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Medicine Year 1 Semester 1 - Part 2 > Local Anaesthetics > Flashcards

Local Anaesthetics Flashcards

1
Q

Give named examples of local anaesthetics

A

Amide-containing

  • Lidocaine
  • Prilocaine
  • Bupivacaine / Levobupicaine
  • Articaine

Ester-containing

  • Tetracaine
  • Chloroprocaine
  • Benzocaine
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2
Q

Lidocaine

A

Medium acting
Rapid onset

[amide containing]

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3
Q

Prilocaine

A

Medium acting
No vasodilation
Used in epidurals

[amide containing]

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4
Q

Bupivacaine

A

Long-acting
Slow onset

[amide containing]

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5
Q

Articaine

A

Short-acting
Rapid onset

[amide containing]

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6
Q

Mepivacaine

A

Dental use

[amide containing]

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7
Q

Tetracaine

A

Long-acting
Very slow onset

[ester containing]

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8
Q

Tetracaine feature

A

Poor solubility
More hydrophobic and so stays around site for longer than procaine.

Longer duration of action

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9
Q

Chloroprocaine

A

Medium acting

[ester containing]

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10
Q

Benzocaine

A

Atypical mechanism of action
e.g. Throat lozenges

[ester containing]

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11
Q

Eryrthroxylon coca

A

Cocaine
Local anaesthetic

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12
Q

Explain the clinical uses of local anaesthetics

A

When loss of consciousness is neither necessary or desirable.

As an adjunct to surgery to avoid high-dose general anaesthetics.

Post-operative analgesia

For major surgery, with sedation.

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13
Q

Basic structure of local anaesthetics

A

They all have :

  • Aromatic region
  • an ester OR amide bond
  • basic amine side-chain
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14
Q

Feature of local anaesthetics

A

They are lipophilic

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15
Q

Why are amides-containing anaesthetics more commonly used in anaesthetics than ester-containing anaesthetics?

A

Esters are more unstable and can be metabolised into compounds associated with allergic reactions.

Amides are more commonly used as they are less likely to be metabolised.

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16
Q

Mechanisms of action of local anaesthetics

A

Local anaesthetics stop the spread of pain and work by reversibly blocking voltage gated sodium channels.

  • stabilises excitable membranes
  • prevents membranes form being depolarised (excited)
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17
Q

Structure of a peripheral nerve

A

Consists of fibres of different function, diameter and insulation.

All can be blocked by Local Anaesthetics, but at different rates.

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18
Q

Order of Loss when using a LA

A

Pain
Temperature
Proprioception
Skeletal muscle tone

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19
Q

Explain the factors that affect the probability that a local anaesthetic will block an impulse.

A
  • Diameter of fibre
  • Myelination status
  • Length of nerve exposed to drug
  • Length of time exposed to drug
  • Concentration of drug

Local anaesthetics have different fibre types and sensitivity to local anaesthetics.

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20
Q

Diameter of fibre effect from LA

A

Smaller nerve fibres blocked more easily than large fibres.

21
Q

Myelination status effect from LA

A

Myelinated fibres are blocked more easily than unmyleinated.

22
Q

Describe voltage gated sodium channels

A

3 stages

  • Resting
  • Open
  • Inactivated
23
Q

Where are sodium gated voltage channels found ?

A

They are not exclusive to pain fibres, they are also found anywhere that has excitable tissue:

  • Sensory, motor and autonomic fibres
  • Skeletal and cardiac muscle
24
Q

What pH do local anaesthetics have ?

A

Almost all local anaesthetics are weak bases.

They can exist as neutral or pronated.
They are pH dependent on the degree of ionisation.

25
Q

Recognise features of weak base structure

A

They can exist as neutral or pronated.
They are pH dependent on the degree of ionisation.

26
Q

Weak base interaction with pH

A

They need to enter the cell to block the voltage gated sodium channels.

Local anaesthetics block when ionised via an intracellular binding site, but can’t pass through membrane when ionised.

27
Q

What is pKa ?

A

The pKa is the pH at which 50% of the drug is ionised.

28
Q

Describe the pKa of local anaesthetics

A

Most local anaesthetics have pKa values between 8 and 9.

BH+ <=> B + H+

Where :

BH+ = The ionised form to block the channel

B = Weak base, i.e. the local anaesthetic
H+ = Ionising agent

29
Q

Henderson - Hasselbalch equation

A

pKa - pH = log10 [BH+]/[B]

30
Q

Explain why different local anaesthetics have varying time courses of actions

A

Many local anaesthetics show use-dependence.

Duration of action dictated by rate of removal.

31
Q

Describe the use-dependence of local anaesthetics

A

They binds to open or inactive channels

Related to the frequency of neuronal firing
- faster onset in faster firing neurones

32
Q

Explain how the duration of action of a local anaesthetic is dictated by the rate of removal

A

Factors such as the following can affect the rate of removal:

Blood flow
Action of plasma esterase (ester-linked LA’s only)
Hydrophobicity of drug

33
Q

What do most local anaesthetics cause ?

A

Vasodilation resulting in :

  • Increased blood flow to the area
  • this increases they chances of destruction
34
Q

How can the varying time courses of action be modified ?

A

To increase the duration of action :

Cocaine and Prilocaine both cause vasoconstriction.
Also ADH and Adrenaline.

Note :

Decreased blood flow to the area means a decreased rate of removal.

35
Q

Describe the function of adrenaline

A

Vasoconstriction (adrenaline)

Can increase the duration of local anaesthetics by around 2x.

Can also decrease bleeding during surgery

Although risk of ischaemic damage at extremities.

36
Q

Different routes of administration of local anaesthetics

A

Surface
Infiltration
Nerve block
Intravenous regional
Extradural (epidural)
Subarachnoid

37
Q

Surface administration examples

A

Nose, Mouth, Bronchial tree, Cornea, Urinary tract

38
Q

Infiltration administration examples

A

Injection into tissues to reach nerve branches / terminals.

39
Q

Nerve block administration examples

A

Small or large regional block by injection around the nerve.

40
Q

Intravenous regional administration examples

A

Double cuff method to contain local anaesthetics to a limb.

41
Q

Extradural (epidural) administration examples

A

Used in thoracic, lumbar and sacral regions.

42
Q

Subarachnoid (intrathecal) administration examples

A

Drug is injected into the subarachnoid space. (CSF)

43
Q

EMLA

A

Eutectic Mixture of Local Anaesthetics

Topical numbing cream
Used for dermal anaesthesia

44
Q

EMLA composition

A

Lignocaine and Prilocaine

45
Q

Features of EMLA

A

Melting point of mixture is lower than that of the individual chemicals.

Therefore, higher concentration of both can be used.

More chance more will get across the skin.

46
Q

Predict the dangers and side-effects of local anaesthetics

A

Local anaesthetics are not specific to nerves, hence side effects.

Unwanted effects due to entry into systemic circulation.

Effects in the Cardiovascular system as well as the CNS

Although, side effects have been sed therapeutically (e.g. epilepsy and some heart conditions)

47
Q

Side effects on the Cardiovascular system

A

Dysrhythmias

Sudden fall in blood pressure, due to effects on the heart and vasculature.

48
Q

Side effects on the CNS

A

Inhibits muscle tissue

Restlessness
Tremors
Convulsions
Respiratory centre depression
Death

Medicine Year 1 Semester 1 - Part 2 (37 decks)

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