Small Animal Clinical Pharmacology - CYF MEDICAL DISTRIBUTION

INJECTABLE ANESTHETIC AGENTS
medetomidine) or benzodiazepines (diazepam, zolazepam,
midazolam) to produce an anesthetic state.
General side effects
Central nervous system effects
Injectable anesthetics induce reversible dose-dependent
depression of the CNS. Most injectable agents decrease
cerebral blood flow, cerebral metabolic oxygen requirements
and intracranial pressure in the presence of both
normal and raised intracranial pressure. The dissociative
agents are the exception to this, having the opposite
effect.
Of the injectable agents, only the dissociative anesthetics
provide significant analgesia. If agents with
minimal analgesic activity are used surgery cannot be
undertaken without excessive CNS depression. Ideally
these agents should be combined with specific analgesic
drugs (e.g. opioids), thereby allowing surgery to be performed
at lighter planes of anesthesia.
Cardiovascular effects
The injectable agents have a variable effect on the cardiovascular
system. In general, these agents cause dosedependent
cardiovascular depression with a decrease in
blood pressure, myocardial contractility and/or peripheral
vascular resistance. At low doses the injectable
agents have only a small effect on cardiovascular parameters
but as the dose is increased depression of myocardial
function and vascular tone may be severe. Etomidate
is much less depressant than the other agents, having a
negligible effect on cardiovascular parameters.
Respiratory effects
All injectable agents can cause respiratory depression of
variable severity. At low doses this depression may be
mild to moderate, depending upon the agent administered,
with hypoventilation increasing as the dosage of
drug increases. Apnea may occur at surgical planes
of anesthesia with some agents. Apnea at induction of
anesthesia is also commonly encountered with the
injectable agents. The duration of apnea depends upon
the agent administered, the rate of drug administration
when given intravenously, the physical condition of the
animal and whether other respiratory depressant drugs
(e.g. opioids) have been given concurrently. In the
animal breathing room air, hypoxia can be a consequence
of hypoventilation, recumbency and vascular
changes induced by the injectable agents.
Hepatic and renal effects
Large doses of some agents have been implicated in the
development of hepatic damage but in general at clinical
doses the injectable agents do not have a direct effect
on hepatic and renal function. However, hepatic and
renal function may be compromised when reduced
blood flow occurs secondary to anesthetic-induced
hypotension. The injectable agents are metabolized in
the liver, with the metabolites excreted in urine or bile,
and a decrease in hepatic or renal function may prolong
the action of these agents.
Skeletal muscle effects
The injectable agents, other than the dissociative anesthetics,
provide some degree of muscle relaxation.
Muscle rigidity can be profound with the dissociative
agents when they are used as the sole agent. Relaxation
is markedly improved with the addition of an α 2 -agonist
or benzodiazepine. The injectable agents, with the
exception of ketamine and tiletamine, have been safely
administered to patients susceptible to malignant
hyperthermia.
Other effects
The injectable agents cross the placenta and depress the
fetus. Generally the agents that are rapidly metabolized
by the dam are also rapidly removed by the neonate
although the duration of effect in the young can be
longer because of immature hepatic and renal function.
Maintenance of anesthesia with injectable agents for a
cesarean section is not recommended.
General guidelines for administration of
injectable anesthetic agents
In general the dose of injectable agent required to
produce anesthesia is reduced in sedated, compromised,
old and pediatric patients. In healthy animals individual
variation in dose requirements also exists. For this
reason the calculated dose of intravenous agent is not
given as a single bolus but is administered to effect.
Typically the calculated dose is drawn up, one-quarter
to half of the dose is given as a bolus and the depth of
anesthesia is assessed after 20–30 seconds. Further
increments, of a quarter-dose or less, can be given as
required until the desired level of anesthesia is achieved.
Administration of the entire calculated dose may not be
necessary.
Thiopental
Clinical applications
Thiopental is an ultra-short-acting thiobarbiturate that
is usually given for induction before gaseous anesthesia
but may be administered as the sole agent for short
procedures that are minimally painful.
Following thiopental administration, induction and
transition to inhalational anesthesia are smooth and
rapid in the premedicated animal. However, if an inadequate
dose is given to an unsedated or poorly sedated
animal, excitement and hypertonus may be seen.
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