1. Drug Dosages and Intravenous Calculations
Chapter 5
Drug Dosages and Intravenous Calculations

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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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4. Interpreting Drug Orders
Physicians use short-hand notations to prescribe drugs
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5. 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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6. Interpreting Drug Labels (cont’d.)
What is the drug?
What is the total volume?
What is the strength?
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7. Dosage Calculation Methods
D = desired dosage H = on-hand dose
V = vehicle A = amount to give
Dosage formula: .
Proportions:
Dimensional analysis

8. 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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9. Dosage Calculation Methods (cont’d.)
Dimensional analysis: .
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10. Parenteral Dosage Calculations
Administered through a non-gastrointestinal route (e.g. syringe)
Figure 5.1 A 3-mL syringe
Figure 5.2 A 1-mL syringe
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11. 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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12. Parenteral Dosage Calculations (cont’d.)
Proportions: .
Dimensional Analysis:
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13. 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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14. Reconstitution of Solutions (cont’d.)
Proportions: .
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15. Reconstitution of Solutions (cont’d.)
Dimensional analysis:
Express:
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16. 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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17. 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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18. Intravenous Flow Rates (cont’d.)
Dimensional analysis: .
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19. 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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20. 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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21. Titration of Intravenous Medications (cont’d.)
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22. Titration of Intravenous Medications (cont’d.)
Using dimensional analysis: .
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23. 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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