Local anaesthetics Flashcards
Define a local anaesthetic
Local anaesthetics reversibly block nerve conduction when applied to a restricted area of the body (to allow for a procedure) without loss of consciousness
Why are local anaesthetics necessary?
To block pain from nociceptors
Pain not perceived
However still detected by sensory receptors
What mechanism is stopped to prevent perception of pain?
Action potentials
Particularly voltage-gated Na+ channels
What suffix suggests a local anaesthetic?
-aine
When was the first synthetic local anaesthetic produced?
1905
Name chemical structures that are common in local anaesthetics
Benzene ring
Amine group
Physical separation of these two structures
Describe the common structure of a local anaesthetic
Aromatic ring - linkage - basic amine group
What is the purpose of the linkage group?
Either an amide or ester
Allows for metabolism, makes them reversible
How might a local anaesthetic with an ester linkage differ to one with an amide linkage?
Esters are metabolised very quickly in the body
The effect will be short-lasting
Amides have a much longer lasting effect as they are metabolised slower
What is the purpose of the aromatic ring?
Aromatic rings make a molecule lipophilic
Allows the molecule to cross the cell membrane
What is the purpose of the amine group
To allow the molecule to go between charged and uncharged state
What pH is a local anaesthetic?
Weak base
What is the ionisation state of a local anaesthetic?
Determined by pH
What happens at the cell membrane?
Unionised version crosses the cell membrane
Ionised form blocks voltage gated channel from the inside
What form of voltage gated channels can the local anaesthetic block?
It can block open channels
“Use-dependent block”
If local anaesthetic blocks open channels, what is the relationship between pain and blocking?
Increase in pain
= increase in open channels
= increase in blocked channels
Name some factors that affect the effectiveness of local anaesthetic
Tissue pH
What could cause a change in tissue pH?
Infection
Bacteria produce acid
Which neurones are targeted by local anaesthetic?
All of them
No specificity
Which neurones are more sensitive to local anaesthetic?
Sensory neurones are more sensitive than motor neurones
Name routes of administration of local anaesthetic
Topical anaesthesia Infiltration anaesthesia Nerve block anaesthesia Epidural anaesthesia Spinal anaesthesia
What are some unwanted effects of local anaesthetic?
Non-specific: hypersensitivity
Specific: high doses, injected into vessels
What could happen if local anaesthetic reached the CNS?
Tremor
Convulsions
Respiratory and cardiac failure (brain stem)
What could happen if local anaesthetic reached the cardiovascular system?
Cardiac muscle contractility decreases
Blood vessels dilate
Blood pressure decreases
