Pharmacology: Medical Calculations Review

Instructor

• Dr. Herndon

Learning Objectives

• Recognize and use different systems of measurement.
• Convert metric units of measurement.
• Convert between different measurement systems.
• Understand appropriate use of decimal points in recording drug doses.

Systems of Measurement

1. Metric System

   • Most commonly used in drug dosage calculations.
   • Units of weight: kilogram (kg), gram (g), milligram (mg), microgram (mcg).
   • Units of volume: liter (L), milliliter (mL).
   • Conversion: Units differ by a power of 1,000.

2. Apothecary System

   • Basic unit of weight: Grain (gr)
     • 1 grain ≈ 65 milligrams (mg)
   • Volume units: dram (approx. 4 mL), minim (drop), fluid ounce (approx. 30 mL).

3. Household System

   • Common in kitchen measurements.
   • Drop (gtt): 1 mL = 15 drops
   • Teaspoon (tsp) = 5 mL
   • Tablespoon (Tbsp) = 15 mL
   • Weight: pound (lb); 1 kg = 2.2 pounds

Metric Conversions

• Use conversion factors or visual arrow system.
• Example: 3,000 mg to grams.
  • 3,000 mg = 3 g (cancel units, use conversion factor: 1,000 mg = 1 g).
• Arrow system helps determine the direction to move decimal points (by 3 places).

Apothecary and Household Conversions

• Need to refer to conversion charts.
• Example conversions:
  • Half a grain = 32.5 mg
  • 45 mL to ounces = 1.5 oz

Decimal Points in Medical Calculations

• Importance of correct decimal placement to avoid dangerous dosing errors.
• Always use a zero before a decimal in values < 1 (e.g., 0.25 mg).
• Avoid trailing zeros in whole numbers (e.g., 25 mg, not 25.0 mg).

Practice Problems

• Convert between systems using provided conversion factors.
• Ensure consistent units before performing dose calculations.

Resources

• Conversion tables available on Canvas and online resources like Plumb's Veterinary Drugs.

Next Steps

• Take medical calculations quiz #1 on Canvas.
• Watch part 2 of medical calculations.
• Complete quiz #2 associated with part 2.