Fundamentals of Pharmacology for Veterinary Technicians Chapter 6

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 18 th 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 = 0. 000001 – Milli- = one thousandth of unit = 0. 001 – 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. 8 C + 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/100 ml) © 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 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 V s x C s = V d x Cd © 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 © 2004 by Thomson Delmar Learning, a part of the Thomson Corporation.