General Anaesthetics

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Slide 1 :

Anaesthesia Belinda Farnfield

Slide 2 :

Some definitions and terms

Slide 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

Slide 4 :

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

Slide 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

Slide 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)

Slide 7 :

Understanding anaesthetic regimens

Slide 8 :

An anaesthetic regimen includes Induction Maintenance Recovery

Slide 9 :

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

Slide 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

Slide 11 :

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

Slide 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

Slide 13 :

Inhalational anaesthesia

Slide 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

Slide 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

Slide 16 :

Equipment used can be complex

Slide 17 :

Induction chambers Induction chambers Useful for small species Minimal restraint needed

Slide 18 :

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

Slide 19 :

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

Slide 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

Slide 21 :

Oxygen cylinder Vaporiser Breathing system/ chamber Scavenging equipment

Slide 22 :

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

Slide 23 :

Isoflurane in the mouse Video: isoflurane in the mouse (6 minutes)

Slide 24 :

Injectable anaesthesia

Slide 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)

Slide 26 :

Practicalities Routes of injection Intraperitoneal Intramuscular Intravenous

Slide 27 :

Many agents and combinations

Slide 28 :

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

Slide 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

Slide 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

Slide 31 :

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

Slide 32 :

Selecting a regimen

Slide 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

Slide 34 :

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

Slide 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

Slide 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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