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Y2 Pharm (LCRS1) > 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

Amide or ester link

Basic amine side-chain

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4
Q

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

A

Ester = COCAINE

Amide = LIDOCAINE

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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)

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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 connective tissue sheath and 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

Describe the pKa of all local anaesthetics.

A

8-9

All local anaesthetics are WEAK BASES

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15
Q

Explain why it is difficult to anaesthetic infected tissue.

A

Infected tissue is ACIDIC

So there will be less anaesthetic that is unionised

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16
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

17
Q

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

A

It can cause systemic toxicity

18
Q

What is infiltration anaesthesia?

A

Injection of anaesthetic directly into the tissue near the sensory nerve terminals

It is used for minor surgery

19
Q

What is often coadministered with infiltration anaesthesia and what are the benefits of this?

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

NOTE: felypressin (V1 agonist) can also be used

20
Q

What is intravenous regional anaesthesia and how can this cause systemic toxicity?

A

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

Anaesthetic is administered intravenously

Removing the pressure cuff too early can lead to a bolus of anaesthetic entering the systemic circulation - leave cuff for at least 20 minutes

21
Q

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

A

Inject anaesthetic close to the nerve trunks

Low doses and slow onset

22
Q

What is coadministered with nerve block anaesthesia?

A

A vasoconstrictor e.g. adrenaline

23
Q

What is another name given to spinal anaesthesia?

A

Intrathecal

24
Q

Where is the anaesthetic inserted in spinal anaesthesia?

A

Into the subarchnoid space (into the CSF) through the dura

25
Q

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

A

Abdomen, pelvis, lower limbs

26
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

27
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

28
Q

Describe the difference in metabolism of lidocaine and cocaine.

A

Lidocaine – hepatic – N-dealkylation

Cocaine – hepatic and plasma by non-specific cholinesterases

29
Q

Describe the difference in half-life between lidocaine and cocaine.

A

Lidocaine – 2 hours

Cocaine – 1 hour

30
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

31
Q

What are the CVS side-effects of lidocaine?

A

Myocardial depression

Vasodilation

Decrease in blood pressure

All because of sodium channel blockade

32
Q

What are the CNS side-effects of cocaine?

A

Euphoria and excitation

Because of blockade of monoamine re-uptake transporters

33
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 re-uptake transporters

34
Q

What properties is the structure of LAs important for?

A

Their mechanism of action

Their pharmacokinetics

35
Q

What kind of local anesthetics utilise the hydrophobic pathway?

A

Very lipid soluble LAs

36
Q

What is the plasma protin binding % of lidocaine and cocaine?

A

Lidocaine - 70%

Cocaine - 90%

37
Q

What are the advantages and disadvantages of epidural over spinal anesthesia?

A

Advantages:
More restricted action
Less effect on bp

This is because it doesn’t have direct access to the spinal cord

Y2 Pharm (LCRS1) (34 decks)

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