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
