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LCRS 1 > Local Anaesthetics > Flashcards

Local Anaesthetics Flashcards

1
Q

Define Local Anaesthetic.

A

Drugs that reversibly block neuronal conduction when applied locally

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2
Q

What is the rapid depolarisation stage of the action potential caused by?

A

Voltage-gated sodium channels

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3
Q

What are the three components that make up all local anaesthetics?

A

Aromatic region
Basic amine side-chain
Amide or ester link

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4
Q

What are the two types of local anaesthetics? Give an example of each.

A 
Ester = COCAINE 
Amide = LIDOCAINE

Esta snorts Cocaine

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5
Q

Name a local anaesthetic that doesn’t fit the structure of all other local anaesthetics.

A

Benzocaine – it has an alkyl group rather than the basic amine side chain
NOTE: this means that it is relatively weak but highly lipid soluble (good for surface anaesthesia-Lozenge)

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6
Q

What are the two pathways of local anaesthesia? State which one is more important.

A

HYDROPHILIC – most important

Hydrophobic

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7
Q

Describe the hydrophilic pathway.

A

Unionised LA from the blood crosses the axon membrane and gets into the axon
Within the axon it forms the cation form of the LA
This cation form then binds to the inside of the voltage-gated sodium channels (when they open) and block sodium entry
This blocks action potential conduction

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8
Q

What feature of local anaesthetics helps make it more selective for nociceptive neurones?

A

Use-dependency

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9
Q

Describe the hydrophobic pathway.

A

Some very lipophilic local anaesthetics will move into the cell membrane (in unionised form) and then drop straight into the sodium channel
It will then become the cation form in the sodium channel
And it will block sodium influx

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10
Q

What effect do local anaesthetics have on resting membrane potential?

A

No effect on resting membrane potential

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11
Q

Explain the effect of local anaesthetics on channel gating.

A

There is some suggestion that local anaesthetics bind more strongly to the sodium channels in their inactive state
Once bound to the sodium channel, it then holds it in the inactive stage for longer thus increasing the refractory period and reducing the frequency of action potentials

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12
Q

Explain the effect of local anaesthetics on surface tension.

A

They lodge into the plasma membrane and reduce surface tension of the membrane
This leads to non-selective expansion of the lipid membrane and leads to non-specific inhibition of ion channels

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13
Q

Describe the selectivity of local anaesthetics.

A

Preference for small diameter axons (e.g. nociception neurones)
Tend to block non-myelinated axons

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14
Q

Explain why it is difficult to anaesthetise infected tissue.

A

Infected tissue is ACIDIC

So there will be less anaesthetic that is unionised

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15
Q

What are the 6 methods of administration of local anaesthetics?

A 
Surface anaesthesia 
Infiltration anaesthesia 
Intravenous regional anaesthesia  
Nerve block anaesthesia 
Spinal anaesthesia  
Epidural anaesthesia
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16
Q

What are the consequences of using high doses in local anaesthesia?

A

It can cause systemic toxicity

17
Q

What is infiltration anaesthesia?

A

Injection of anaesthetic directly into the tissue near the sensory nerve terminals
It is used for minor surgery

18
Q

What is often coadministered with infiltration anaesthesia and why?

A

Adrenaline – this causes vasoconstriction and increases the duration of action of the anaesthetic meaning that a lower dose can be used
It also slows bleeding at the site of injection and reduces the amount of local anaesthetic going into the systemic circulation

19
Q

What is intravenous regional anaesthesia?

A

Pressure cuff is used to cut off the blood supply downstream of it
Anaesthetic is administered intravenously

20
Q

What is nerve block anaesthesia? Describe the dosage and onset.

A

Inject anaesthetic close to the nerve trunks

Low doses and slow onset

21
Q

What is coadministered with nerve block anaesthesia?

A

A vasoconstrictor e.g. adrenaline

22
Q

What is another name given to spinal anaesthesia?

A

Intrathecal

23
Q

Where is the anaesthetic inserted in spinal anaesthesia?

A

Into the subarchnoid space (into the CSF)

24
Q

Which parts of the body can be anaesthetised effectively with spinal and epidural anaesthesia?

A

Abdomen, pelvis, lower limbs

25
Q

How does spinal anaesthesia affect blood pressure and why does it have this effect?

A

It can cause a drop in blood pressure because it anaesthetises the nerve roots and the preganglionic sympathetic nerves are particularly sensitive to blockade by local anaesthetics
This leads to reduced sympathetic output and hence a drop in blood pressure

26
Q

What trick can anaesthetists do to get better control over the location of the spinal anaesthesia?

A

Add glucose to the anaesthetic mixture
This increases the specific gravity of the local anaesthetic meaning that the patient can be tilted to move the bolus of anaesthetic to the right place

27
Q

Describe the difference in metabolism of lidocaine and cocaine.

A

Lidocaine – hepatic – N-dealkylation

Cocaine – hepatic and plasma by non-specific cholinesterases

28
Q

What are the CNS side-effects of lidocaine? Explain why it has these effects.

A 
CNS stimulation 
Restlessness  
Confusion 
Tremor  
This is because the GABA system (inhibitory effect on CNS) is very sensitive to local anaesthetics
29
Q

What are the CVS side-effects of lidocaine?

A

Myocardial depression
Vasodilation
Decrease in blood pressure
All because of sodium channel blockade

30
Q

What are the CNS side-effects of cocaine?

A

Euphoria and excitation

Because of blockade of monoamine transporters

31
Q

What are the CVS side effects of cocaine? Explain why it has these effects.

A

Increased cardiac output
Vasoconstriction
Increased blood pressure
Due to increased sympathetic drive caused by blockade of monoamine transporters

32
Q

Are local anaesthetics acids or bases

A

Weak bases

33
Q

Why do local anaesthetics only affect sensory neurones

A

They cant affect motor neurones due to myelin

34
Q

Where do surface anaesthetics affect

A

Mucosal surfaces such as mouth which can be sprayed on

35
Q

Problems with surface anaethesia

A

High concentrations lead to systemic toxicity

36
Q

Problem with IV regional anaesthesia

A

When pressure cuff removed a bolus of anaesthetic can enter systemic circulation

37
Q

Differences between spinal anaesthetics and epidural

A

Subarach space vs epidural
Surgery vs childbirth
Effect on BP vs no effect
Low dose vs high dose as slow onset

LCRS 1 (58 decks)

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