Local Anaesthetics

Question 1

What is the function of local anaesthetics and how do they achieve this

Answer 1

To stop nerve conduction by blocking the voltage gated Na channels

Question 2

Which part of the nerve pathway does LA block

Answer 2

Anaesthetises the receptor and the axon before it reaches the spinal dorsal horn or trigeminal nucleus

Question 3

In what order are the different nerve fibres blocked by LA

Answer 3

As
C
Ab
Aa

Question 4

Why may patients feel discomfort but not pain during injection of LA

Answer 4

Because of the activity of proprioceptors which are the last fibres to be anaesthetised

Question 5

Describe the mechanisms of LA

Answer 5

LA binds to a site in the Na channel
LA blocks the channel and prevents NA influx
This blocks AP generation and propagation
The block persists so long as a sufficient number of Na channels are blocked

Question 6

What effect does LA have on the heart

Answer 6

LA blocks Na channels in the heart as it is an excitable tissue
This can cause bradycardia and hypotension

Question 7

What are the 3 components of a LA molecule

Answer 7

Aromatic region - hydrophobic
Ester or amide bond
Basic amine side chain - hydrophilic

Question 8

How does a LA molecule present

Answer 8

Base.HCl

Question 9

How does LA pass through the cell membrane

Answer 9

The active form - B.H+ - must breakdown so the base can diffuse through the membrane

Question 10

Why must LA molecules be both hydrophilic and hydrophobic

Answer 10

Hydrophilic as they need to be soluble in water to move around in the tissues
Hydrophobic as they need to enter the membrane to work inside the cell

Question 11

What will make it more difficult for LA to cross the cell membrane

Answer 11

In acidic areas with high H+ concentration, the dissociation of B.H+ will not happen quickly

Question 12

How does diameter affect how an axon is susceptible to LA block

Answer 12

Na channels are distributed evenly so axons with a larger diameter will have more Na channels
This means you will need more LA to act on all these channels and it will take more time to occur

Question 13

How does myelination of an axon affect LA block

Answer 13

Na channels are concentrated at the nodes of Ranvier where the axon is exposed to LA

Question 14

Why is the term safety factor used when blocking myelinated axons with LA

Answer 14

If LA only acts upon a small region, the saltatory effect can jump across the anaesthetised area and produce and continue an AP
Must have a higher level of LA to act in several nodes to block the AP

Question 15

What does a LA preparation include

Answer 15

LA base
Reducing agent (sodium metabisulphide)
Preservatives and fungicide
A vasoconstrictor

Question 16

What may allergy from LA be due to

Answer 16

Most likely the a preservative or fungicide
May be the reducing agent
Rarely the LA

Question 17

What are the different ester LAs

Answer 17

Cocaine
Procaine
Benzocaine

Question 18

What are the different amide LAs

Answer 18

Lignocaine (lidocaine)
Prilocaine
Articaine
Mepivacaine
Bupivacaine
Ropivacaine

Question 19

How are the different types of LAs applied

Answer 19

Ester LAs are topical

Amide LAs are applied by injection

Question 20

Why are vasoconstrictors used in LA

Answer 20

Most LAs are vasodilators and increased blood flow will increase wash out of LA
Vasoconstrictors increase duration of action of LA and as LA isn’t washed out, the concentration will be greater, so you can use less

Question 21

What different vasoconstrictors are used in LA

Answer 21

Adrenaline

Felypressin (synthetic vasopressin)

Question 22

Which receptors do vasoconstrictors act upon

Answer 22

Adrenoreceptors:
Alpha receptors - generate vasoconstriction
Beta 2 receptors - generate vasodilation
Beta 1 receptors in cardiac muscle - have a positive chronotropic and inotropic effect so increase heart rate and force

Vasopressin acts upon ADH receptors

Question 23

How does adrenaline act as a vasoconstrictor

Answer 23

Locally it acts on alpha receptors causing vasoconstriction

Question 24

How does noradrenaline act as a vasoconstrictor

Answer 24

Locally it has a vasoconstrictive effect as it acts on alpha receptors
Systemically it increases TPR as it acts more on alpha receptors
This increases cardiac output and raises the mean arterial BP
This results in a fall in BP

Question 25

How does adrenaline act systemically

Answer 25

Systemically it lowers TPR as it acts more on beta receptors
It increases cardiac output
Overall, it has little or no effect on mean arterial BP

Question 26

How can LA be inactivated

Answer 26

Washout from tissues by blood supply
Countered by presence of vasoconstrictor agent
Ester types broken down by tissue ester ashes - brief action
Amide types broken down by liver amid ashes - long action

Question 27

What are the different modes of administration of LA

Answer 27

Surface application - topical
Injection
Local infiltration
Regional nerve block
Nerve root block (spinal, epidural)
Intravenous

Question 28

Describe the lignocaine preparations for dental injections

Answer 28

2% lignocaine HCl
or
2% lignocaine HCl + 1:80,000 adrenaline

Question 29

Describe the prilocaine preparations for dental injections

Answer 29

4% prilocaine HCl
or
3% prilocaine HCl + felypressin (0.03U/ml)

Question 30

How is a percentage solution calculated

Answer 30

X% solution = X mass/volume

Question 31

What is the maximum dose of lignocaine

Answer 31

Approximately 4mg per kg body weight

Question 32

What is the maximum dose of adrenaline

Answer 32

500ug

Question 33

How much adrenaline does a cartridge contain

Answer 33

27.5