Pharmacology of LA

Question 1

What is LA

Answer 1

When used of a specific nerve pathway, effects such as analgesia and paralysis can occur

Question 2

What can LA be produced

Answer 2

• Cooling with ethyl chloride
• Pressure (to reduce discomfort from injection in palatal tissue)
• Hypoxia
• Irreversible blockade

OR

• True local anaesthetics

Question 3

What are the different ways of applying LA

Answer 3

• Topical application
• Subcutaneous injection (1 or more)
• Nerve block
• Epidural
• Intrathecal

Question 4

What area has the fastest absorption

Answer 4

• Intravenous
• Mucous membrane
• Intercostal block
• Caudal block
• Epidural block
• Brachial plexus block
• Peripheral nerve block
• Subcutaneous infiltration

Question 5

What are the ideal properties of LA

Answer 5

• Stable in solution
• Non-irritating to tissues
• No permanent damage to the tissues
• No systemic toxicity
• No allergic response
• Potent
• Rapid onset

Question 6

What does mean by the aromatic terminal being lipophilic

Answer 6

It has no positive or negative charge and can penetrate the fatty tissues such as the lipid sheath of nerves to gain access to the nerve cell membrane to reach its site of action

Question 7

Why is water solubility of the amino terminal of LA important

Answer 7

1. To allow for the dissolution in a solvent to permit injection
2. To allow penetration through interstitial fluid following administration

Question 8

Answer 8

Question 9

What is Dyclonine

Answer 9

A ketone-type liquid topical anaesthetic agent that may be applied with a swab or used as a mouth-rinse or throat lozenge
- Has bactericidal and fungicidal properties

Question 10

Draw Dyclonine

Answer 10

Question 11

What is the sequence of block onset by the LA

Answer 11

• B fibres (autonomic)
• C fibres (unmyelinated)
• Aγ fibres (sensory)
• Aβ fibres (sensory)
• Aα (sensory)

Question 12

What is the hierarchy of LA effects on sensory function

Answer 12

1. Cold
2. Warmth
3. Pain (first pain, then second pain)
4. Touch
5. Deep pressure
6. Motor function

Question 13

How does LA work?
What happens if the Na+ inflxux is interrupted

Answer 13

• LA acts by inhibiting an influx of Na+ through sodium channels
• When the Na+ influx is interrupted an action potential cannot be generated and signal conduction is inhibited

Question 14

What are some symptoms of hypersensitivity

Answer 14

Results in anaphylactic reaction and can range from skin rash to full anaphylactic shock - its a problem with esters but rare with amides

Question 15

What does Local anaesthetic system toxicity effect

Answer 15

Heart and brain

Question 16

What is methemoglobinemia

Answer 16

Main toxic effect of prilocaine and articaine when administered in high doses, symptoms are cyanosis, lethargy and respiratory depress which doesn’t respond to oxygen

Question 17

What does vasoconstriction restrict

Answer 17

The diffusion of local anaesthetic away from the site of injection and thus allows the LA to stay at the site longer, producing a long duration of action

Question 18

Why are LAs given with vasoconstrictor

Answer 18

• Prolong action
• Reduce plasma levels
• ‘Greater anaesthesia’ or reduced dose
• Reduced operative haemorrhage

Question 19

What is the action of the vasoconstrictor

Answer 19

• Delays distribution in circulation away from site of action
• Decreases the peak serum concentration and toxic effect and allows metabolism to keep pace with absorption
• Greatly intensifies and prolongs the duration of action

Question 20

What do preparations with vasoconstrictors usually contain

Answer 20

• Epinephrine (adrenaline) 1:200,000
• Norepinephrine (noradrenaline) 1:100,000

Question 21

When are vasoconstrictors not used with LAs

Answer 21

When injected into extremities like fingers and toes - limited circulation so may result in hypoxia