Local Anaesthetics Flashcards
What is the definition of a local aesthetic
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
What is the chemical structure of a local anaesthetic ?
aromatic ring- linkage and amine group
What is significant about the aromatic ring in the structure of local anaesthetics?
allows lipid solubility
What is significant about the linkage in the chemical structure of a LA?
usually amine or ester link
esters are metabolised faster
amines stick around longer
metabolites produced from breakdown may have adverse effects
What is significant about the amine group?
acts as a weak base can accept protons to be ionised
Why is procaine not commonly used any more what type of link is it
Ester link
produces unfavourable metabolites
Give 2 examples of LA’s
Bupivacaine used in labour long acting amide link
lidocaine amide link medium acting
What is the significance of LA’s being weak bases?
What is the LA present as at physiological pH?
In acidic conditions equilibrium shifts to favour ionised form
in ionised form they cannot cross the hydrophobic lipid bilayer
At physiological pH 6-7 more ionised than unionised molecules
What is the mechanism of action of a LA
Outside nociceptive neuron there is a wqulibirum between ionised and unionised form
unoised form can pass through the lipid bilayer via diffusion into the neuron
one inside the axoplasm different equilibrium is established pH inside the cell causing ionisation of some of the LA
the ionised form can block voltage gated Na+ ion channels in the membrane preventing sodium influx and the generation of an action potential and therefore pain
What is the use dependant block mechanism?
the ionised form of the LA is the only one that can block the Vg Na+ channels
The more Na+ ion channels that are open (more pain) the more blocking that occurs preventing action potentials
How does tissue pH affect the effectiveness fo LA’s
Inflammation infection increased acidity results in lots of ionised form being present not a lot of unionised form
poor take up and therefore poor blocking of channels
this results in poorer anaesthesia –> clear up infection first
How does the differential sensitivity of neurones influence LA
C fibres and A delta fibres are the small fibres that transmit sensory signals to the SC that allow us to detect pain
Some larger motor neurones are also blocked leading to some temporary loss of motor control however are luckily less sensitive to LA than the C fibres and A delta fibres
What is Topical anaesthesia
is it effective
when is it used
via the skin or mucosa
not very effective hard for molecules to get to the skin
used in rubbing creams and throat sweets also useful in ophthalmology laser eye surgery
What is infiltration anaesthesia?
Is it effective
When is it used?
Injection or multiple injections all the way round an area targeting local nocioceptors
used in dental work or when stitching up
What is nerve block anaesthesia
when is used?
LA injected proximally on the nerve effecting multiple sensory and motor neurones
results in whole area being anaesthetised
used in major dental work
