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BDS2 BAMS > Local Anaesthetics > Flashcards

Local Anaesthetics Flashcards

1
Q

basics of how LA work

A

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

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2
Q

what part of the nervous system do LAs work on

A

peripheral nervous system (doen’t touch CNS)

work on first order afferent receptors

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3
Q

what do the connective tissue layers in peripheral nerves act as

A

diffusion barriers

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4
Q

what factor determines which nerve will be affect by LA first , when they are in similar proximity to the LA

A

number of membranes that are barriers

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5
Q

what is the general rule for anaesthetising and weaning out

A

in general anaesthetised first, tends to be weaned out first

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6
Q

what type of molecule can cross connective tissue nerve membranes

A

lipophillic

aromatic ring

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7
Q

how can the lipophilic characteristics of LA influence how they perform

A

adheres, penetrates the membrane easier, like to be around fat - take longer to leave, will work in certain areas longer

fat allows to stay in nerve for longer

Cannot be too great affinity as can stay in fat and not reach nerve

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8
Q

what is the order of blockage by LA in different nerve fibres

A

A delta
C
A beta
A alpha

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9
Q

how can we assess whether a nerve fibres have been blocked or not

A

different nerve fibres have different functions so can assess if worked effectively

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10
Q

myelination status of A alpha

A

myelinated

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11
Q

myelination status of A beta

A

myelinated

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12
Q

myelination status of A gamma

A

myelinated

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13
Q

myelination status of A delta

A

myelinated

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14
Q

myelination status of C fibres

A

unmyelinated

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15
Q

function of A alpha fibres

A

sensory (proprioception)

motor (skeletal muscle)

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16
Q

function of A beta fibres

A

sensory (mechanoreceptors)

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17
Q

function of A gamma fibres

A

motor (muscle spindles)

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18
Q

function of A delta fibres

A

sensory (mechano-, themo-, noci- and chemoreceptor)

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19
Q

function of C fibres

A

sensory (noci-, thermo- and chemoreceptors)

autonomic (post ganglionic)

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20
Q

what function of C nerve fibres do you not want to anaesthetise

A

autonomic (post ganglionic)

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21
Q

mechanism of LA action

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

Block persists so long as a sufficient number of Na+ channels are blocked

  • Enough blocked but not necessarily all
  • Enough to avoid AP reaching maximum level

Local anaesthetics block Na+ channels in other excitable tissue, e.g. heart muscle
- LA can cause bradycardia and hypotension
Distressing for pt
- could faint

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22
Q

what group of nerve fibres are you aiming to anaethetise

A

A beta (3rd in order)

sensory (mechanoreceptors)

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23
Q

what class of molecule is LA

A

organic

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24
Q

what are the 3 basic components of LA

A

aromatic ring (hydrophobic)

ester or amide bond

basic amide side chain (hydrophilic)

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25
Q

what is evidence that the amine base in more water soluble than the chloride?

A

the LA is presented as a hydrochloride (B.HCl)

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26
Q

what is the active form of LA

A

B.H+

partially dissociated

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27
Q

what is the diffusible form of LA

A

B (inactive)

cannot cross membrane in active B.H+ form

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28
Q

what form of LA renders and block sodium channels in nerve

A

B.H+

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29
Q

how many B.H+ active molecules are needed to deactivate each sodium channel

A

one per channel

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30
Q

why are small diameter axons more susceptible to LA block

A

more channels are blocked proportionally

  • less number of total channels
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31
Q

what is the distribution of sodium channels like in myelinated nerves

A

greater concentration at nodes of Ranvier

- more channels to block

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32
Q

is anaethetism by LA a linear process

A

no

33
Q

what is the ‘safety factor’ of myelinated nerves

A

The local currents are strong enough to flow past the blocked region, and to regenerate the AP at the next node of Ranvier

  • Block one node of ranvier
  • Not able to block nerve or that axon
  • Can bypass that LA block

Need to block a greater area

Need to dissolve to greater extent to anaesthetised axons that are myelinated

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

34
Q

what must the LA do in order to be effective on a myelinated nerve

A

Need to dissolve to greater extent to anaesthetised axons that are myelinated

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

35
Q

what is the role of the LA base present as hydrochloride in LA preparations

A

needed to increase solubility in aqueous solutions

36
Q

what is the % range for dental preparations of LA

A

2-4%

37
Q

what is the cartridge volume for dentistry LA use in UK

A

1.8 or 2.2ml

38
Q

what is a common reducing agent in LA

A

sodium metabisulphide

39
Q

4 basic constituents of LA preparations for dental use

A

LA base as hydrochloride

reducing agent (sodium metabisulphide)

preservative(s) and fungicide

and/or vasoconstrictor

40
Q

what is the role of vasoconstrictor in LA preparations

A

prolong effect of aesthetic

  • More time in certain area
  • less LA volume needed

thus

  • Reduce clearance
  • Reduce need for larger volume
41
Q

2 disadvantages of adding a vasoconstrictor

A

increase HR

Reduces the blood flow - needed in certain treatments
- Periodontal treatment - recreate healthy tissue (flap - stick gingiva from other area over)

Need to consider if affected by vasoconstrictor
- Tissue may not survive

42
Q

what is the more likely cause of pt being allergic to LA

A

they are allergic to brand of LA

Preservative and reducing agent (preservative vary brand to brand, reducing less so)

E.g. Latex glaze on plug - common allergy

43
Q

what should you do if you recognise that a pt is having an allergic reaction to a LA

A

note the product and manufacturer

both need to be ID to work out true cause of reaction
- rare to react to lidocaine

44
Q

2 families of LA

A

esters

amides

45
Q

3 ester LAs

A

benzocaine

(cocaine)

(procaine)

