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Anaesthesia

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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  1. Anaesthesia Belinda Farnfield

  2. Some definitions and terms

  3. 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

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

  5. 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

  6. 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)

  7. Understanding anaesthetic regimens

  8. An anaesthetic regimen includes • Induction • Maintenance • Recovery

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

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

  11. Recovery • Rapid return to normal physiology & behaviour • Abnormal physiology leads to poor animal model

  12. 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

  13. Inhalational anaesthesia

  14. 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

  15. 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

  16. Equipment used can be complex

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

  18. Face masks • Facemasks • Need close restraint • Or • Induction chamber first • Or • Injectable agents first

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

  20. 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

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

  22. Other equipment you may see/ use • Endotracheal tubes • Protect and maintain airway • Can control ventilation • Essential for some species • Skill needed to place

  23. Isoflurane in the mouse • Video: isoflurane in the mouse (6 minutes)

  24. Injectable anaesthesia

  25. 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)

  26. Practicalities • Routes of injection • Intraperitoneal • Intramuscular • Intravenous

  27. Many agents and combinations

  28. And • Lots of dose tables and “Recipes” • Species differences • Strain differences

  29. 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

  30. 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

  31. One common combination is xylazine and ketamine • Video: mouse xylazine and ketamine (5 minutes)

  32. Selecting a regimen

  33. 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

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

  35. 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

  36. 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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