Intravenous Flow Rates

Rate of flow: number of drops (gtt) that flow into patient per minute• .

• Drop factor: number of drops it takes to produce 1 cc

• Amount of solution: total amount of solution administered

• Time: time it takes for solution administration

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Intravenous Flow Rates (cont’d.)

An IV bag contains 1,000 mL solution• Drop factor is 90 gtt/mL• Flow rate is 125 gtt/min• Find infusion time:

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Intravenous Flow Rates (cont’d.)

Dimensional analysis:• .

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Titration of Intravenous Medications

Four-step process:• If order has different units than strength of

solution, convert the strength • Determine lower limit by using lower range

given together with patient’s weight• Use step 1 result to convert lower limit in step 2

into units of milliliters per hour• Use step 3 result together with ratio of upper

titration range to lower titration range, to find upper limit

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Titration of Intravenous Medications (cont’d.)

IV line was ordered to titrate between 2 and 4 mcg/kg/min• Patient weighs 60 kg • IV solution contains 60 mg of drug in 300 mL

solution• Determine rate of flow in milliliters per hour

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Titration of Intravenous Medications (cont’d.)

.

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Titration of Intravenous Medications (cont’d.)

Using dimensional analysis:• .

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Dosages Based on Weight

Convert patient’s weight to kilograms Find dose (range) for body weight

• Example: 400 mg/kg/day for 99 lbs – How many milligrams a day?– How many every 6 hours?

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Dosages Based on BSA

Formulas:• .

• West Nomogram: common chart used to estimate BSA

– See Figure 5.3

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Summary

Physicians use short-hand notations Drug labels contain a lot of information Dosages can be calculated using formulas,

proportions and dimensional analysis Many parenteral dosages and reconstitution

of solutions can be determined by using:

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