16a - Local Anaesthetics

Question 1

Local anaesthetic mechanism of action

Answer 1

Interrupts APs of peripheral nerves. Blocks Na+ channels to prevent Na+ entering.
- Introduced alongside HCl to be water soluble which makes it diffuse in tissue easily
- Reacts w/ Sodium bicarbonate (NaHCO3) to lose it’s HCI and become lipid soluble so it enters cell. (NaHCO3 is lowered during inflammation due to lower pH so more anaesthetic is needed if inflammed).
- Reacts w/ H2O to get H+ attached.
- Now has +ve charge which moves towards and blocks Na+ channels.

Question 2

2 classes of local anaesthetics?
- Difference in effects?
- Examples?

Answer 2

Esters – Can be broken down by plasma cholinesterase to PABA causing hypersensitivity. E.g. Cocaine, oxybuprocaine…
Amides – Metabolised in liver so safer. Lidocaine (which is also a class 1 antiarrhythmic.

Question 3

Describe cocaine
- 3 features
- Why is it addictive?

Answer 3

• Constricts pupils
• Vasoconstrictor (can kill nose cells when sorted)
• Local anaesthetic
• Addictive due to one of receptors it uses is dopamine receptors.

Question 4

List 4 esters local anaesthetics and what they do

Answer 4

Procaine:
- Vasodilator (not good since makes it less localised)
- Slower/weaker than cocaine (1st alternative to cocaine)
Amethocaine/tetracaine
- Very potent. Good for stuff like spine or eye drop.
Oxybuprocaine
- Excellent surface anaesthetic like mucous membranes, bronchi, eye.
Proxymetacaine
- Stings less than amethocaine/tetracaine.

Question 5

1) An example of an amide local anaesthetic?
2) Compared w/ esters, amides are typically used how?

Answer 5

1) Lidocaine
2) surface anaesthetics or epidural.

Question 6

1) Why add vasoconstrictors to local anaesthetic preparations? (4 total)
2) Most common vasoconstrictor we use?
3) Where not to use that vasoconstrictor?

Answer 6

1)
- Enhanced potency
- Prolong duration of action
- Decreased systemic toxicity
- Decreased bleeding (when applied to site of injury).
2 Adrenaline
3 Don’t infiltrate around end-arteries like fingers, toes, and penis. Can reduce blood supply and cause gangrene.

Question 7

Ocular LA side effects?

Answer 7

• Allergic dermatitis/conjunctivitis (esters)
• Delayed hypersensitivity
• Corneal eroision + Keratitis (rare and w/ prolonged proxymetacaine)
• Anaphylaxis (rare)

Question 8

Systemic side effects of local anaesthetics?

Answer 8

Brain and Heart (heart requires 4-6x more dose than brain)
- Nervous, anxious, confusion
- Followed by convulsions.
- Maybe anaphylatic.

Question 9

How to treat systemic toxicity of local anaesthetics? (5 things)

Answer 9

• Open airways + O2
• Lift legs to bring blood to brain
• Diazepam IV for convulsions
• Beta-stimulant for heart
• Adrenaline for allergies.