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P A R T 1
Introduction to nursing pharmacology
dysfunction can lead to toxic levels of a drug in the body
because the drug cannot be excreted. Figure 2.3 outlines
the pharmacokinetic processes that occur when a drug
is administered orally.
Pharmacology:
Excretion
Half-life
The
half-life
of a drug is the time it takes for the amount
of drug in the body to decrease to half of the peak level
it previously achieved. For instance, if a person takes
20 mg of a drug with a half-life of 2 hours, 10 mg of
the drug will remain 2 hours after administration.
Two hours later, 5 mg will be left (half of the previous
level); in 2 more hours, only 2.5 mg will remain. This
information is important in determining the appropri
ate timing for a drug dose or determining the duration
of a drug’s effect on the body. (See Box 2.1 Focus on
calculations.)
The absorption rate, the distribution to the tissues,
the speed of biotransformation and how fast a drug is
excreted are all taken into consideration when deter
mining the half-life of a drug. The half-life that is
indicated in any drug monograph is the half-life for
a healthy person. Using this information, one can
estimate the half-life of a drug for a person with kidney
or liver dysfunction (which could prolong the biotrans
formation and the time required for excretion of a
drug), allowing the prescriber to make changes in the
dosing schedule.
The timing of drug administration is important to
achieve the most effective drug therapy. Nurses and
midwives can use their knowledge of drug half-life to
explain the importance of following a schedule of drug
administration in the hospital or at home. Figure 2.4
shows the effects of drug administration on the effective
concentration of a drug.
Drug
swallowed
Drug dissolved in
gastrointestinal fluids
Dissolved drug
reaches intestine
Drug absorbed by
portal system
Drug in liver
Drug in
circulation
Drug distributed
throughout body
Drug
“does its thing”
Not dissolved, lost in faeces
Lost in acid
Lost in food, acid, digestion
Biotransformed to non-effective state
Bound to plasma proteins
Broken down in tissues
Bound to plasma proteins
Reaches reactive tissue
Excreted by kidneys, lungs, skin, etc.
Bound to fat tissue
FIGURE 2.3
Pharmacokinetics affects the amount of a drug that
reaches reactive tissues. Very little of an oral dose of a drug actually
reaches reactive sites.
Safe medication administration
The liver is very important in metabolising drugs in the
body and the kidneys are responsible for a large part of the
excretion of drugs from the body. One should get into the
habit of always checking a person’s liver and renal function
before they start a drug regimen. If the liver is not functioning
properly, the drug may not be metabolised correctly and
may reach toxic levels in the body. If the kidneys are not
functioning properly, the drug may not be excreted properly
and could accumulate in the body. Dose adjustment needs to
be considered if a person has problems with either the liver
or the kidneys.
Determining the impact of half-life on
drug levels
A person is taking a drug that has a half-life of 12 hours.
You are trying to determine when a 50-mg dose of the
drug will be gone from the body:
• In 12 hours, half of the 50 mg (25 mg) would be in the
body.
• In another 12 hours (24 hours), half of 25 mg (12.5 mg)
would remain in the body.
• After 36 hours, half of 12.5 mg (6.25 mg) would remain.
• After 48 hours, half of 6.25 mg (3.125 mg) would
remain.
• After 60 hours, half of 3.125 (1.56 mg) would remain.
• After 72 hours, half of 1.56 (0.78 mg) would remain.
• After 84 hours, half of 0.78 (0.39 mg) would remain.
• Twelve more hours (for a total of 96 hours) would
reduce the drug amount to 0.195 mg.
• Finally, 12 more hours (108 hours) would reduce the
amount of the drug in the body to 0.097 mg, which
would be quite negligible.
• Therefore, it would take 4½ to 5 days to clear the drug
from the body.
Calculations
BOX 2.1
