Small Animal Clinical Pharmacology - CYF MEDICAL DISTRIBUTION

CHAPTER 17 DRUGS USED IN THE MANAGEMENT OF HEART DISEASE AND CARDIAC ARRHYTHMIAS
atropine administration or may have a partial response
(e.g. the heart rate may increase to 110 beats/min).
Second-degree AV block disappears following atropine
administration to dogs with vagally mediated seconddegree
AV block. Although we commonly administer
atropine to dogs with third-degree AV block to assess
their response, we have never identified a dog that had
a significant response.
Vagal tone can be increased by numerous factors.
Anesthesia, central nervous system lesions, abnormal
carotid sinus function (hypersensitive carotid sinus syndrome
in humans), respiratory disease and abdominal
disease are common. Often the cause is unknown (idiopathic).
Parenteral anticholinergic therapy can be used
to control bradyarrhythmias in situations where vagal
tone is increased for only a short period (e.g. during
anesthesia) or can sometimes be used for home therapy
if the owner can administer an injection. This is no more
involved than teaching a client to administer insulin to
a pet with diabetes mellitus.
Oral administration of anticholinergic agents can also
be tried in these patients. Some patients do very well on
oral anticholinergic therapy. However, oral anticholinergic
therapy is not always successful and parenteral
administration, administration of a sympathomimetic
or pacemaker implantation may be required. The oral
anticholinergic drugs can be ranked in order of effect.
Drugs with weak anticholinergic effects are commonly
used as antidiarrheal drugs in veterinary medicine. They
include isopropamide iodide and propantheline bromide.
These drugs are only rarely effective for chronically
treating vagally induced bradyarrhythmias. Atropine
and glycopyrolate are more potent vagolytics and much
more effective agents.
Formulations and dose rates
Atropine
When administered subcutaneously, 0.04 mg/kg should be administered
and the dog should be placed in a cage for 30 min before
reassessing the cardiac rhythm. For intravenous administration,
0.04 mg/kg is also administered but the rhythm can be reassessed
in 5–10 min.
Atropine tablets used to be available and in the authors’ experience
were often effective. They are no longer manufactured but can occasionally
be found. The parenteral atropine solution can also be administered
PO but is extremely bitter. To administer it PO, it must be
diluted in a sweet substance, such as corn syrup, to disguise the taste.
The authors have found that a dose of 0.04 mg/kg q.8 h can be
effective.
Glycopyrolate is available as 1 mg and 2 mg tablets. Although this
product should be effective, the authors have little experience with its
use.
Adverse effects
Vagolytic substances can produce side effects. These
include:
● mydriasis
● constipation
● dry mouth
● keratoconjunctivitis sicca.
In the authors’ experience, however, these side effects
are often remarkably inapparent.
Sympathomimetic drugs
Isoprenaline (isoproterenol)
Clinical applications
Sympathomimetics can also be used to treat bradyarrhythmias.
Isoprenaline (isoproterenol) is a pure β-
agonist that stimulates both β 1 - and β 2 -adrenergic
receptors (see Chapter 4). In so doing, it increases the
sinus node rate, increases the rate of subsidiary pacemakers
in the heart and increases conduction velocity
in the AV node.
Isoprenaline can be used temporarily to increase the
heart rate in dogs with sick sinus syndrome or thirddegree
AV block. This is done only in dogs that are
severely bradycardic or are symptomatic. Isoprenaline
is infrequently used in the author’s clinic to increase the
heart rate in dogs that are waiting to have a pacemaker
implanted. It is more frequently used in dogs that
become severely bradycardic under anesthesia prior to
pacemaker implantation.
Formulations and dose rates
Isoprenaline is administered intravenously as a CRI at a dose of
0.05–0.2 µg/kg/min. The dose must be titrated and the lowest effective
dose should be used. Oral administration of isoprenaline is not
effective because it is almost completely metabolized by the liver
before it reaches the systemic circulation.
Adverse effects
● Isoprenaline stimulates β-receptors in systemic arterioles,
producing vasodilation. This can cause
hypotension.
● Isoprenaline can also stimulate tachyarrhythmias.
b 2 -Agonists
Clinical applications
Numerous drugs that stimulate β 2 -receptors are available.
These drugs are used as bronchodilators and are
effective after oral administration. They are generally
formulated not to produce many cardiac effects.
However, this is impossible since β 2 -receptors are
present in the heart and play an important role in modulating
the sinus rate. Consequently, these drugs can also
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