Chapter 6 Systems of Measurement in Veterinary Pharmacology Copyright © 2011 Delmar, Cengage Learning

Chapter 6Systems of Measurement

in Veterinary Pharmacology

Copyright © 2011 Delmar, Cengage Learning

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


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


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


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 = 0.000001– Milli- = one thousandth of unit = 0.001– Centi- = one hundredth of unit = 0.01– Kilo- = one thousand units = 1,000


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


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


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


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


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


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


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


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


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


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


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


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


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)


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


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


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


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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Chapter 6 Systems of Measurement in Veterinary Pharmacology Copyright © 2011 Delmar, Cengage Learning