Chapter 6 Systems of Measurement in Veterinary Pharmacology L. VanValkenburg, RVT, BASVT

Practice! Practice! Practice! Refer to the CD that came with your textbook for practice problems. And… Medical Mathematics and Dosage Calculations for Veterinary Professionals CHAPTER 6 CHAPTER 7 CHAPTER 8 What are my advisement times? 

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 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 A.k.a. the international system (SI) 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 conversion factors Converting from larger to smaller units – multiply Converting from smaller to larger units – divide

Converting Within the Metric System Use dimensional analysis (a.k.a. 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 Use 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 Use dimensional analysis 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

Converting 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

Converting 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 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