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Calculating Drug Dosages & Pharmacologic Math

Calculating Drug Dosages & Pharmacologic Math. Scientific Measurement. Standard Forms Precise and uniform system Space—length, volume Mass—weight Energy Time. Categories of Measurement. Weight (mass) Quantity of matter present in a substance Space Length Distance between two points

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Calculating Drug Dosages & Pharmacologic Math

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  1. Calculating Drug Dosages & Pharmacologic Math

  2. Scientific Measurement • Standard Forms • Precise and uniform system • Space—length, volume • Mass—weight • Energy • Time

  3. Categories of Measurement • Weight (mass) • Quantity of matter present in a substance • Space • Length • Distance between two points • Volume • Space occupied by a substance • Time

  4. Systems of Measurement • Metric system • Apothecary system • Household system

  5. Metric System • French Academy of Science in 1872 • Official international measure system • Decimal system • Based upon multiples of 10 • Basic unit • Names of secondary units by adding Greek or Latin prefix to basic unit name

  6. Units of mass • Standard • Gram (g) • The weight of 1 ml water • Larger units • Kilo= 1000x • Kilogram (kg) 1,000 grams • Dekagram (dkg)= 10g, hectogram (hg)= 100g

  7. Units of mass • Smaller units • Milligram (mg) = 0.001 gram • Microgram (mg) = 0.000001 gram

  8. Units of length • Standard • Meter (m) • Larger units • Kilometer (km) = 1,000m • Dekameter (dkm) = 10m • Smaller units • Centimeter (cm) = 0.01m • Millimeter (mm) = 0.001m • Micron (m) = 0.000001m

  9. Units of volume • Milliliter vs. cubic centimeter • Milliliter larger than cubic centimeter by about 28 parts in 1,000,000 • Two units generally mean the same • Milliliter (ml) = 0.001 liter • Deciliter (dl) = 0.1 liter • Microliter (ml) = 0.000001 liter

  10. Apothecary system • Rarely used today • Less precise and less convenient • Confusing abbreviations • Replaced by metric system

  11. Apothecary Units of mass • Standard • Grain (gr) • Based on a single grain of wheat • Larger • Scruple () = 20 grains • Dram (dr) = 60 grains or 3 scruples • Ounce (oz) = 8 drams • Pound (lb) = 12 ounces

  12. Apothecary Units of volume • Standard • Minim (m) • Volume of water that would weigh 1 grain • Larger units • Fluid dram (f dr) = 60 m • Fluid ounce (f oz) = 8 f dr • Pint (pt) = 16 f oz • Quart (qt) = 2 pts

  13. Household Units of length • Standard • Foot (ft) = 12 inches • Larger units • Yard = 3 ft • Mile • Statute = 5,280 ft • Nautical = 6076.042 ft

  14. Household Units of volume • Drops (gtt) • Teaspoon (tsp or t) = 60 drops • Tablespoon (T) = 3 teaspoons • Ounce (fl oz) = 2 tablespoons

  15. Conversion between systems • Common conversions • 1 meter = 39.37 inches • 1 centimeter = 0.3937 inches • 1 inch = 2.54 cm • 1 liter = 1.057 quarts • 1 oz = 28 g • 1 lb = 453.6 g • 1 kg = 2.204 lbs

  16. International System of Units • Periods should not be used with abbreviations of units • kg not k.g. or kg. • Units are not capitalized • kilogram not Kilogram • A single space is used between the number and the unit abbreviation • 10 kg not 10kg

  17. International System of Units • Abbreviations are not pluralized • kg not kgs • Fractions are not used with metric units • 0.50 mg not 1/2mg • Values less than 1 should have a zero to the left of the decimal for metric units • 0.25 mg not .25 mg

  18. Calculations • Ratios • A constant relationship between two values • Example 60 min : 1 hour • The relationship between the two values does not change • Written with a colon • 2 pints : 1 quart • Read 2 pints to 1 quart

  19. Calculation formula • Volume to be administered = D(dose desired) X V(volume on hand) H (concentration on hand) • Dose desired = amount (usually in mg) • Volume on hand = amount of fluid in the drug container (usually in ml) • Concentration on hand = amount of drug present in container (usually in grams or mg)

  20. Calculation formula • All units above the line must be the same as the units below the line, in other words, you cannot divide grams by micrograms • Concentration = total dose of drug (mg) total volume (ml)

  21. IV infusion calculations • Necessary to know how many drops your administration set contains • Minidrip = 60 gtt/ml • Macrodrip = 10 gtt/ml, 15gtt/ml, 20gtt/ml • For drug infusions, use minidrip set • More exact • Bigger margin for error

  22. IV infusion calculations • To calculate gtt/min to deliver when the order is in ml/hr • Volume to be infused X gtt/ml Time of infusion in minutes

  23. IV infusion calculations • Calculating drug infusions in mg/min 1. Calculate volume to deliver in ml/min using the first formula Volume= D X V H 2. Convert ml/min to gtt/min gtt/min = ml/min X gtt/ml

  24. IV infusion calculations • Calculating infusions based on pt weight • Dopamine and other vasopressors 1. Convert lbs to kg: kg = 2.2 lbs 2. Calculate mcg (or mg) desired for this pt. mcg (or mg) = desired dose X wt in kg

  25. IV infusion calculations 3. Calculate volume to be delivered in ml/min based on the first formula Volume = D X V H 4. Calculate drops/ min. gtt/min = ml/min X gtt/ml

  26. Let’s Practice And everyone stay calm.

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