L18: Local anaesthetics

Question 1

What is general anaesthetics?

Answer 1

Act on the brain to numb the whole body

Question 2

What is local anaesthetics?

Answer 2

Involves numbing an area of the body

Question 3

Name of the specialised sensory nerves that detect tissue damage

Answer 3

Nociceptors

Question 4

How do we sense & feel pain?

Answer 4

Nociceptors detect tissue damage
- This triggers AP along sensory nerve to the brain via spinal cord
- When AP reaches brain, we perceive pain

Question 5

How does local anaesthetics prevent pain?

Answer 5

Stopping action potentials being triggered/propagating along sensory nerves

Question 6

Action of local anaesthetics blocking APs

Answer 6

Molecular target (voltage-dependent Na+) channel
- Drugs ‘plug’ the channel pore, preventing Na+ entry & initiation of AP

Question 7

What does local anaesthetic molecules consist of?

Answer 7

Aromatic head linked to a basic side chain by an amide OR an ester bond

Question 8

What can the ester bond be destroyed by to be shorter acting?

Answer 8

Esterase enzymes

Question 9

Are local anasthetics weak or strong bases?

Answer 9

Weak bases (pKA = 8.0)

Question 10

Why is lignocaine longer acting than amethocaine?

Answer 10

Lignocaine has amide/ester bond

Question 11

Action of outer/inner mouth of voltage-dependent Na+ channels

Answer 11

Outer mouth: Too narrow to allow drug molecules through

Inner mouth: Guarded by activation/inactivation gates

Question 12

Describe hydrophobic pathway of drug

Answer 12

Non-ionised form of the drug can penetrate membrane

Question 13

Describe the hydrophilic pathway

Answer 13

Ionised form of the drug enters ONLY when the Na+ channels are open

Question 14

Describe *use dependence *

Answer 14

More the ‘use’ of the channel, the more readily the drug can gain access

Question 15

Why is the hydrophobic pathway NOT use-dependent?

Answer 15

Drug enters the Na+ channnel from lipid membrane and doesn’t matter as the it can enter either it is closed/open

Question 16

Which pathway is available when the channel is closed?

Answer 16

Only hydrophobic pathway

Question 17

Which pathway is available when the channel is open?

Answer 17

Both hydrophobic & hydrophilic

Question 18

What is the molecular target for local anaesthetic drugs?

Answer 18

Voltage-dependent Na+ channels

Question 19

Describe surface anaesthesia

Answer 19

Drug applied to the surface of the body
- Not very affective as it takes ~1 hour to take effect

Side effects:
If applied over large area/damaged skin

Question 20

Describe infiltration anaesthesia

Answer 20

Local anesthetic injected into tissues around sensory nerve teminals & branches

e.g minor surgery, dentistry

Question 21

Describe nerve-block anaesthesia

Answer 21

Local anaesthetic is injected around nerve trunks
- Requires less drug than infiltration but affects wider area
- Requires accurate placement

e.g brachial plexus in the neck to anaesthetise the arm

Question 22

What is the brachial plexus?

Answer 22

Bundle of nerves that supplies the shoulder, arm & hand with feeling & movement

Question 23

Describe spinal/epidural anaesthesia

Answer 23

Local anaesthetic is injected into spinal OR around epidural the spinal cord
- Blocks nerve as they enter/rise within spinal cord
- Widespread effect (BELOW site of injection blocked)

Question 24

Describe intravenous regional anaesthesia (Bier block)

Answer 24

Local anaesthetic injected into a limb isolated from the rest of circulation using a pressure cuff

• Limit duration of procedure

e.g limb surgery

Question 25

Side effects of local anaesthetics

Answer 25

All are toxic
- High concentrations can cause life-threatening side effects

Question 26

How are drugs removed from their site of action?

Answer 26

Being broken down (ester bond) OR entering into general circulation to be diluted

Question 27

How can duration of action using ‘nerve block’ or ‘infiltration’ anaesthesia be increased?

Answer 27

Co-administrating a vasoconstrictor (e.g adrenaline) to prevent the drug entering the general circulation

Question 28

Why does spinal anaesthesia have the most widespread effects?

Answer 28

All nerves entering the cord BELOW the site of injection are blocked

Question 29

Which form of local anaesthesia might be used to reset a broken arm?

Answer 29

Intravenous regional anesthesia (Bier block)

Question 30

Why do local anaesthetic preparations sometimes contain adrenaline?

Answer 30

Prevent drug entering the general circulation