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BDS2 LA > Local Anaesthetic > Flashcards

Local Anaesthetic Flashcards

1
Q

What do local anaesthetics do?

A

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

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2
Q

What nerve in nerve pathways do local anaesthetics work on?

A

The first order afferent nerve receptors (we dont touch the CNS)

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3
Q

What acts as a diffusion barrier for local anaesthetics in a peripheral nerve?

A

Connective tissue layers

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4
Q

What connective tissue layers are found in a peripheral nerve and where?

A
  • Epineurium = around full nerve
  • Perineurium = around bundles of axons
  • endoneurium = found around the myelin shealth in myelinated nerve fibres (axons)
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5
Q
A

A will be blocked first because it is closer to the injection site but also because the number of membranes it has to diffuse through are the same as B (or possibly less)
Those in close proximity to the LA are anaethetised first

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6
Q

Which nerve from the previous card will the LA wear out on first?

A

A - in general the nerve that is anaesthetised first will wear out first
NOTE: however, there can be other factors

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7
Q

What characteristic does LA need to have to be able to cross the membranes?

A

Lipophilic

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8
Q

Nerve axons differ in their susceptibility to block by LA. What is the order of block of different nerve fibres by LA?

A
  • Aδ
  • C
  • Aβ
  • Aα
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9
Q

Describe Aα nerve axons.

A

-myelinated
Function = sensory (proprioception) Motor (skeletal muscle)

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10
Q

Describe Aβ axons.

A

-myelinated
Function = sensory (mechanoreception)

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11
Q

Describe Aδ nerve axons.

A

-myelinated
Function = sensory (mechano- thermo-, noci- and chemo-receptors)

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12
Q

Describe C-fibres.

A

-unmyelinated
Function = sensory (noci-, thermo- and chemo-receptors) Autonomic (post-ganglionic)

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13
Q

Due to the order of block of different axons, what senses are therefore blocked first and last?

A 
Sensory functions (A-delta) [mechano-, thermo-, noci- and chemoreception]
Proprioception blocked last (A -alpha) - some patients feel movement but wont feel pain but this can make some P's anxious
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14
Q

What is the mechanism of action for local anaesthetics?

A
  • LA binds to a site in the Na+ channel
  • LA blocks the channel and prevents Na+ influx
  • This blocks action potential generation and propagation
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15
Q

How long does the block on axons from LA persist?

A

As long as a sufficient number of Na+ channels are blocked on the axon

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16
Q

Do all the Na+ channels on an axon need to be blocked to stop AP generation and propagation?

A

No - just a sufficient number

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17
Q

Where else can local anaesthetics block Na+ channels and what can this cause?

A
  • Can block Na+ channels in other excitable tissues such as heart muscle
  • can cause bradycardia and hypotension
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18
Q

What might happen to a patient if they get hypotension as a result of an LA injection?

A

Could faint

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19
Q

What are local anaesthetics made up from?

A

Organic molecules

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20
Q

What are the 3 components of LA’s?

A
  • aromatic region (hydrophobic)
  • ester or amide bond
  • basic amine side chain (hydrophilic)
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21
Q

In what form is the local anaesthetic presented? (comes in)

A

-hydrochloride (B.HCL)

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22
Q

Whatsd fn

A

renders thte base more water soluble

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23
Q

The LA is partly what?

A

Dissociated

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24
Q

In what form is the LA active?

A

ionised form (B.H+)

