Small Animal Clinical Pharmacology - CYF MEDICAL DISTRIBUTION

INJECTABLE ANESTHETIC AGENTS
marked muscle rigidity and is rarely administered as the
sole agent. It is usually combined with an α 2 -agonist
such as medetomidine or a benzodiazepine such as
diazepam.
Ketamine does not produce a true anesthetic state but
induces dissociation from the environment with analgesia
and sensory loss. It does not suppress laryngeal and
pharyngeal reflexes and swallowing persists to a variable
degree, even when ketamine is combined with other
drugs. An active swallow reflex should not be equated
with an ability to protect the airway and the trachea
should be intubated as with other anesthetic agents.
Increased muscle tone and open eyes are additional
features of the dissociative state.
More recently there has been an increased interest in
the use of ketamine as an analgesic. As an NMDA receptor
antagonist it is able to reverse the enhanced pain
sensitivity that frequently accompanies major trauma
or surgical injury. Subanesthetic doses, usually administered
by infusion, may be beneficial in sensitized
individuals especially when combined with more conventional
analgesics such as opioids and NSAIDs.
Mechanism of action
Unlike other injectable anesthetics, ketamine has no
effect at the GABA A receptor. Its main effects, i.e. dissociative
anesthesia and analgesia, result from an antagonistic
action at the NMDA receptor.
Formulations and dose rates
Ketamine belongs to the cyclohexamine group of dissociative injectable
agents. It is a water-soluble acidic drug (pH of solution is 3.5–4.1)
that should not be mixed with alkaline solutions. It exists as two
optical isomers and is manufactured as the racemic mixture. The S(+)
isomer produces greater CNS depression and analgesia but less
muscular activity compared to the R(−) isomer.
• Ketamine should not be used alone to produce anesthesia in
mammalian species, especially dogs. It should be combined
with other sedative-type drugs to reduce the incidence of
muscle rigidity and seizures.
• Assessment of anesthetic depth is more diffi cult when ketamine
is used. Typical dissociative effects, such as increased muscle
tone, an open eye and active refl exes, would indicate
inadequate depth of anesthesia if a conventional anesthetic
agent were administered. Therefore care is needed in deciding
when additional drugs are required.
INTRAVENOUS KETAMINE AND BENZODIAZEPINE COMBINATIONS
• Ketamine 5–10 mg/kg + diazepam/midazolam 0.25–0.5 mg/kg
in dogs and cats.
• Equal volumes of ketamine (100 mg/mL) and diazepam/
midazolam (5 mg/mL) can be mixed and administered slowly to
effect. Approximately 1 mL/10 kg of the mixture (0.25 mg/kg
diazepam/midazolam + 5 mg/kg ketamine) will produce
anesthesia of short duration suffi cient to allow endotracheal
intubation. Administration of twice this dose produces 15–
20 min of anesthesia.
INTRAMUSCULAR KETAMINE AND BENZODIAZEPINE
COMBINATIONS
• Midazolam 0.2 mg/kg + ketamine 5–10 mg/kg.
• This combination can be used to produce 20–30 min of heavy
sedation to light anesthesia in cats.
INTRAVENOUS KETAMINE AND α 2 -AGONIST COMBINATIONS
IN CATS
• Medetomidine 40 µg/kg + ketamine 1.25–2.5 mg/kg ±
butorphanol 0.1 mg/kg.
• α 2 -agonist and ketamine combinations are licensed for
intravenous use in cats and produce 20–30 min of anesthesia.
INTRAMUSCULAR KETAMINE AND a 2 -AGONIST COMBINATIONS
Dogs
• Medetomidine 40 µg/kg + ketamine 5–7.5 mg/kg.
• Medetomidine 25 µg/kg + butorphanol 0.1 mg/kg + ketamine
5 mg/kg.
• Xylazine 1 mg/kg + ketamine 10–20 mg/kg.
Cats
• Medetomidine 80 µg/kg + ketamine 2.5–7.5 mg/kg.
• Medetomidine 80 µg/kg + butorphanol 0.4 mg/kg + ketamine
5 mg/kg.
• Xylazine 1 mg/kg + ketamine 10–20 mg/kg.
• Combinations with an α 2 -agonist (± butorphanol) provide 30–
50 min of anesthesia when administered IM. Use of xylazine
may be associated with a slightly shorter duration whereas
higher doses of ketamine may extend the effect. The α 2 -agonist
(± butorphanol) can be given fi rst, followed 10–20 min later by
the ketamine. This practice allows time for the patient to vomit
before anesthesia is induced and is recommended when
xylazine is used in cats. Alternatively drugs can be combined in
the same syringe and administered concurrently. Use of
atipamezole to reverse the α 2 -agonist is not recommended
following ketamine use in the dog.
INTRAMUSCULAR KETAMINE AND PHENOTHIAZINE
COMBINATIONS
• Low doses of ketamine (2–3 mg/kg) may be combined with
acepromazine and opioid and administered IM (or SC) to
premedicate or sedate painful or fractious cats.
USE OF KETAMINE AS AN ANALGESIC
• Ketamine 0.1–1 mg/kg IV or 1–2.5 mg/kg IM in dogs and cats.
• Ketamine 2–10 µg/kg/min as an IV infusion ± 0.5 mg/kg as a
loading dose in dogs.
• Supplemental analgesia of short duration, e.g. suffi cient for a
dressing change, can be produced by administration of an IV
bolus or IM dose of ketamine. To achieve a more prolonged
effect in sensitized canine patients an IV infusion can be given,
usually in combination with opioid analgesics. The use of
analgesic ketamine infusions has not been reported in the cat.
Administration of analgesic doses of ketamine to anaesthetized
patients may be associated with respiratory depression and
respiratory parameters should be monitored carefully.
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