Transcript Chapter 6 - Systems of Measurement in Veterinary Pharmacology

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 =
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 = 1.8C + 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:
• 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
© 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.