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Medicine Notes Neurology Notes

General Anaesthetics Notes

Updated General Anaesthetics Notes

Neurology Notes

Neurology

Approximately 117 pages

These notes helped me achieve a mark of 76% in my neurology exam, which is the equivalent of a 1st. The notes are based on a series of 49 lectures on the subject. This is a very good, thorough and in depth review of the nervous system. They are very clearly laid out and easy to follow. They cut out unnecessary information on the topic, making the notes very concise, and fast to get through. Anyone studying medicine, or any other subject requiring knowledge of the nervous system (e.g. physiology o...

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Lecture 13

General anaesthetics

  • Examples

    • Injection/Intravenous Anaesthetics

      • Thipentone

      • Propofol (better than above)

      • Etomidate

      • Midazolam (sedative)

      • Ketamine (dissociative anaesthetic)

    • Inhalation anesthetics

      • Nitrous oxide

      • Isoflurane, desflurane, sevoflurane (halothane)

  • Effects

    • Loss of consciousness (not asleep)

    • Analgesia (many, but not all, produce pain relief)

    • Muscle relaxation (important in surgery)

    • Associated with loss of reflexes

  • Stages of anaesthesia (occur with increasing drug in brain)

    • STAGE 1- ANALGESIA

      • Drowsiness (not yet unconscious)

    • STAGE 2- EXCITEMENT

      • Loss of consciousness

      • Respiration is irregular, patient is thrashing (unwanted)

      • Need to get through this stage as quickly as possible

    • STAGE 3- SURGICAL ANAESTHESIA

      • Regular respiration

      • Decline in muscle tone

      • Loss of reflexes (planes

    • STAGE 4- MEDULLARY PARALYSIS

      • Too much general anaesthetic

      • Stop breathing=death

  • Drug combinations

    • Premedication- sedative

    • Rapid induction- propofol, thiopental (anaesthesia induced)

    • Maintenance of anaesthesia- isoflurane, nitrous oxide

    • Analgesic supplement- opiate (give during anaesthesia to stop pain going to spinal cord that causes adaptation which would cause pain after surgery)

    • Muscle relaxant- NMJ blocking agent e.g. atracurium

    • Muscarinic antagonist- Atropine (operations could increase secretion as PSNS stimulate, so atropine)

    • (Anticholinesterase)

  • Mechanisms of action (not certain)

    • No structure activity relationship

    • No obvious receptor they would all bind to

    • Unitary theory

      • Lipid theories

        • Membrane expansion

          • GA dissolves into lipid membrane and expands lipid

        • Membrane fluidity

          • GA goes into membrane and make more fluid

        • Problem as active ingredient in cannabis is highly lipid soluble but does not give GA effect, so theory wrong

      • Protein theory

        • Binding to hydrophobic regions

          • GA lipid soluble to get into membrane, then bind to proteins in lipid and disrupt function

  • Effects on ion channels

    • General

      • Not all GAs do all of these

      • Antagonists

    • Mechanisms

      • Enhance GABA(A) receptor function

        • Volatile anaesthetics- bind at interface between a & B subunits

        • IV anaesthetics- bind to B subunits (some don’t bind at all)

      • Activate K channels

        • (Two pore K channels- TREK, TASK)

        • Volatile and gaseous anaesthetics only

      • Block NMDA receptors

        • NO, xenon and ketamine (only on this receptor)

      • Other channels

        • Inhibit voltage-sensitive Ca channels

        • Inhibit voltage-sensitive Na channels

        • Glycine, nicotinic and 5-HT ligand gated ion channels

  • Effects on nervous system

    • Inhibit excitatory synaptic transmission

      • Decrease NT release (presynaptic)

      • Decrease action of NT (postsynaptic)

      • Decrease excitability (opening of K channels) of postsynaptic neurone

    • Enhance or inhibit inhibitory synaptic transmission

  • Injectable anaesthetics

    • Types

      • General

        • Barbiturates (thiopental)

        • Propofol

        • Imidazole derivatives (etomidate)

        • Steroids (veterinary)

      • Dissociative

        • Ketamine

      • Benzodiazepine

        • Midazolam (also used in endoscopy + analgesic (e.g. fentanyl) where full anaesthesia not required)

    • Advantages and disadvantages

      • Advantages

        • Easy too administered

        • Rapid/instant induction

      • Disadvantages

        • Complex pharmokinetics

        • Slow elimination

        • Side effects

  • Barbiturate anaesthetics (thiopental)

    • Distribution of blood (and therefore drug)

      • Brain (majority of CO; brain will equilibrate with drug)

      • Heart, lungs, liver, kidneys

      • Muscle and fat (if not exercising, get small amount)

      • Liver metabolism (for thiopentone=smallest)

    • Distribution of thiopentone following IV injection

      • Blood and brain equilibrate fairly quickly

      • Problem as stores are finite and so can get full (in fat etc.)

      • After 2 hours, lots of drug in body but barely any in blood, so little in brain

    • Repeated doses...

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