Small Animal Clinical Pharmacology - CYF MEDICAL DISTRIBUTION

DRUGS ACTING ON THE AUTONOMIC NERVOUS SYSTEM AND THEIR CLINICAL APPLICATION
reactions to bites and stings. It also relaxes bronchial
smooth muscle, as well as reducing blood flow and
hence congestion in the lungs.
The reduced blood flow to mucous membranes is also
exploited to relieve the symptoms of allergic rhinitis.
Ephedrine and pseudoephedrine have similar but weaker
vasoconstrictive actions and are often used for this
purpose.
Vasoconstrictor actions are also the reason why
adrenaline (epinephrine) is often coadministered with
local anesthetics when these are given by injection. Here
adrenaline (epinephrine) functions both to prolong the
action of the local anesthetic and to prevent the escape
of high concentrations into the systemic circulation with
the attendant risk of causing cardiac arrhythmia.
Very occasionally adrenaline (epinephrine) is used in
cardiovascular emergencies – acute severe hypotension
and cardiac arrest. It may be useful in the short term
but it often masks the progress of the underlying problem
and should be used cautiously. It is also proarrhythmogenic
and increases the workload of the heart so other
catecholamines such as dopamine and dobutamine are
usually preferred (see Chapter 17).
No approved veterinary formulations are available in
the USA or UK.
Formulations and dose rates
Adrenaline (epinephrine) is available in an injectable formulation as
0.1 mg/mL (1:10000) and 1 mg/mL (1:1000) solutions.
DOGS AND CATS
• Cardiac resuscitation: 0.05–0.5 mg (0.5–5 mL) of 1:10000
solution intratracheally, IV or intracardially
• Anaphylaxis: 0.02 mg/kg IV. Dose may be doubled and given
intratracheally
– Dilute 1 mL of 1:1000 solution in 10 mL saline and give
1 mL/5 kg IV or IM. May repeat q.5–15 min
• Feline asthma: 0.1 mL of 1:1000 solution SC or IV
– Dilute 1 mL of 1:1000 solution in 10 mL saline and give
1 mL/10 kg IV or IM. May repeat q.5–15 min
Pharmacokinetics
Adrenaline (epinephrine) is well absorbed from IM and
SC sites; absorption can be enhanced by massaging the
injection site and absorption is slightly faster from IM
sites compared to SC. Onset of action after IV administration
is immediate and after SC administration
within 5–10 min. Oral administration is not effective as
it is rapidly metabolized in the gut. Adrenaline (epinephrine)
crosses the placenta and enters milk but does not
cross the blood–brain barrier.
Action is terminated by uptake and metabolism in sympathetic
nerve endings. It is also metabolized in liver and
other tissues by MAO and catechol-O-methyltransferase
(COMT) to yield inactive metabolites.
Adverse effects
Adverse effects associated with adrenaline (epinephrine)
administration include:
● anxiety
● tremor
● excitability
● vomiting
● hypertension
● arrhythmias.
Repeated injections can cause necrosis at the injection
site.
Contraindications and precautions
● Contraindicated in narrow angle glaucoma, during
halothane anesthesia, during labor and in patients
with heart disease and heart failure.
● When combined with local anesthetics, it should not
be administered to small body appendages as the
ensuing vasoconstriction can cause tissue hypoxia,
necrosis and slough.
● Use with caution in patients with:
– hyperthyroidism
– diabetes mellitus
– hypertension.
Known drug interactions
● In solution, adrenaline (epinephrine) hydrochloride
is incompatible with aminophylline, hyaluronidase,
sodium bicarbonate and warfarin sodium.
● Adrenaline (epinephrine) should not be used concurrently
with other sympathomimetic drugs.
● Some antihistamines (e.g. diphenhydramine,
chlorphenamine (chlorpheniramine)) and levothyroxine
may potentiate the effects of adrenaline
(epinephrine).
● The concomitant use of β-blockers may potentiate
hypertension and antagonize the bronchodilatory
and cardiostimulatory effects of adrenaline
(epinephrine).
● Nitrates, α-blocking agents and diuretics may abolish
or reduce the pressor effects of adrenaline
(epinephrine).
● Use of adrenaline (epinephrine) concurrently with
drugs that sensitize the myocardium to arrhythmias
(e.g. halothane, digoxin) may increase the risk of
arrhythmias.
● Use of oxytocin concurrently with adrenaline (epinephrine)
or other sympathomimetic drugs can cause
postpartum hypertension.
Phenylpropanolamine
Clinical applications
Phenylpropanolamine is one of the mainstays of treatment
of urinary incontinence in the bitch. Stimulation
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