Local Anaesthetics Flashcards
What is a local anaesthetic?
local anaesthetics reversibly block nerve conduction when applied to a restricted area of the body without loss of consciousness.
Why would we want to use local anaesthetics?
Reduce pain that individuals would or are feeling.
an - without
aesthesia - feeling
Why do anaesthetics block nerve conduction?
They prevent the message of pain from reaching the brain so the individual has no feeling of nociception.
How do anaesthetics block nerve conduction?
They block the voltage gated Na channels preventing action potentials from being transmitted and therefore no impulse sent to the brain perceiving nociception.
What was the first local anaesthetic?
Using cocaine by squirting it into eyes to anesthetise the eye without causing the hyperactive functions
What is the prefix for most local anaesthetics?
Caine.
eg. cocaine
procaine
What is the basic structure of a local anaesthetic?
Aromatic ring - linkage - basic amine group
Why is the linkage group important in the structure of the anaesthetic?
The linkage group is the site of metabolism.
Often an amide or an ester. (ester faster than amide)
Also ensures that the local anaesthetic is reversible.
give some examples of anaesthetics and their linkage and duration.
Procaine/Cocaine - E - short duration
Lidocaine - A - medium duration
Prilocaine - A - Medium duration
Bupivacaine - A - long duration.
What is the importance of the aromatic ring in the basic structure of local anaesthetics?
Aromatic rings show lipophilicity and hydrophobicity.
This means that the local anaesthetic is a weak base because it can be charged or neutral (ionised or unionised)
Therefore the drug is dependent on the bodies pH
What happens to the ionisation of a local anaesthetic when the environment gets more alkaline or acidic?
Increase alkalinity - more unionised
Increase acidity - More ionised.
Which type of form of anaesthetic is more prominent at physiological pH?
Both ionised and unionised forms of the anaesthetic are present at physiological pH however there will be a greater proportion of ionised anaesthetic.
How can anaesthetic move into axons?
At physiological pH there will be both unionised and ionised forms of the anaesthetic.
The unionised form is lipophilic and therefore can diffuse through the membrane where they can then become ionised inside the axon to begin interacting within the axon prevent the sodium channels from functioning and action potentials form being produced.
What is use-dependent block?
Local anaesthetics only block open or recently opened channels and therefore target the channels active and thus blocking the channels that are transmitting nociception.
What factors affect the effectiveness of local anaesthetics?
Tissue pH:
- inflammation and infection –> causes acidic conditions
- therefore acidity is increasing and pH drops meaning that more of the anaesthetic becomes ionised (on top of the already greater proportion).
- This means that there is a less effective anaesthesia as a greater proportion of the drug will be unable to interact within the axon due to its ionised state.
What is the differential sensitivity of local anaesthetics for different sizes of axon diameters?
Local anaesthetics find it easier to block smaller diameter neurones such as the Adelta or C fibres.
These fibres have a higher sensitivity compared to the large motor neurones which are less sensitive.
The benefit to this is that the smaller fibres are the ones that convey nociception.
What are the various routes of administration of local anaesthetics?
Topical Infiltration Nerve block Spinal Epidural
What are the unwanted effects of local anaesthetics?
Non-specific - Hypersensitivity reactions
Specifc - High doses / injected into the wrong vessels.
Leads to other excitable tissues being anaesthetised where it finds Na channels.
What could be caused if the CNS is accidentally anaesthetised?
Cause convulsions, tremors, respiratory failure.
What could be caused if the cardiovascular system is accidentally anaesthetised?
Cardiac muscle loses contractility
blood vessels dilate
what type and why are other types of drugs administered with Local anaesthetics?
Vasoconstrictors (eg. adrenaline) enable us to localise the anaesthetics to areas we want to target it and also prevents the drug from leaving the tissues leading to its own breakdown or causing unwanted effects.
What are the properties of a good local anaesthetic?
Reversible
Block nerve conduction in nociceptive neurones
effective for time of procedure
low toxicity.
What is topical administration?
application to body surfaces (isnt very effective)
What is infiltration administration?
Injection of LA that surround the nerve terminals of the nociceptive neurones.