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25
In what form can the LA diffuse across membranes?
un-ionised form only
26
Describe LA's mechanism of action. (getting to the sodium channels)
basically the B.H+ needs to become un-ionised to cross the membrane then activate again (ionise)
27
Why are smaller diameter axons more susceptible to LA blocks?
They have less Na+ channels so takes less LA to sufficiently block the nerve
28
In myelinated axons, where are the Na+ channels found?
The Na+ channels (and K+) are concentrated at the Nodes of Ranvier
29
To be able to block a myelinated axon, what needs to be done? Why?
The LA needs to act on several nodes of Ranvier along the axon This is because the local currents are stronge enough to flow past a blocked Node of Ranvier and regenerate the AP at the next Node of Ranvier
30
The LA base is present as a hydrochloride, why?
To increase solubility in aqueous solution
31
For dental injections, what % solutions are the preparations?
32
What cartridge size is normally used? Why is knowing the cartridge size important?
Get 1.8 and 2.2 - 2.2 normally used Important as it changes how you calculate the max dosage
33
What else is present in an LA injection other than the hydrochloride?
-reducing agent (sodium metabisulphide) -preservatives an fungicide - + or - a vasoconstrictor
34
What does a vasoconstrictor in LA do/provide?
- used to prolong the effect of the LA - means you can use less LA - BUT reduces blood flow
35
\*\*you will hear about P's that are allergic to LA History of P - they have has LA before without an allergic reaction They could be allergic to a brand This is the reducing agent and the preservative is what differs between brands Presence of these 2 components - important to know risk of allergic reaction to them If have a reaction make sure to note the brand, batch and manufacture - could be the key to identify the real cause of the allergic reaction
36
What are the 2 subtypes of LA?
- esters - amides
37
What do ester LA's tend to be?
Topical anaesthetics
38
What is an ester used for topical anaesthetic?
Benzocaine
39
What amides are used as LA's?
- lignocaine (lidocaine) - Prilocaine - Articaine - Bipivacaine
40
In terms of vasodilation/vasoconstriction, what are most local anaesthetics?
Vasodilators
41
What is the problem with local anaesthetics being vasodilators?
-will cause increased blood flow and will increase the 'wash-out' of LA
42
How can the duration of action of an LA be increased?
-including a vaso-constrictor
43
What are some vasoconstrictors that can be used in LA?
- adrenaline (most common) - Felypressin (synthetic vasopressin)
44
What do vasoconstrictors act on?
On receptors on vascular smooth muscle
45
What different adrenoreceptors do you get?
- α receptors and β2 receptors on blood vessels - β1 receptors on cardiac muscle
46
What do alpha receptors cause?
Vasoconstriction
47
What is the effect of activation of beta-2 receptors?
Vasodilation
48
What are the effects of beta-1 receptor activation?
- positive chrontropic effect (increased heart rate) - positive inotropic effect (increased force of heart)
49
What adrenoreceptors is adrenaline more effective on?
Is equally effective on both alpha and beta receptors
50
Adrenaline given locally has what effect?
Vasoconstrictor effect (action on alpha receptors)
51
Systemically, what effect does adrenaline have?
works on beta 2 receptors and lowers total peripheral resistance
52
What effect does adrenaline have on cardiac output?
increases caridac output
53
What might the patient complain of due to increased cardiac output?
feelings of palpitations due to increased HR and force
54
What effect does adrenaline have on mean arterial BP?
has little or no effect on mean arterial BP
55
What adrenoreceptor is noradrenaline more effective on?
More effective on alpha receptors than beta
56
What effect does noradrenaline have when given locally?
vasoconstrictor effect (alpha receptors)
57
What does noradrenaline do systemically?
Increases total peripheral resistance (works on alpha recptors)
58
What effect does noradrenaline have on cardiac output?
increases CO
59
What effect does noradrenaline have on mean arterial blood pressure?
raises it
60
The raise in mean arterial BP from noradrenaline can lead to a fall of BP. Why?
Bodys response to the increase in mean arterial BP to compensate for this change
61
How is LA inactivated?
Washes out from the tissues by the blood supply to then be broken down
62
How are ester LA's broken down?
Broken down by tissue esterases
63
How are amide type LA's broken down?
By liver amidases
64
Which type of LA has a longer duration of action?
Amides (esters action is quite brief)
65
What might be problem with LA's in a patient with a liver problem?
Liver important for breakdown of amide LA's so safe doses will need to adjusted
66
What are the modes of administration for LA?
- surface application (topical) - injection - local infiltration - regional nerve block - nerve root block (spinal, epidural) - intravenous
67
What is the formula for percentage solution?
X% solution = X mass/volume
68
How much prilocaine HCl will be in a 2ml cartridge of a 3% solution?
e.g. 3% prilocaine HCl solution: 3% = 3g/100ml =30mg/1ml A 2ml cartridge of 3% prilocaine HCL will contain 2x30 = 60mg of prilocaine HCL
69
As the solution of vasoconstrictors are very dilute in LA, how are they expressed?
As ratios e.g. 1:80,000 1 part of adrenaline in 80,000 parts liquid
70
What is the maximum dose for Lignocaine?
Approx. 4mg per kg body weight
71
What is the maximum dose of adrenaline?
500 ug (B.N.F.)
72
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BDS2 LA (4 decks)

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