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9 Cards in this Set
- Front
- Back
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What are local anaesthetics
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Drug causing reversible local anaesthesia, mostly to:
- remove pain sensation - paralysis on specific nerve pathway - effect brought by blocking Na channels (prevent nerve conduction) |
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2 major classes of LA?
- give examples |
1. ester: cocaine (only one in use)
procaine, tetracaine, benzocaine, 2-chloroprocaine 2. amides: lidocaine (lignocaine), ropivacaine, bupivacaine - mepivacine, prilocaine, etidocaine |
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Pharmacodynamics of LA [2]
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Membrane stabilizing properties:
- bind to voltage-gated NA channel: prevent sodium ion influx: no AP, so no depol - acts on nociceptors (pain receptor) |
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Pharmacokinetics of LA [3]
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LA are weak bases:
1. - ↓pKa *↑unprotonated form, ↑faster onset speed - ↑acidic will ↑protonated form so less readily diffuse (hence acidosis of infection tissue ↓LA efficacy) - ↑alkaline will ↑unprotonated form so LA more readily diffuse across cell membrane 2. ↑lipid solubility ↑potency: ability to dissolve in fatty myelin sheath 3. Nerve fibres: all diff susceptibility to LA - gives diff onset time & recovery time - achieve differential block (pain sensation blocked more readily than other sensation) - Most sensitive: A gamma, A delta |
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Routes of LA admin [5]
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1. Topical
- EMLA (eutectic mixture of LA): mixture of chemicals with m.p. lower than individual components - topical patch: effectives depends on skin thickness - mix lidocaine & prilocaine 2. Infiltration - inject w. needle directly into tissue 3. Nerve/plexus [block large area w. small amount) - around peripheral nerve/nerve plexus 4. Epidural/spinal [large area w. small amount] - epidural space → affect spinal nerve roots - CSF → spinal roots & parts of spinal cord 5. IV [occasionally] - Bier's block: *tourniquet to interrupt limb's blood circulation *LA → peripheral vein → diffuse into surr. nerves *effect limit to area excluded from blood circulation |
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Dose limits for
- lignocaine - bupivacaine - adrenaline |
Lignocaine:
- 4 mg/kg, 7mg/kg with adrenaline Bupivacaine - 2.5 mg/kg Adrenaline - 1-2 µg/kg (allows higher threshold for LA admin) ∵vasocon → slows LA absorption→ higher dose enabled ↑duration of action, ↓toxicity - affected by patient factor & injection site |
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*what can adrenaline NOT be used in?
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Penile block and digital block
∵presence of end arteries → vasoconstriction → ischemia |
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ADR [4]
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Toxicity: Muscular twitch → unconscious → convsulsion → coma → resp. arrest → CVS depression
- unintended block of other sensory modules (motor block, sympathetic block) - toxic metabolites *lignocaine: monoethylglycinexylidide *prilocaine: o-toluidine which ↓Hb for O2 transport - hypersensitivity [uncommon] |
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factors on choice of La, [4]
and factors on concentration [3] |
choosing LA:
onset? duration? admin route? safety/toxicity? Concentration motor block? duration? vol required & dose limit |
