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© 2004 by Thomson Delmar Learning, a part of the Thomson Corporation. Fundamentals of Pharmacology for Veterinary Technicians Chapter 6 Systems of Measurement in Veterinary Pharmacology
© 2004 by Thomson Delmar Learning, a part of the Thomson Corporation. 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
© 2004 by Thomson Delmar Learning, a part of the Thomson Corporation. 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 = –Centi- = one hundredth of unit = 0.01 –Kilo- = one thousand units = 1,000
© 2004 by Thomson Delmar Learning, a part of the Thomson Corporation. 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
© 2004 by Thomson Delmar Learning, a part of the Thomson Corporation. 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
© 2004 by Thomson Delmar Learning, a part of the Thomson Corporation. 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
© 2004 by Thomson Delmar Learning, a part of the Thomson Corporation. 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
© 2004 by Thomson Delmar Learning, a part of the Thomson Corporation. 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
© 2004 by Thomson Delmar Learning, a part of the Thomson Corporation. 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
© 2004 by Thomson Delmar Learning, a part of the Thomson Corporation. 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
© 2004 by Thomson Delmar Learning, a part of the Thomson Corporation. 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
© 2004 by Thomson Delmar Learning, a part of the Thomson Corporation. 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
© 2004 by Thomson Delmar Learning, a part of the Thomson Corporation. 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)
© 2004 by Thomson Delmar Learning, a part of the Thomson Corporation. 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
© 2004 by Thomson Delmar Learning, a part of the Thomson Corporation. Another Way to Determine Volume Volume concentration method: V s = volume of the beginning or stock solution C s = concentration of the beginning or stock solution V d = volume of the final solution C d = concentration of the final solution V s x C s = V d x C d
© 2004 by Thomson Delmar Learning, a part of the Thomson Corporation. 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
© 2004 by Thomson Delmar Learning, a part of the Thomson Corporation. 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
© 2004 by Thomson Delmar Learning, a part of the Thomson Corporation. Additional Practice Check the book, CD-ROM, and on-line material for calculation problems
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