20
P A R T 1
Introduction to nursing pharmacology
The therapeutic index usually measures the differ
ence between:
• An effective dose for 50% of the people treated
• The minimal dose at which adverse reactions occur
A drug with a low or narrow therapeutic index has
a narrow range of safety between an effective dose and
a lethal one. Examples of drugs with narrow margins
of safety are warfarin, digoxin and gentamycin. On the
other hand, a drug with a high therapeutic index has a
wide range of safety and less risk of toxic effects.
Loading dose
Some drugs may take a prolonged period to reach an
effective concentration. If their effects are needed quickly,
a loading dose is recommended. Digoxin (
Lanoxin
)—a
drug used to increase the strength of heart contrac
tions—and many of the xanthine bronchodilators (e.g.
aminophylline, theophylline) used to treat asthma attacks
are often started with a
loading dose
(a higher dose than
that usually used for treatment) to reach the effective con
centration. The effective concentration is then maintained
by using the recommended dosing schedule.
Dynamic equilibrium
The actual concentration (generally this concentration
refers to blood or plasma concentration) that a drug
reaches in the body results from a dynamic equilibrium
involving several processes:
• Absorption from the site of entry
• Distribution to the active site
• Biotransformation (metabolism) in the liver
• Excretion from the body
These processes are key elements in determining
the amount of drug (dose) and the frequency of dose
repetition (scheduling) required to achieve the effec
tive concentration for the desired length of time. When
administering a drug, the nurse or midwife needs to
consider the phases of pharmacokinetics so that the
drug regimen can be made as effective as possible.
Absorption
To reach responsive tissues, a drug must first make its way
into the circulating fluids of the body.
Absorption
refers to
what happens to a drug from the time it is introduced to
the body until it reaches the circulating fluids and tissues.
Drugs can be absorbed from many different areas in the
body: through the GI tract either orally or rectally, through
mucous membranes, through the skin, through the lung,
or through muscle or subcutaneous tissues (Figure 2.2).
Pharmacology:
Absorption
Routes of administration
Drug absorption is influenced by the route of admin
istration. Generally, drugs given by the oral route are
absorbed more slowly than those given parenterally. Of
the parenteral route, intravenously administered drugs
are ‘absorbed’ the fastest.
1
The oral route is the most frequently used drug
administration route in clinical practice. Oral adminis
tration is not invasive and, as a rule, is less expensive
than drug administration by other routes. It is also the
safest way to deliver drugs. People can easily continue
their drug regimen at home when they are taking oral
medications.
Oral administration subjects the drug to a number
of barriers aimed at destroying ingested foreign com
pounds including drugs. The acidic environment of the
stomach is one of the first barriers to foreign chemicals.
The acid breaks down many compounds and inactivates
others. This fact is taken into account by pharmaceuti
cal companies when preparing drugs in capsule or tablet
form. The binders that are used are often designed to
break down in a certain acidity and release the active
drug to be absorbed.
When food is present, stomach acidity is higher
and the stomach empties more slowly, thus exposing
the drug to the acidic environment for a longer period.
Certain foods that increase stomach acidity, such as milk
products, alcohol and protein, also speed the breakdown
of many drugs. Other foods may chemically bind drugs
or block their absorption. To decrease the effects of this
acid barrier and the direct effects of certain foods, oral
drugs ideally are to be given 1 hour before or 2 hours
after a meal.
Some drugs that cannot survive in sufficient quantity
when given orally are administered by injection directly
into the body. Drugs that are injected intravenously (IV)
reach their maximum plasma concentration almost at
the time of injection, as there is no initial breakdown
of the drug nor delay due to absorption. Basically, these
drugs have an immediate onset and are fully “absorbed”
at administration because they directly enter the blood
stream. These drugs are more likely to cause toxic
effects because much higher peak concentration of the
drug is reached and the margin for error in dose is much
smaller.
Drugs that are injected intramuscularly (IM) are
absorbed directly into the capillaries in the muscle. This
takes time because the drug must be picked up by the
capillaries and transferred into the veins and the general
circulation. Men have more vascular muscles than
women do. As a result, drugs administered to men via
the IM route reach a peak level faster than they do in
women. Subcutaneous injections deposit the drug just
1
In case of intravenous administration the drug does not need to be absorbed through any membranes, thus using the term absorption in
relation to intravenous administration of a drug is somewhat of a misnomer.
