LOCAL ANAESTHETIC

Question 1

How do local anaesthetics work?

Answer 1

They block the initiation and propagation of the action potentials

Question 2

How do local anaesthetics block action potentials?

Answer 2

They act directly on the sodium channel proteins

Question 3

What are local anaesthetics strongly dependent on?

Answer 3

pH

Question 4

In alkaline conditions, local anaesthetic is…

Answer 4

lipid soluble and less ionised

Question 5

In what condition is local anaesthetic able to penetrate the myelin sheath?

Answer 5

-In non-ionised form (due to alkaline pH)

Question 6

Once within the myelin sheath, which form of the local anaesthetic gives rise to the LA effect? Ionised/non-ionised

Answer 6

Ionised form

Question 7

If the Local anaesthetic is unable to penetrate the myelin sheath, then at what sites can it still act?

Answer 7

Nodes of Ranvier (gaps in the myelin sheath along the nerve)

Question 8

How much of an unmyelinated nerve fibres needs to be anaesthetised to block its function?

Answer 8

small part

Question 9

Do local anaesthetics block more myelinated or unmyelinated fibres?

Answer 9

myelinated

Question 10

What is the structure of local anaesthetics commonly used in ophthalmology?

Answer 10

Aromatic residues linked to tertiary amide chains

Question 11

Are local anaesthetics more stable in acidic or alkaline form?

Answer 11

Acidic form (because they are in cationic -positively charged- state)

Question 12

What percentage of local anaesthetic used is in the non-cationic (and therefore lipid soluble) state?

Answer 12

15%

Question 13

What is the order of neurone blockade following local anaesthetic use?

Answer 13

Firstly: parasympathetic and sympathetic fibres are blocked

Secondly: Sensory fibres are blocked (pain and temperature)

Lastly: motor fibres are blocked (large, myelinated nerves)

Question 14

When is epinephrine added to local anaesthetic in ocular surgery?

Answer 14

In infiltrative regional anaesthesia e.g. retrobulbar or peribulbar

Question 15

What concentration of epinephrine is added to local anaesthetic for the vasoconstrictive effect?

Answer 15

1:200 000

Question 16

Why is epinephrine not available pre-packaged with local anaesthetic?

Answer 16

Epinephrine is broken down by heat, so may have reduced potency in pre-prepared solutions

Question 17

What is the main local side effect of local anaesthetic?

Answer 17

Poor wound healing, especially in cornea

Question 18

How do local anaesthetics inhibit wound healing?

Answer 18

They disrupt tight junctions between cells and interfere with corneal metabolism

Question 19

What are the systemic effects of local anaesthetics?

Answer 19

• Numbness
• Tingling
• Aggressive behaviour
• Dizziness
• Slurred speech

Question 20

What are signs of central nervous system toxicity in local anaesthetic use?

Answer 20

• convulsions
• respiratory depression
• myocardial side effects

Question 21

What precautions does the Royal college of opthalmologists advise are taken during Regional Anaesthetic use to monitor side effects of central nervous system depressio?

Answer 21

• IV access is available

- Monitor heart rate and saturations

Question 22

Biochemically, what is the composition of local anaesthetics?

Answer 22

aromatic residue

amide ring or side chain

Question 23

Which part of the structure of local anaesthetic is hydrophilic?

Answer 23

Amide group

Question 24

Which part of the structure of the local anaesthetics is hydrophobic?

Answer 24

aromatic residue