Practical anaesthesia

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Practical anaesthesia
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Premedication

• Drugs given before anaesthesia is induced
• Good time to given an analgesic drug
• Carprofen (a NSAID) good choice for many species
  and procedures
• Opioids as well if a more painful procedure is
  planned
• Sedative to allow calm handling
• Historically, premedicants included drugs to
  reduce salivation
• Not usually necessary with more modern
  anaesthetic drugs

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Anaesthetics may be given by various routes

• Possible routes are
• Inhalation
• Injection
• Local
• Routes may be combined
• Injectable agent to begin with (induction)
• Then inhalational anaesthesia for maintenance

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Monitoring peri-operative care

• Depth of anaesthesia must be checked before any
  painful procedure starts
• Depth then monitored continuously
• To ensure anaesthesia is adequate for the
  procedure
• BUT without being too deep, when risk from
  cardiovascular and respiratory side effects
• Important part of peri-operative care is to
  minimise hypothermia at all stages of the
  anaesthetic

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Recovery

• Rapid return to normal physiology behaviour
• Humane to minimise impact of the procedure on the
  animal
• Abnormal physiology leads to poor animal model

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Example 1 inhalation anaesthesia in the mouse
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Principles of inhalation anaesthesia

• Animal breathes in anaesthetic vapour delivered
  in a carrier gas (oxygen /- nitrous oxide)
• Crosses from alveoli to blood stream
• Reaches CNS and has anaesthetic effect
• Side effects
• Recovery process
• animal breathes off anaesthetic vapour
• minimal metabolism

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Agents used

• Two main agents
• Isoflurane
• Halothane
• Both are liquids at room temperature, placed in a
  vaporiser
• Oxygen passed through the vaporiser
• Vapour delivered in oxygen to the animal

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Carrier gases for isoflurane/ halothane

• Usually isoflurane/ halothane vapour delivered
  using oxygen as the carrier gas
• Nitrous oxide may be used as a carrier gas in
  addition to oxygen
• Not often in small species
• May be used in larger species
• Some analgesic effect
• Comes in and out of solution in the blood very
  rapidly

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(mouse isoflurane videos x 2)
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Self-study
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Equipment used can be complex
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The anaesthetic machine

• Find the oxygen cylinder
• learn to change cylinder, ask about alarms
• Find the vaporiser
• learn how to fill it
• Learn where to connect a breathing system/
  chamber
• Learn how to connect up and use scavenging
  equipment

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Example 2Injectable anaesthesiain the mouse
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Principles of injectable anaesthesia

• Drug is administered subcutaneous, intravenous,
  intraperitoneal, intramuscular
• Absorption and circulation
• depending on route
• Anaesthetic effect once it reaches the central
  nervous system (CNS)
• Recovery process
• Specific reversing agent for some injectable
  anaesthetics
• Metabolism (liver) excretion of drug (kidneys)

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Many agents and combinations
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Alpha 2 adrenoceptor agonists

• Xylazine and medetomidine
• Act at the alpha 2 receptor
• Produce sedation, muscle relaxation and some
  analgesia
• Side effects hypoxia, general cardiovascular and
  respiratory depression, hypergylcaemia, diuresis
• Specific antagonist available atipamezole

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Ketamine

• A dissociative anaesthetic
• NMDA receptor antagonist
• On its own can be used to restrain higher species
  eg primates
• For other species, used in combination with the
  alpha-2 adrenoceptor agonists and will then
  provide surgical anaesthesia

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Pentobarbitone

• Widely used in the past
• Poor analgesic, light anaesthesia only
• Narrow safety margin, with risk of respiratory
  arrest if higher doses are used to try to obtain
  surgical anaesthesia
• Main use now is as a euthanasia agent

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Remember 3 aims of anaesthesia

• Single injectable agent often unable to produce
  analgesia, loss of consciousness, muscle
  relaxation abolition of reflexes on its own
• Hence combinations are used
• Eg ketamine if used alone, poor muscle
  relaxation and analgesia
• Add xylazine improved analgesia and muscle
  relaxation
• Dose rates vary with species and strain

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(mouse injectable agent video)
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Self-study
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Find out which injectable anaesthetic agents you
are likely to use

• Background reading for each agent
• Use standard text to find out about each agent
• Which drug family does it belong to?
• Mechanism of action?
• Dose rates and usual routes of administration
• Is additional analgesia needed before and during
  anaesthesia for a painful procedure?