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Flashcards in Local Anaesthetics Deck (33)
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1

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

To stop nerve conduction by blocking the voltage gated Na channels

2

Which part of the nerve pathway does LA block

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

3

In what order are the different nerve fibres blocked by LA

As
C
Ab
Aa

4

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

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

5

Describe the mechanisms of LA

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

6

What effect does LA have on the heart

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

7

What are the 3 components of a LA molecule

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

8

How does a LA molecule present

Base.HCl

9

How does LA pass through the cell membrane

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

10

Why must LA molecules be both hydrophilic and hydrophobic

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

11

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

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

12

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

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

13

How does myelination of an axon affect LA block

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

14

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

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

15

What does a LA preparation include

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

16

What may allergy from LA be due to

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

17

What are the different ester LAs

Cocaine
Procaine
Benzocaine

18

What are the different amide LAs

Lignocaine (lidocaine)
Prilocaine
Articaine
Mepivacaine
Bupivacaine
Ropivacaine

19

How are the different types of LAs applied

Ester LAs are topical
Amide LAs are applied by injection

20

Why are vasoconstrictors used in LA

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

21

What different vasoconstrictors are used in LA

Adrenaline
Felypressin (synthetic vasopressin)

22

Which receptors do vasoconstrictors act upon

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

23

How does adrenaline act as a vasoconstrictor

Locally it acts on alpha receptors causing vasoconstriction

24

How does noradrenaline act as a vasoconstrictor

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

25

How does adrenaline act systemically

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

26

How can LA be inactivated

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

27

What are the different modes of administration of LA

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

28

Describe the lignocaine preparations for dental injections

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

29

Describe the prilocaine preparations for dental injections

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

30

How is a percentage solution calculated

X% solution = X mass/volume

31

What is the maximum dose of lignocaine

Approximately 4mg per kg body weight

32

What is the maximum dose of adrenaline

500ug

33

How much adrenaline does a cartridge contain

27.5