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BDS2 BAMS - Oral biology > LA symposium 2 - Local anaesthetics > Flashcards

LA symposium 2 - Local anaesthetics Flashcards

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1
Q

How do local anaesthetics stop nerve conduction

A

by blocking voltage-gated Na channels

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2
Q

which receptor in the nerve pathway does LA work on

A

first order afferent receptor (we don’t interfere with CNS)

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3
Q

Why will the nerve axon closest to the LA injection site be the first to be anaesthetised

A

it’s closer and the number of membranes that it has to cross is the same as axons further in the bundle

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4
Q

what very important characteristic does LA need to be to cross membranes in peripheral nerves

A

lipophilic

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5
Q

what impact does fat within the peripheral nerve have on the LA

A

allows it to stay longer

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6
Q

Nerve axons differ in their susceptibility to block by LA, what is the order that the fibres block in

A
  1. A delta
  2. C
  3. A beta
  4. A alpha
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7
Q

Why might a dentist tell a patient the tooth will be numb but you might feel pressure

A
  • because nerve axons differ in their susceptibility to block by LA
  • A alpha fibres blocked last are involved in proprioception
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8
Q

What are the functions of the different types of nerve axon

A

A alpha

  • sensory (proprioception)
  • motor (skeletal muscle)

A beta
- sensory (mechanoreception)

A gamma
- motor (muscle spindles)

A delta
- sensory (mechano-, thermo- noci- and chemo-receptors)

C

  • sensory (noci-, thermo- and chemo- receptors)
  • autonomic (post-ganglionic)
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9
Q

What is the mechanism of action of LA

A
  • LA binds to a site in the Na channel
  • LA blocks the channel and prevents Na+ influx
  • this blocks AP generation and propagation
  • block persists so long as a sufficient number of Na channels are blocked
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10
Q

what effect does LA have on the heart and why

A
  • can cause bradycardia and hypotension

- LA block Na channels in other excitable tissue e.g. heart muscle

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11
Q

what are the components of LA

A

3 components:

  • aromatic region (hydrophobic)
  • ester or amide bond
  • basic amine side chain (hydrophilic)
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12
Q

Why are LA’s presented as hydrochloride (B.HCl)

A

renders the amine base more water soluble (so increases solubility)

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13
Q

What form do LA’s have to be to cross the membrane

A

partly dissociated

  • active in ionised form
  • can cross membrane only in un-ionised form
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14
Q

Why are small diameter axons more susceptible to LA block

A

because of number of channels which are blocked. Small diameter axon has less channels to block

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15
Q

How do myelinated axons impact susceptibility to LA

A
  • more channels to block
  • Na+ channels (and K+ channels) are concentrated at the nodes of ranvier
  • so need safety factor
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16
Q

why do we need safety factor

A
  • on myelinated axons the local currents are strong enough to flow past the blocked region, and to regenerate the AP at the next node of ranvier
  • to block the AP, the LA needs to act on several nodes of ranvier along the axon
17
Q

How should LAs be prepared

A
  • LA base present as hydrochloride, to increase solubility in aqueous solution
  • for dental injections, 2-4% solutions
  • reducing agent (sodium metabisulphide)
  • preservative(s) and fungicide
  • +/- vasoconstrictor
18
Q

When people say they are allergic to LA what do they usually mean

A
  • that it’s the preservative/ reducing agent that they are allergic to
  • check manufacturer
  • possible but rare to be allergic to the actual LA
19
Q

what are the 2 types of LA

A
  • esters

- amides

20
Q

What is the ester that we normally use

A

Benzocaine (topical LA)

21
Q

What amides do we use

A
  • Lignocaine (lidocaine)
  • prilocaine
  • articaine
  • bupivacaine (surgical procedures)
22
Q

Why are vasoconstrictors added to most LAs

A
  • most LAs are vasodilators
  • increased blood flow will increase ‘wash-out’ of LA
  • to increase duration of action, LA preparations often include a vaso-constrictor
23
Q

what are used as vasoconstrictors

A
  • adrenaline

- felypressin (synthetic vasopressin)

24
Q

What do vasoconstrictors act on

A

receptors on vascular smooth muscle

  • adrenoreceptors (alpha and beta)
  • ADH receptors (vasopressin)
25
Q

What effect does adrenaline have as a vasoconstrictor

A
  • EQUALLY effective on alpha and beta receptors
  • given locally, it has a vasoconstrictor effect (alpha receptors)
  • systemically, it LOWERS TPR (as beta more than alpha)
  • increases cardiac output
  • has little or no effect on mean arterial bp
  • more force, more flow, less peripheral resistance
26
Q

what effect does NA have as a vasoconstrictor

A
  • NA is MORE effective on alpha than on beta receptors
  • given locally, it has a vasoconstrictor effect (alpha receptors)
  • systemically it INCREASES TPR (alpha more than beta)
  • NA increases cardiac output
  • overall raises mean arterial BP
  • BUT this can result in a fall of bp…
27
Q

why does the fact that NA raises mean arterial blood pressure actually potentially result in a fall in blood pressure

A

response of body that isn’t necessary is trying to compensate and reduce peripheral resistance - compensation overshoot

28
Q

How is LA inactivated

A
  • washout from tissues by blood supply
  • countered by presence of vasoconstrictor agent
  • ester types broken down by tissue esterases (action brief)
  • amide types broken down by liver amidases (action longer duration)
29
Q

What are the different modes of administration of LA

A
  • surface application (‘topical’)
  • injection
  • local infiltration
  • regional nerve block
  • nerve root block (‘spinal’, ‘epidural’)
  • intravenous
30
Q

What are the LA preparations for dental injections

A

Lignocaine

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

Prilocaine

  • 4% prilocaine HCl
  • 3% prilocaine HCl + felypressin (0.03U/ml)
31
Q

How do you work out what a solution should contain from percentages

A

X% solution = 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

32
Q

why are vasoconstrictors given as a ratio rather than a percentage

A

because very small amounts of vasoconstrictor are present in LA preparations

e.g. rather than <0.001%, 1:80,000 (same for fluoride concentrations)

33
Q

what is the max dose of lignocaine

A

approx 4mg per kg body weight

34
Q

what is the max dose of adrenaline

A

500µg (BNF)

35
Q

how much adrenaline does a cartridge contain

A

27.5µg

  • if this entire amount was injected into 5 litres of blood –> 5µg/l
  • plasma levels of adrenaline following dental injections increase with amount injected
  • the plasma levels following ‘normal’ injections are within the physiological range (up to 0.5µg/litre)
  • max physiological levels (intense exercise) can reach 0.5µg/litre

BDS2 BAMS - Oral biology (9 decks)

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