Uses and Actions of Local Anaesthetics* Flashcards
define a local anaesthetic
Local anaesthetics reversibly block nerve conduction when applied to a restricted area of the body to enable a procedure to be carried out without loss of consciousness.
why is a local anaesthetic reversible?
Reversible: so your sense can return afterwards – last until necessary
This is hugely contrasting to general (body wide) anaesthesia
Why might we want to use local anaesthetics?
They relieve or reduce pain in a specified area e.g. mouth, throat, eye.
what are receptors that detect pain called?
nociceptors
how would local anaesthetics help prevent pain?
Sensory receptors detect stimulus –> generate APs –> brain
We must stop that AP reaching the brain to prevent pain from being registered
They target the voltage-gated Na+ channels
what is the naming convention of local anaesthetics?
-caine
This is because the very first local anaesthetic was cocaine so they now all end the same way
Cocaine is no longer used due to the psychoactive effects
synthetic version of cocaine (procaine) has been produced which will not exhibit these effects
what is the chemical nature of local anaesthetics?
All local anaesthetics the same common structure:
- similar functional groups
- similar bonds
what is the chemical structure of a LA?
aromatic ring
linkage
amine group
what properties does the aromatic ring have that makes the drug the way it is?
Makes the drug lipophilic
Cells are surrounded by phospholipid bilayer; this allows the drug needs to cross the cell membrane of neurones
what are the 2 linkages and what properties do they have?
Linkage = amide or ester
Site of metabolism - where the molecule is broken down into the aromatic ring and amine group by enzymes. This stops the drug from being present in the body forever (reversible)
Ester linkage:
metabolised more quickly - doesn’t last in the body for long so it has limited clinical use.
Metabolites formed from drugs with ester linkage gave people allergic reactions.
Amide linkage:
Used more commonly
what are the properties of an amine group in a drug?
Amine groups can be neutral or have a net positive charge by associating with another hydrogen ion. Charged molecules are lipophobic and hydrophilic.
name:
4 drugs
their linkages
their duration
what does equilibrium in the context of LA mean?
Equilibrium means that both charged and uncharged forms are present in the extracellular space.
how do LA exist in equilibrium?
The amine groups are weak bases – they can accept H+ ions
Therefore local anaesthetics exists in an equilibrium of ionised and non-ionised forms
If alkalinity increases (increase in pH), eqm shifts to left (unionised form)
If acidity increases (decrease in pH), eqm shifts to right (ionised form)
At physiological pH (body pH), what form would the LA be in?
more ionised form of local anaesthetic