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Flashcards in Local Anaesthetics Deck (45)
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1

what do local anaesthetics do

stop nerve conduction by blocking the voltage-gated Na+ channels

2

where on the nerve pathway does LA affect

first order afferent neuron

3

where can you get information on the toxicity of LA and the maximum dosage

BNF

4

where are blood vessels found within the peripheral nerve

within the bundles but also surrounding the nerve and whole structure

5

what nerves are anaesthetised first

Nerves in the proximity of the local anaesthetic will be anaesthetised first
the number if membranes the LA has to pass through plays a part as well

6

molecules that cross membranes need to be what

> lipophilic
> have an aromatic ring - this is in LA which gives the characteristics of being lipophilic

7

what benefit is there to having fat in the nerve bundles

Allows LA to remain for longer
LA Cannot be too hydrophilic as it will stay in the fat and not really reach the nerve
Needs to have an affinity but not a great affinity

8

if the LA acts fast then does it tend to leave the space faster or stay longer

leave the space faster

9

which axons are most affected by LA (most to least)

A delta
C
A beta
A alpha

10

describe the structure and function of a alpha axons

group I

myelinated

Sensory (proprioception)
Motor (skeletal muscle)
Last ones for LA to work on
Important to reassure patient that the tooth is numb but can still feel a pressure

11

describe the structure and function of a beta axons

group II

myelinated

Sensory (mechanoreception)

12

describe the structure and function of a gamma axons

myelinated

motor (muscle spindles)

13

describe the structure and function of a delta axons

group III

myelinated

Sensory (mechano-, thermo-, noci- & chemo-receptors)
Most affected - as they are sensorial they are what we are looking for the LA to work on along side the a delta and c

14

describe the structure and function of c axons

group IV

unmyelinated

Sensory (noci-, thermo- & chemo-receptors)
Autonomic (post-ganglionic) [we do not want to anaesthetise these]

15

describe the mechanism of the action of LA

• LA binds to a site in the NA+ channel
• LA blocks the channel and prevents Na+ influx
• This blocks action potential generation and propagation
• Block persists so long as a sufficient number of Na+ channels are blocked
○ This is important - don’t need to have all the Na channels blocked but you do need to have enough blocked to stop the AP from reaching the maximum level

16

what happens when LA blocks Na channels in other excitable tissues like the heart muscle

• NB: LA block Na+ channels in other excitable tissue eg heart muscle
• LA can cause bradycardia and hypotension
○ This is a worrying / stressing side effect / symptom
○ Injecting LA with or without vasoconstriction - want to stop it entering the main system due to this
○ Can cause the patient to faint
§ Hypotension is what is generated for the fainting but bradycardia is what you want to look out for?

17

what is in LA

• Organic molecules, three components:
○ Aromatic region (hydrophobic)
○ Ester or amide bond
○ Basic amine side chain (hydrophilic)

18

what is LA presented as

• Presented as hydrochloride (B.HCl)
○ Renders the amine base more water soluble
B = base, HCl = hydrochloride

• Partly dissociated
○ Active in ionised form
Can cross membrane only in un-ionised form

19

describe B.H+ in the LA

pharmacologically active
non-diffusible
base hydrogen can dissociate
both B and B.H can bind
BH has greater affinity

20

describe B. in the LA

diffusible
non-active
no way for LA to cross membrane to be active if it is ionised - membrane will not allow this

21

LA mechanisms of action picture

dont think i understand this enough to ask a question about it in words cos it looks so confusing but go look at it and test that you know where everything goes cos i havent a clueeee hhahaha sorry xoxo

22

why are small diameter axons more susceptible to LA block

- Due to the number of channels that are blocked
○ Don’t need to block all the channels but need to block a great proportion to stop the signal

- Small diameter axons = have less number of channels
○One molecule per blocking will act better on a smaller diameter than a larger diameter as there are less channels to block

23

how does LA work on myelinated axons

• Na+ channels (and K+ channels) are concentrated at the nodes of Ranvier
Nodes = have a greater concentration of channels as they need to generate AP in a saltatory way - so it can jump from one to another

24

what is the safety factor for using LA on myelinated axons

• The local currents are strong enough to flow past the blocked region and to regenerate PA at the next node of Ranvier
Ie: blocking one node of Ranvier will not be enough the block the whole axon / nerve because the jumping is so strong it can just by pass the LA

To block the AP, the LA needs to act on several nodes of Ranvier along the axon

25

what is included in LA preparations

• LA base present as hydrochloride, to increase solubility in aqueous solution
• For dental injections, 2-4% solutions ○ UK: 2.2% but cannot guarantee this
• Reducing agent ○ Sodium metabisulphide
• Preservative(s) and fungicide
± vasoconstrictor

26

what is the vasconstrictor used for

Used to prolong the effects of the anaesthetic and reduces the need for a higher concentration of LA

27

what are the disadvantages of using vasoconstrictors

§ There are problems with adrenaline
□ Reduces blood flow

This is a problem when doing a procedure like moving a piece of gingiva tissue to another area (flap / periodontal treatment) and it is not beneficial to have less blood flow as it might mean the tissue won't survive

28

can patients be allergic to LA

yes but these are very rare situations (usually have other allergies)

usually ingredients in the LA are what cause the allergic reaction
such as the preservative or the reducing agent

some LA use latex and patients can be allergic to these but a lot of places are latex free now a days

29

name local anaesthetics that are esters

○ Cocaine
§ Good topical anaesthetic
§ not usually used
§ Hard to use

○ Procaine
§ Toxic effects
§ not usually used

○ Benzocaine
§ Topical anaesthetic we use
□ Stings
□ Bad taste
□ But does reduce pain

○ Some clinicians don’t like to use topical anaesthetics and wouldn’t use it but others wouldn’t do LA without it
Technique is more important than topical anaesthetics to reduce sensitivity without the need for topical as it doesn’t really solve the problem anyway

30

name local anaesthetics that are amides

○ Lignocaine (lidocaine)
§ Used

○ Prilocaine
○ Articaine
§ These 2 above are used more in specific situations

○ Mepivacaine
§ not usually used

○ Bupivacaine
§ Used in certain procedures

○ Ropivacine
§ not usually used