46
Q

use of benzocaine

A

ester LA

Topical anaesthetic reduce pain but sting

Taste unpleasant

Varies between clinicians whether to use
- Technique can reduce need for topical

47
Q

use of cocaine as LA

A

ester LA

good topical LA
hard to use

48
Q

use of procaine as LA

A

ester LA

has toxic effects

49
Q

6 amide LAs

A

ligocaine (lidocaine)

prilocaine

articaine

bupivacaine

(mepivacaine)

(ropivacaine)

50
Q

what is a commonly used LA

A

ligocaine (lidocaine)

amide LA

51
Q

common action of LA on circulation

A

most LAs are vasodilators

52
Q

what is the effect of increased blood flow on the action of LA

A

increase ‘wash-out’

53
Q

what can be added to LA to increase their duration of action

A

a vasoconstrictor

e.g. adrenaline, felypressin (synthetic vasopressin, for prilocaine)

54
Q

what do vasoconstrictors act on

A

receptors in vascular smooth muscle

adrenoreceptors
- alpha
- beta 2
- beta 1
ADH receptors
55
Q

in general what is the effect of vasoconstrictors

A

heart rate and force increased

56
Q

alpha adrenoreceptor vasoconstrictor effect

A

vasoconstriction

57
Q

beta 2 adrenoreceptor vasoconstrictor effect

A

vasodilation

58
Q

beta 1 adrenoreceptor vasoconstrictor effect

A

on cardiac muscle

  • positive chronotropic effect (increase rate)
  • positive inotropic effect (increase force)
59
Q

effect of adrenaline as a vasoconstrictor

A

Adrenaline is equally effective on alpha and beta receptors equal effect
- not preferential

Given locally, it has a vasoconstrictor effect (action on alpha receptors)

Systemically, it lowers TPR (beta > alpha - works on beta-2 more than alpha)

increases Cardiac Output

Overall, adrenaline has little or no effect on mean arterial BP
- More force and flow and less peripheral resistance

increase in HR and force gives feelings of palpitations

60
Q

effect of noradrenaline as a vasoconstrictor

A

More effective on alpha than beta receptors

Given locally, it has a vasoconstrictor effect (alpha receptors)

Systemically, it increases TPR (alpha > beta)

  • Effect lesser on beta
  • So increase in arterial BP

increases Cardiac Output

Overall, NA raises mean arterial BP
- can cause a FALL of BP –Response of body when have increase in BP us not necessary or physiological
- Drive blood flow to other areas
- Reduce peripheral resistance
Can overshoot if go too high can cause fall in BP (overshoot)

61
Q

what is a possible side effect of noradrenaline as a vasoconstrictor

A

Overall, NA raises mean arterial BP
- can cause a FALL of BP –Response of body when have increase in BP us not necessary or physiological
- Drive blood flow to other areas
- Reduce peripheral resistance
Can overshoot if go too high can cause fall in BP (overshoot)

62
Q

what process occurs to inactivate LA

A

Washout’ from tissues by blood supply

Countered by presence of vasoconstrictor agent

63
Q

how are ester LAs broken down

A

by tissue esterases

64
Q

in general what is the duration of action of ester LAs

A

action is quite brief

topical anaesthetic

65
Q

how are amide LAs broken down

A

by liver amidases

66
Q

in general what is the duration of action of amide LAs

A

longer duration of action

67
Q

6 modes of LA administration

A

Surface application (‘topical’)

Injection

Local infiltration

Regional nerve block

Nerve root block (‘spinal’, ‘epidural’)

Intravenous

68
Q

why is it important to ask about pt liver medical history

A

amide LAs are broken down by liver amidases

re-evaluate how to use LA as process of removal is effected

69
Q

what type of nerve blocks do dentists use

A

local nerve blocks
- ID nerve block

rare to use root nerve blocks (maxillofacial)

70
Q

2 common drugs used for LA preparations in dentistry

A

ligocaine

prilocaine

71
Q

common LA dental preparation of ligocaine

A

2% ligocaine HCl

2% ligocaine HCl and 1:80,000 adrenaline

72
Q

common LA dental preparation of prilocaine

A

4% prilocaine HCl

3% prilocaine HCl + felypressin (0.03Uml)

73
Q

why would there need to be a higher concentration of prilocaine in LA preparation without a vasoconstrictor

A

reduced clearance with vasoconstrictor hence lesser volume required

74
Q

% solution =

A

X mass/volume

e.g.
3% Prilocaine HCl solution
- 3% = 3g / 100ml
- = 30mg / 1ml

A 2ml cartridge of 3% prilocaine HCl will contain
2 x 30 = 60mg prilocaine HCl

In clinic 2.2 calculate

75
Q

1ml=

A

1g

76
Q

how are vasoconstrictors volumes given for LAs

A

very small volumes are present

content is expressed as a ratio:

e. g. 1:80,000
- 1 part of adrenaline in 80,000 parts of liquid

77
Q

maximum dose of ligocaine

A

4mg per kg body weight

Reach max dose of LA first before adrenaline
- But symptoms tend to be more associated with adrenaline
Although bradycardia is serious

78
Q

maximum dose adrenaline

A

500 micrograms (ug)

Cartridge contains 27.5 ug adrenaline
- If this entire amount was injected into 5 litres of blood then 5ug/l

Plasma levels of adrenaline following dental injections increase with amount injected
- Increase in force and speed of heat

The plasma levels following ‘normal’ injections are within the physiological range (up to 0.5 ug /litre) *
- Not inject BV – entering blood stream

Maximum physiological levels (intense exercise) can reach 0.5 ug /litre

Reach max dose of LA first before adrenaline
- But symptoms tend to be more associated with adrenaline
Although bradycardia is serious

BDS2 BAMS (54 decks)

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