C H A P T E R 3 0
Adrenergic agonists
465
■■
a
-specific adrenergic agonists, such as phenylephrine
and clonidine, stimulate only the
a
-receptors within
the SNS. Clonidine stimulates
a
2
-receptors and is
used to treat hypertension because its action blocks
the release of noradrenaline from nerve axons.
■■
Care must be taken to prevent extravasation when
using IV; the vasoconstrictive properties of the drug
can cause necrosis and cell death in the area of
extravasation.
■■
These drugs should be tapered over 2 to 4 days when
discontinued because the adrenergic receptors will be
very sensitive and rebound hypertension, tachycardia,
arrhythmias and even death can occur.
β-SPECIFIC ADRENERGIC AGONISTS
Most of the drugs that belong to the class of
b
-specific
adrenergic agonists (Table 30.3), or
b
-agonists, are
KEY POINTS
b
2
-specific agonists and are used to manage and treat
bronchial spasm, asthma and other obstructive pul-
monary conditions. These drugs, including salbutamol
(
Butamol
,
Ventolin
), salmeterol (
Serevent
) and terbu-
taline, are discussed in Chapter 55, which deals with
drugs used to treat obstructive pulmonary diseases.
This section specifically addresses isoprenaline (
Isuprel
),
which is used as a sympathomimetic for its overall
stimulatory properties.
Therapeutic actions and indications
Therapeutic effects of isoprenaline are related to its stim-
ulation of all
b
-adrenergic receptors. Desired effects of
the drug include increased heart rate, conductivity and
contractility; bronchodilation; increased blood flow to
skeletal muscles and splanchnic beds; and relaxation of
the uterus. Its use has decreased over the years as more
specific drugs with less toxicity have been developed to
treat the cardiac problems isoprenaline was developed
to treat. Some research has shown that isoprenaline
exerts a “coronary steal” effect, diverting blood away
from injured or hypoxic areas of the heart muscle, an
effect that can increase the size and extent of an evolving
myocardial infarction, further decreasing its usefulness
in the clinical setting. There are some emergency situ-
ations however, that respond well to isoprenaline. See
Table 30.3 for usual indications.
Pharmacokinetics
Isoprenaline is rapidly distributed after injection; it is
metabolised in the liver and excreted in the urine. The
half-life is relatively short—less than 1 hour.
Evaluation
■
■
Monitor response to the drug (improvement in
condition being treated).
■
■
Monitor for adverse effects (GI upset, CNS and
cardiovascular changes).
■
■
Monitor the effectiveness of comfort measures and
compliance with the regimen.
■
■
Evaluate the effectiveness of the teaching plan
(person can name drug, dosage, adverse effects to
watch for and specific measures to avoid them).
TABLE 30.3
DRUGS IN FOCUS Beta-specific adrenergic agonists
Drug name
Dosage/route
Usual indications
isoprenaline (Isuprel)
Adult: IV injection, 0.02–0.06 mg; IV infusion,
5 mcg/minute; 0.2 mg IM or SC
Paediatric: 0.1–1.0 mcg/kg/minute IV has been
used
Treatment of shock, cardiac arrest, and
certain ventricular arrhythmias; treatment
of heart block in transplanted hearts;
prevention of bronchospasm during
anaesthesia
salbutamol (Butamol,
Ventolin)
Adult: 2–4 mg t.d.s.–q.i.d. PO, or
1–2 inhalations every 4–6 hours
Paediatric: 2 mg t.d.s.–q.i.d. PO, or
1.25–2.5 mg b.d. by inhalation
Treatment and prevention of
bronchospasm; treatment of
acute bronchospasm and exercise-
induced bronchospasm when used by
inhalation
salmeterol (Serevent)
Adults and children >12 years: 1 inhalation b.d.;
30 minutes before exercise with exercise-
induced bronchospasm
Treatment and prevention of bronchial
asthma and reversible bronchospasm,
including exercise-induced
bronchospasm (people ≥4 years)
terbutaline (Bricanyl)
Adults and children >12 years: 3–4 mg PO up
to t.d.s. 0.5 mL SC q 6 hours prn inhalation:
500 mcg–5 mg q 6 hours prn
Paediatric: 0.075 mg/kg/dose PO q 4–6 hours
prn; Inhalation: 500 mcg q 4–6 hours prn
Treatment and prevention of bronchial
asthma and reversible bronchospasm
