Transcript Chapter 6 - Delmar Cengage Learning

Chapter 6
Systems 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
Copyright © 2011 Delmar, Cengage Learning
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
Copyright © 2011 Delmar, Cengage Learning
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
Copyright © 2011 Delmar, Cengage Learning
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
Copyright © 2011 Delmar, Cengage Learning
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
Copyright © 2011 Delmar, Cengage Learning
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
Copyright © 2011 Delmar, Cengage Learning
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
Copyright © 2011 Delmar, Cengage Learning
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
Copyright © 2011 Delmar, Cengage Learning
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
Copyright © 2011 Delmar, Cengage Learning
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
Copyright © 2011 Delmar, Cengage Learning
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
Copyright © 2011 Delmar, Cengage Learning
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
Copyright © 2011 Delmar, Cengage Learning
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
Copyright © 2011 Delmar, Cengage Learning
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
Copyright © 2011 Delmar, Cengage Learning
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
Copyright © 2011 Delmar, Cengage Learning
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
Copyright © 2011 Delmar, Cengage Learning
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)
Copyright © 2011 Delmar, Cengage Learning
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
Copyright © 2011 Delmar, Cengage Learning
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
Copyright © 2011 Delmar, Cengage Learning
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
Copyright © 2011 Delmar, Cengage Learning
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
Copyright © 2011 Delmar, Cengage Learning