Anaesthesia

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Anaesthesia

• Belinda Farnfield

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Some definitions and terms
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Anaesthesia Analgesia

• Anaesthesia
• State of controllable, reversible insensibility
• general - loss of consciousness
• local - loss of sensory and motor function
  confined to a specific region
• Analgesia
• Temporary abolition or diminution of pain
  perception

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Sedatives Tranquillisers

• Sedatives
• Drowsiness
• Reduced fear apprehension
• Tranquillisers
• Calming effect
• No sedation
• Overlap, species variation
• Think of them as one group

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Muscle relaxants

• Muscle relaxant
• Many drugs used as part of an anaesthetic regimen
  produce varying degrees of muscle relaxation
• Specific term muscle relaxant refers to
• neuromuscular blocking agent
• produce paralysis of skeletal muscles

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Premedication

• Historical
• drugs given before anaesthesia to reduce side
  effects
• eg excess secretions, ether anaesthesia
• Modern
• pre-emptive analgesia
• before painful stimulus occurs
• sedate/ tranquillise (in larger species)

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Understanding anaesthetic regimens
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An anaesthetic regimen includes

• Induction
• Maintenance
• Recovery

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Induction

• Refers to general anaesthesia
• Animal loses consciousness
• Should be
• Calm, stress free (and so humane)
• Safe for animal and handler

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Maintenance

• Keeping animal asleep
• The animal should be
• Unable to perceive painful stimuli
• Relaxed muscles
• Asleep (or immobile)
• This is called the anaesthesia triad

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Recovery

• Rapid return to normal physiology behaviour
• Abnormal physiology leads to poor animal model

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Anaesthetics may be given by various routes

• Possible routes are
• Inhalation
• Injection
• Local
• Routes may be combined
• Injectable agent for induction
• Then inhalational anaesthesia for maintenance

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Inhalational anaesthesia
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Inhalational anaesthesia principles

• 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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Practicalities

• Two main agents used
• 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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Equipment used can be complex
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Induction chambers

• Induction chambers
• Useful for small species
• Minimal restraint needed

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Face masks

• Facemasks
• Need close restraint
• Or
• Induction chamber first
• Or
• Injectable agents first

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The anaesthetic machine

• Take some time with this
• Well before you are first due to use the machine

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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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• Oxygen cylinder
• Vaporiser
• Breathing system/ chamber
• Scavenging equipment

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Other equipment you may see/ use

• Endotracheal tubes
• Protect and maintain airway
• Can control ventilation
• Essential for some species
• Skill needed to place

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Isoflurane in the mouse

• Video isoflurane in the mouse (6 minutes)

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Injectable anaesthesia
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Injectable anaesthesia principles

• Drug is administered
• Absorption and circulation
• depending on route
• Anaesthetic effect once it reaches the central
  nervous system (CNS)
• Recovery process
• Metabolism (liver) excretion of drug (kidneys)

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Practicalities

• Routes of injection
• Intraperitoneal
• Intramuscular
• Intravenous

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Many agents and combinations
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And

• Lots of dose tables and Recipes
• Species differences
• Strain differences

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Why are combinations of agents used?

• One agent can be used to produce induction and
  all 3 desired effects of the triad (unable to
  perceive painful stimuli, relaxed muscles,
  asleep)
• Eg isoflurane
• Injectable agents are often used in combination
• Balanced anaesthesia
• Eg ketamine if used alone, poor muscle
  relaxation
• Add xylazine improved analgesia and muscle
  relaxation

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Information overload?

• Invest some time
• Get to know the basic drug families
• plus key examples of each
• Laboratory Animal Anaesthesia - vital reading
• Which agents are you likely to be using
• In which species
• Get familiar with one protocol at a time

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One common combination is xylazine and ketamine

• Video mouse xylazine and ketamine (5 minutes)

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Selecting a regimen
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Selecting a regimen

• May be done for you
• Join another research group
• Evaluate question
• Based on a publication
• Essential to contact the authors if you are
  adapting something reported in a journal

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Species procedure

• Species, strain, age, sex, weight
• Procedure
• Depth duration
• Experience/ skill
• Equipment available
• Legal permission

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During all parts of the regimen

• A minimum of side effects
• Normal physiology maintained
• All body systems
• This is impossible
• Desirable effects and side effects must be
  balanced

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Make a list

• Which routes are practical?
• Make a list of possible agents/ combinations
• Likely interactions with your procedure?
• Cross off undesired agents
• Now have one or two left
• Recovery time
• Familiarity
• Cost