458
P A R T 5
Drugs acting on the autonomic nervous system
Table 30.1 discusses usual indications for each of these
agents.
Dopamine, a naturally occurring catecholamine,
is the sympathomimetic of choice for the treatment
of shock. It stimulates the heart and increases blood
pressure but also causes a renal and splanchnic arteri
ole dilation that increases blood flow to the kidneys.
This prevents the diminished renal blood supply and
possible renal shutdown that can occur with adren-
aline or noradrenaline, which are also naturally
occurring catecholamines that interact with both
a
- and
b
-adrenergic receptors and are used for the treatment of
shock and to stimulate the body after cardiac arrest (see
Table 30.1 for additional indications for adrenaline and
noradrenaline).
Dobutamine, ephedrine and metaraminol are syn-
thetic catecholamines. Dobutamine, although it acts at
both
b
-receptor sites, has a preference for
b
1
-receptor
sites. It is used in the treatment of heart failure because
it can increase myocardial contractility without much
change in rate and does not increase the oxygen demand
of the cardiac muscle, an advantage over all of the other
sympathomimetic drugs.
Ephedrine stimulates the release of noradren-
aline from nerve endings and acts directly on
adrenergic receptor sites. Although ephedrine was once
used for situations ranging from the treatment of shock
to chronic management of asthma and allergic rhinitis,
its use in many areas is declining because of the avail-
ability of less toxic drugs with more predictable onset
and action.
Many OTC cold products contain ephedrine or
pseudoephedrine. These products can be used to produce
methamphetamine, an often-abused street drug. By
law, the sale of these products is now restricted. The
products are found behind the counter at pharmacies,
not on open shelves; the amount that can be purchased
at any given time is limited and in Australia and New
Zealand pharmacies maintain a record of multiple pur-
chases of pseudoephedrine by any one individual.
Metaraminol is very similar to noradrenaline. It is
given as a single parenteral injection to manage hypo-
tension by increasing myocardial contractility and
causing peripheral vasoconstriction. Its use is limited to
situations in which dopamine or noradrenaline cannot
be used.
Pharmacokinetics
These drugs are generally absorbed rapidly after injec-
tion or passage through mucous membranes. They
are metabolised in the liver and excreted in the urine.
When used in emergency situations, they are given intra
venously (IV) to achieve rapid onset of action.
TABLE 30.1
DRUGS IN FOCUS Alpha- and beta-adrenergic agonists
Drug name
Dosage/route
Usual indications
adrenaline (Anapen,
EpiPen)
Adult: 0.5–1.0 mg IV for acute treatment;
0.3–0.5 mg SC or IM for respiratory distress;
may be used in a nebuliser or as topical nasal
drops
Paediatric: 0.005–0.01 mg/kg IV, base dose
on age, weight and response; do not repeat
more than q 6 hours; topical nasal drops for
children >6 years as needed
Treatment of shock when increased blood
pressure and heart contractility are
essential; to prolong effects of regional
anaesthetic; primary treatment for
bronchospasm; as an ophthalmic agent;
to produce a local vasoconstriction that
prolongs the effects of local anaesthetics
dobutamine (Dobutrex)
2.5–10 mcg/kg/min IV with dose adjusted
based on response
Treatment of heart failure
dopamine (generic)
Initially 5–10 mcg/kg per minute IV with
incremental increases up to 20–50 mcg/kg
per minute based on response
Treatment of shock
ephedrine (generic)
Adult: 25–50 mg IM, SC or IV for acute
treatment; 25–50 mg PO for asthma
maintenance
Paediatric: 25–100 mg/m
2
IM or SC in four
to six divided doses; 3 mg/kg per day in
four to six divided doses PO, SC or IV for
bronchodilation
Treatment of hypotensive episodes
metaraminol (generic)
15–100 mg in 500 mL NaCl or 5% Dextrose,
adjust rate according to blood pressure
Paediatric: 0.01 mg/kg IV as a single dose
Prevention and management of acute
hypotension
noradrenaline (Levophed)
8–12 mcg base/min IV; base rate and dose on
response
Treatment of shock; used during cardiac
arrest to get sympathetic activity
