Local anaesthetics

Question 1

What are the three subtypes of local anaesthesia (and what do they mean)?

Answer 1

regional anaesthesia (loss of sensation to a region or part of body)
local infiltration (cut, skin incision)
topical (eye, skin)

Question 2

Name some non-reversible pharmacological methods of local anaesthesia?

Answer 2

phenol, ethanol, radiofrequency, surgical (kills the nerve)

Question 3

Define local anaesthetic

Answer 3

a drug which REVERSIBLY prevents transmission of the nerve impulse in the REGION to which it is applied without affecting CONSCIOUSNESS

Question 4

How do local anaesthetics prevent conduction of action potential?

Answer 4

blocking sodium channels

Question 5

Where is local anaesthetic injected?

Answer 5

AROUND the nerves (so must pass through the perineurium and endoneurium before having effect, hence the delay in action)

Question 6

Where does the local anaesthetic bind?

Answer 6

on the sodium channel INSIDE the cell (therefore must pass through cell membrane)

Question 7

Why can local anaesthetic pass through the cell membrane?

Answer 7

They are not ionised.

Question 8

What are the ideal characteristics of local anaesthetic?

Answer 8

• reversible
• good therapeutic index
• quick onset
• suitable duration
• no local irritation
• no side effects
• no potential to induce allergy
• applicable by all routes
• cheap, stable, soluble

Question 9

What does the onset of action depend on?

Answer 9

pKa (when pKa=pH, quickest onset)

Question 10

What is the pKa?

Answer 10

pH at which the ionised and non-ionised forms of local anaestheticas are equal

Question 11

Why doesn’t local anaesthetic work as well in areas of inflammation or where there is pus?

Answer 11

the pH of pus is much lower than the rest of the body (about 6.9) so the pKa and pH will be too different for action and there will be much more of the non-ionised form

Question 12

What does duration of action depend upon?

Answer 12

protein binding (more binding, longer duration of action)

Question 13

What is protein binding dependant on?

Answer 13

the length of the intermediate chain joining the aromatic and amide/ester group

Question 14

Define potency

Answer 14

dose required to produce a desired effect

Question 15

What does potency of LA depend on?

Answer 15

lipid solubility (more lipid soluble will be more potent)

Question 16

What does ability to block neuronal conduction depend upon?

Answer 16

type of nerve fibre (larger the fibre, slower the onset)

location of nerve fibre (deeper within nerves will take longer to be blocked)

Question 17

what are the advantages of using vasoconstrictors?

Answer 17

• prolong action
• reduce plasma levels (less risk of CNS effects)
• ‘greater anaesthesia’ or reduced dose
• reduced operative haemorrhage

Question 18

What are the contraindications of vasoconstrictors?

Answer 18

body parts that are supplied by end-vessels (fingers, toes, penis, ear lobe, alae of nose)

Question 19

How does adrenaline act as a vasoconstrictor?

Answer 19

stimulation of alpha adrenoreceptors constrict blood vessels

Question 20

Does adrenaline decrease the safe dose of anaesthetics?

Answer 20

NO

Question 21

Which type of LA are more likely to induce a hypersensitivity reaction?

Answer 21

ester LAs

Question 22

What is the main toxic effect of prilocaine?

Answer 22

methaemoglobinaemia (it oxidises ferropus to ferric ions –> hypoxia)

Question 23

What is the treatment of methaemoglobinaemia?

Answer 23

methylene blue

Question 24

What is the treatment for LA toxicity?

Answer 24

STOP INJECTING

• maintain airway
• give oxygen
• confirm/establish IV access
• control seizures
• consider drawing blood for analysis
• give intravenous lipid emulsion

Question 25

What is the toxic dose of lidocaine (with and without adrenaline)?

Answer 25

3 mg/kg

with adrenaline 7 mg/kg