1. Chapter 6 Systems of Measurement in Veterinary Pharmacology
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2. Systems of Measurement
Household system: lacks standardization; not accurate for measuring medicine
Metric system: developed in late 18th century to standardize measures and weights for European countries
Units based on factors of 10
Prefixes denote increases or decreases in size of unit
Apothecary system: system of liquid units of measure used chiefly by pharmacists
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Household System
Uses household measures when an approximate dose is acceptable
Uses a system of weights and measures based on 1 pound containing 16 fluid ounces
This system lacks standardization, but is calibrated in units that most people are familiar with such as teaspoon and cup
The drop, tablespoon, and teaspoon are the only household measures still used in pharmacology
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Apothecary System
System of liquid measure used by pharmacists; also called the common system
Derived from the British apothecary system of measures
Units in the apothecary system:
Minim = liquid volume of a drop of water from a standard medicine dropper
60 minims = 1 fluid dram
Grain = basic unit of weight measurement
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Metric System
Units are based on factors of 10
Base units are meter (length), liter (volume), and gram (weight)
Prefixes commonly used:
Micro- = one millionth of unit =
Milli- = one thousandth of unit = 0.001
Centi- = one hundredth of unit = 0.01
Kilo- = one thousand units = 1,000
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6. Converting Within Systems
Convert from one unit to another within the same system
Use of conversion factors
Converting from larger to smaller units – multiply
Converting from smaller to larger units – divide
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7. Converting Within the Metric System
Use dimensional analysis (unit calculation)
Must know metric equivalents called conversion factors
Conversion factors are used to change between units and always have a value of one
Cancel units to achieve answer in desired unit of measure
Desired unit of measure should be on top of the conversion factor
Always validate answer
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Shortcut Method
Move decimal point appropriate direction based on units
Examples:
kg to g = move decimal point 3 places to the right
g to kg = move decimal point 3 place to the left
l to ml = move decimal point 3 places to the right
ml to l = move decimal point 3 places to the left
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Remember…
When converting from larger units to smaller units, the quantity gets larger
When converting from smaller units to larger units, the quantity gets smaller
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10. Converting within the Apothecary System
Using the dimensional analysis method
Determine the apothecary equivalents
Create a conversion factor
Determine what format to write the conversion factor in
Set up the conversion in an equation
Perform the calculation
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11. Converting within the Household System
Determine the household equivalents
Create a conversion factor
Determine what format to write the conversion factor in
Set up the conversions in an equation
Perform the calculation
Make sure the correct answer is determined by proving the work
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12. Conversions Between Metric and Household Systems
Use the dimensional analysis method
Determine the conversion factor between the two systems
Determine what format to write the conversion factor
Set up the conversions into an equation
Perform the calculation
Make sure you have the correct answer by proving the work
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13. Conversions between Metric and Apothecary Systems
At times, you may need to make conversions between systems
Need relationship between two systems to serve as a bridge
Bridges are found in Table 6-6
A conversion factor is a number used within either multiplication or division to change a measurement from one unit of measurement to it’s equivalent in another unit of measure
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14. Temperature Conversions
In the Fahrenheit system, water freezes at 32 degrees; water boils at 212 degrees
In the Celsius system, water freezes at 0 degrees; water boils at 100 degrees
Comparison:
212 – 32 = 180
100 – 0 = 100
180 ÷ 100 = 1.8
C = F – 32/1.8
F = 1.8C + 32
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Dose Calculations
Must know correct amount of drug to administer to a patient
Must be in same system of measurement
Weight conversion factor: 2.2 lb = 1 kg
Remember that drugs can be measured in mcg, mg, g, gr, ml, l, units
Remember that drugs can be dispensed or administered in tablets, ml, l, capsules
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Solutions
Solutions are mixtures of substances not chemically combined with each other
The dissolving substance of a solution is referred to as the solvent (liquid)
The dissolved substance of a solution is referred to as the solute (solid or particles)
Substances that form solutions are called miscible
Substances that do not form solutions are called immiscible
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17. Working with Solutions
The amount of solute dissolved in solvent is known as the concentration
Concentrations may be expressed as parts (per some amount), weight per volume, volume per volume, and weight per weight
Usually reported out as percents or percent solution
Remember that a percent is the parts per the total times 100
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18. Rules of Thumb When Working with Solutions
Parts: parts per million means 1 mg of solute in a kg (or l) of solvent (1:1000)
Liquid in liquid: the percent concentration is the volume per 100 volumes of the total mixture (1 ml/100 ml)
Solids in solids: the percent concentration is the weight per 100 weights of total mixture (60 mg/100 mg)
Solids in liquid: the percent concentration is the weight in grams per 100 volume parts in milliliters (dextrose 5% = 5 g/100ml)
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19. Percent Concentration Calculations
Pure drugs are substances that are 100% pure
Stock solution is a relatively concentrated solution from which more dilute solutions are made
Ratio-proportion method: one method of determining the amount of pure drug needed to make a solution
Amount of drug/amount of finished solution = % of finished solution/100% (based on a pure drug)
Remember that the amount of drug used to prepare a solution is added to the total volume of the solvent
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20. Another Way to Determine Volume
Volume concentration method:
Vs = volume of the beginning or stock solution
Cs = concentration of the beginning or stock solution
Vd = volume of the final solution
C d = concentration of the final solution
Vs x Cs = Vd x Cd
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21. Drug Concentrations in Percents
Drug concentrations are sometimes listed in percents
Parts per total = parts (in g) per 100
The front of the vial specifies the concentration (for example, 2% lidocaine)
Use X g/100 ml to determine dose
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22. Reconstitution Problems
Drug is in powder form because it is not stable when suspended in solution
Such a drug must be reconstituted (liquid must be added to it)
The label should state how much liquid to add
Powder may add to the total final volume of liquid being reconstituted
Label a reconstituted drug with the date prepared, the concentration, and your initials
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