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7 Cards in this Set
- Front
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Local anaesthetics. Describe the MOA and how the pharmacokinetics may affect its potency.
Is it basic or acidic? Why does it preferentially affect nociceptor? |
Stopping the pain signal being transmitted from the PNS to the CNS
Blocking conduction of nerve action potentials (APs) Local anaesthetic (LA) binds to receptor site on the neuron’s Na channel ® sodium channel closes ® no nerve conduction LA’s are amphiphiles (lipophilic end & hydrophilic end) • The lipophilicity of the molecule allows diffusion through the neuron’s plasma membrane where the LA can bind to the receptor – as such…anaesthetic potency of LAs as lipophilicity • ‘use-dependent blockade’ – LAs traverse sodium channels whilst open, so target the most active neurons pKa = 7.8-9.0 (weak bases) & quite lipophilic Implications for drug preparation – Lipophilicity makes the drug relatively insoluble in aqueous solutions so must make the drug a salt (usually acidic), which improves solubility and stability, and therefore permits injection Implications on clinical effect – Most of the LA molecules absorbed become hydrophilic in the body (pH = 7.4) – The few that have remained lipophilic create the clinical effect Readily absorbed into circulation DOA by adding a vasoconstrictor to preparation eg adrenaline Pharmacodynamics in vivo preferentially affects nociceptive nerve fibres > sensory > motor Theory: the nociceptive fibres have less myelin and are closer to surface of nerve bundles :we can often still walk with an epidural Toxicity = high dose is convulsions. |
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Name the local anesthetic drugs. (5)
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Lignocaine, mepivacaine, bupivacaine, proxymetacaine, lignocaine with phenylephrine
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Lignocaine. 3 points
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• VERY COMMONLY used
• irritant • for cow caesarians, stitch ups |
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Mepivacaine
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less irritant than lignocaine
for nerve blocks in lame horses |
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Bupivacaine
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Long acting Good for epidurals
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Proxymetacaine
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excellent topical anaesthetic for ocular examination
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Lignocaine with Phenylephrine
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Topical LA to spray onto larynx of cat prior to intubation
prevents potentially fatal laryngospasm in the cat |
