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P A R T 5
Drugs acting on the autonomic nervous system
A
n
adrenergic agonist
is also called a
sympathomimetic
drug because it mimics the effects of the sympathetic
nervous system (SNS). The therapeutic and adverse
effects associated with these drugs are related to their
stimulation of adrenergic receptor sites. That stimula-
tion can be either direct, by occupation of the adrenergic
receptor, or indirect, by modulation of the release of
neurotransmitters from the axon. Some drugs act in
both ways. Adrenergic agonists also can affect both the
a
- and
b
-receptors, or they can act at specific receptor
sites.
The use of adrenergic agonists varies from ophth
almic preparations for dilating pupils to systemic
preparations used to support individuals experiencing
shock. They are used in people of all ages (Box 30.1).
α- AND β-ADRENERGIC AGONISTS
Drugs that are generally sympathomimetic (Figure 30.2)
are called
a
-agonists
(stimulate
a
-receptors) and
b
-agonists
(stimulate
b
-receptors). These agonists stimu
late all of the adrenergic receptors, that is, they affect
both
a
- and
b
-receptors (Table 30.1). Agents that
affect both
a
- and
b
-receptor sites include adrenaline
(
Anapen, EpiPen
), dobutamine (
Dobutrex
), dopamine
(generic), ephedrine (generic), metaraminol (generic) and
noradrenaline (
Levophed
). Several of these drugs are
naturally occurring catecholamines.
Subdivision of α- and β-receptors
There is a further subdivision of adrenergic receptors
into
a
1
,
a
2
,
b
1
,
b
2
and
b
3
receptors. The
a
1
-receptors
are found in the peripheral blood vessels, the eye and
the heart. Stimulation of
a
1
-receptors causes vasocon-
striction of peripheral blood vessels and contraction
of the dilator muscle of the pupil resulting in dilation
of the pupil. In the heart, stimulation of
a
1
-receptors
increases the contractility of the heart.
b
1
-receptors are
also found in the heart and their stimulation causes
increased heart rate as well as increased contractility
BOX 30.1
Drug therapy across the lifespan
Adrenergic agonists
CHILDREN
Children are at greater risk for complications associated
with the use of adrenergic agonists, including tachycardia,
hypertension, tachypnoea and gastrointestinal
complications.The dosage for these agents needs to
be calculated from the child’s body weight and age.
It is good practice to have a second person check the
dosage calculation before administering the drug to
avoid potential toxic effects. Children should be carefully
monitored and supported during the use of these drugs.
Phenylephrine is often found in over-the-counter (OTC)
allergy and cold preparations, and parents need to be
instructed to be very careful with the use of these drugs—
they should check the labels for ingredients, monitor the
recommended dose and avoid combining drugs that
contain similar ingredients.
ADULTS
Adults being treated with adrenergic agonists for shock or
shock-like states require constant monitoring and dosage
adjustments based on their response. People who may
be at increased risk for cardiac complications should be
monitored very closely and started on a lower dose. Adults
using these agents for glaucoma or for seasonal rhinitis
need to be cautioned about the use of OTC drugs and
alternative therapies that might increase the drug effects
and cause serious adverse effects.
PREGNANCY AND BREASTFEEDING
Many of these drugs are used in emergency situations
and may be used during pregnancy and breastfeeding.
In general, there are no adequate studies about their
effects during pregnancy and breastfeeding, and in those
situations, they should be used only if the benefit to the
mother is greater than the risk to the fetus or neonate.
OLDER ADULTS
Older people are more likely to experience the adverse
effects associated with these drugs—central nervous
system, cardiovascular, gastro-intestinal and respiratory.
Because older people often have renal or hepatic
impairment, they are also more likely to have toxic
levels of the drug related to changes in metabolism
and excretion. Older people should be started on lower
doses of the drugs and should be monitored very closely
for potentially serious arrhythmias or blood pressure
changes.
They also should be cautioned about the use of OTC
drugs and complementary therapies that could increase
drug effects and cause serious adverse reactions.
Safe medication administration
Administering ophthalmic medications
Some of the adrenergic agonists are applied in the eye; it is
important to review the administration technique. First, wash
hands thoroughly. Do not touch the dropper to the eye or to
any other surfaces. Have the person tilt their head back or lie
down and stare upwards. Gently grasp the lower eyelid and
pull the eyelid away from the eyeball. Instill the prescribed
number of drops into the lower conjunctival sac and then
release the lid slowly (Figure 30.1). Have the person close
the eye and look downwards. Apply gentle pressure to the
inside corner of the eye for 3 to 5 minutes. Do not rub the
eyeball and do not rinse the dropper. If more than one type of
eye drop is being used, wait 5 minutes before administering
the next one.
