Anaesthesia
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Anaesthesia Belinda Farnfield Some definitions and terms 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

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Anaesthesia

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Anaesthesia

Belinda Farnfield


Some definitions and terms


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


Sedatives & Tranquillisers

  • Sedatives

    • Drowsiness

    • Reduced fear & apprehension

  • Tranquillisers

    • Calming effect

    • No sedation

  • Overlap, species variation

  • Think of them as one group


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


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)


Understanding anaesthetic regimens


An anaesthetic regimen includes

  • Induction

  • Maintenance

  • Recovery


Induction

  • Refers to general anaesthesia

  • Animal loses consciousness

  • Should be

    • Calm, stress free (and so humane)

    • Safe for animal and handler


Maintenance

  • Keeping animal asleep

  • The animal should be

    • Unable to perceive painful stimuli

    • Relaxed muscles

    • Asleep (or immobile)

  • This is called the anaesthesia triad


Recovery

  • Rapid return to normal physiology & behaviour

  • Abnormal physiology leads to poor animal model


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


Inhalational anaesthesia


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


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


Equipment used can be complex


Induction chambers

  • Induction chambers

    • Useful for small species

    • Minimal restraint needed


Face masks

  • Facemasks

    • Need close restraint

    • Or

      • Induction chamber first

    • Or

      • Injectable agents first


The anaesthetic machine

  • Take some time with this

  • Well before you are first due to use the machine


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


  • Oxygen cylinder

  • Vaporiser

  • Breathing system/ chamber

  • Scavenging equipment


Other equipment you may see/ use

  • Endotracheal tubes

    • Protect and maintain airway

    • Can control ventilation

    • Essential for some species

    • Skill needed to place


Isoflurane in the mouse

  • Video: isoflurane in the mouse (6 minutes)


Injectable anaesthesia


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)


Practicalities

  • Routes of injection

    • Intraperitoneal

    • Intramuscular

    • Intravenous


Many agents and combinations


And

  • Lots of dose tables and “Recipes”

  • Species differences

  • Strain differences


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


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


One common combination is xylazine and ketamine

  • Video: mouse xylazine and ketamine (5 minutes)


Selecting a regimen


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


Species & procedure

  • Species, strain, age, sex, weight

  • Procedure

  • Depth & duration

  • Experience/ skill

  • Equipment available

  • Legal permission


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


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


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