LOCAL ANAESTHETIC Flashcards
How do local anaesthetics work?
They block the initiation and propagation of the action potentials
How do local anaesthetics block action potentials?
They act directly on the sodium channel proteins
What are local anaesthetics strongly dependent on?
pH
In alkaline conditions, local anaesthetic is…
lipid soluble and less ionised
In what condition is local anaesthetic able to penetrate the myelin sheath?
-In non-ionised form (due to alkaline pH)
Once within the myelin sheath, which form of the local anaesthetic gives rise to the LA effect? Ionised/non-ionised
Ionised form
If the Local anaesthetic is unable to penetrate the myelin sheath, then at what sites can it still act?
Nodes of Ranvier (gaps in the myelin sheath along the nerve)
How much of an unmyelinated nerve fibres needs to be anaesthetised to block its function?
small part
Do local anaesthetics block more myelinated or unmyelinated fibres?
myelinated
What is the structure of local anaesthetics commonly used in ophthalmology?
Aromatic residues linked to tertiary amide chains
Are local anaesthetics more stable in acidic or alkaline form?
Acidic form (because they are in cationic -positively charged- state)
What percentage of local anaesthetic used is in the non-cationic (and therefore lipid soluble) state?
15%
What is the order of neurone blockade following local anaesthetic use?
Firstly: parasympathetic and sympathetic fibres are blocked
Secondly: Sensory fibres are blocked (pain and temperature)
Lastly: motor fibres are blocked (large, myelinated nerves)
When is epinephrine added to local anaesthetic in ocular surgery?
In infiltrative regional anaesthesia e.g. retrobulbar or peribulbar
What concentration of epinephrine is added to local anaesthetic for the vasoconstrictive effect?
1:200 000
Why is epinephrine not available pre-packaged with local anaesthetic?
Epinephrine is broken down by heat, so may have reduced potency in pre-prepared solutions
What is the main local side effect of local anaesthetic?
Poor wound healing, especially in cornea
How do local anaesthetics inhibit wound healing?
They disrupt tight junctions between cells and interfere with corneal metabolism
What are the systemic effects of local anaesthetics?
- Numbness
- Tingling
- Aggressive behaviour
- Dizziness
- Slurred speech
What are signs of central nervous system toxicity in local anaesthetic use?
- convulsions
- respiratory depression
- myocardial side effects
What precautions does the Royal college of opthalmologists advise are taken during Regional Anaesthetic use to monitor side effects of central nervous system depressio?
- IV access is available
- Monitor heart rate and saturations
Biochemically, what is the composition of local anaesthetics?
aromatic residue
amide ring or side chain
Which part of the structure of local anaesthetic is hydrophilic?
Amide group
Which part of the structure of the local anaesthetics is hydrophobic?
aromatic residue
