Lecture 26 : Local Anaesthesia Flashcards
(12 cards)
What is it? Def
Reversibly block nerve conduction when applied in particular area of body, enabling procedure to be carried out. DO NOT MAKE YOU UNCONSCIOUS.
Blocks Na+ channels.
Eg. Face in dentist
Nociception
Where nociceptors (sensory pain receptors) detect harmful stimuli and send signals to CNS
Structure of LA
- Aromatic ring
- Linkage = amide / ester
- Basic amine group
Chemical nature of local anaesthetics
Ester link is hydrolysed / metabolised faster than amide link. Hence esters have a shorter duration of action, does not affect honest of LA.
Weak bases
ionisation state determined by pH
- When pH < pKa, molecule is ionised
- When pH > pKa, molecule is unionised
What is use dependent block
When a drug like an LA binds more effectively to ion channels (like Na+ channels) when they are more frequently opening and closing, ie. More frequently active.
Drug has higher affinity for open or inactivated states of the channel.
Factors affecting effectiveness of local anaesthetic
pH of tissue
- Infections / inflammation causes acidity
- So LA wont act as well here, decreasing pH will
Fact about axon diameters and pain
Nerves which carry nociceptive messages are more sensitive to pain.
Less myelinated nerve -> Easier for LA
What are the routes of local anaesthesia
Specificity and non-specificity of LA
Unwanted side effects of LA
- Can affect other tissues in CNS
- Causes tremors, respiratory failure, reduces cardiac muscle contractility, blood vessels may dilate
Other drugs administered with LA
- Administering vasoconstrictors with LA, eg. Adrenaline, reduces distribution of LA around the body because blood is kept more within the target tissue
- Localises LA, reduces unwanted effects, increases duration of action and drug wont be metabolised as quick
- So lidocaine + adrenaline is good
Properties of good local anaesthetics
- Reversible
- Blocking nerve conduction in NOCICEPTIVE neurones
- Effective for time procedure (amide linked)
- Low toxicity
