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YR2 LCRS Pharm > 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

Opening of voltage-gated sodium channels

Influx of Na+

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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
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5
Q

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

A

Benzocaine: has an alkyl group rather than an amine side chain
Thus it’s 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 axon membrane + gets into the axon
Within the axon it forms the cation form of the LA
Cation then binds to the inside of the VGSCs (when open) + block Na+ entry
Blocks AP conduction

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8
Q

What feature of local anaesthetics helps make it more selective for nociceptive neurones? Can they block motor neurones?

A

Use-dependency

Can block MN’s but less intensely (fire less + are myelinated)

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9
Q

Describe the hydrophobic pathway. What is advantageous about this pathway?

A

Very lipophilic LAs move into the cell membrane (in unionised form) + drop straight into the sodium channel
Then transition to cation form in the sodium channel
+ will block Na+ influx
VGSCs don’t need to be open for LA to bind

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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

Suggestion: LA’s bind more strongly to the VGSCs in their inactive state
Once bound it holds it in the inactive stage for longer thus increasing the refractory period + reducing the frequency of APs

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12
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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13
Q

Describe the pKa of all local anaesthetics. What does this mean for local anaesthetics acting at our physiological pH?

A

8-9
All local anaesthetics are WEAK BASES
Only small % will be unionised + can pass into neurones

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14
Q

Explain why it is difficult to anaesthetise infected tissue.

A

Infected tissue is ACIDIC

So less anaesthetic will be 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

Can cause systemic toxicity- go to heart then brain

17
Q

What is infiltration anaesthesia?

A

Injection of anaesthetic directly into tissue near the sensory nerve terminals
Used for minor surgery

18
Q

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

A

Adrenaline: causes vasoconstriction + increases the duration of action of LA meaning a lower dose can be used
Also slows bleeding at site of injection + reduces the amount of LA going into the systemic circulation
Avoided in LA of extremities as risk of ischaemic damage

19
Q

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

A

LA administered intravenously, distal to pressure cuff (which restricts blood flow)
Used in limb surgery
Removing pressure cuff too early can lead to a bolus of anaesthetic entering the systemic circulation

20
Q

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

A

Inject anaesthetic close to the nerve trunks

Low doses + 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

Can cause drop in BP due to effects on preganglionic sympathetic nerves (have small diameter)
Leads to reduced sympathetic output + hence a drop in BP
Can cause prolonged headaches

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

Increases specific gravity of the LA meaning the patient can be tilted to move the bolus of anaesthetic to target site

27
Q

Describe the difference in metabolism of lidocaine and cocaine.

A

Lidocaine: hepatic – N-dealkylation
Cocaine: hepatic + plasma by non-specific cholinesterases

28
Q

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

A

Lidocaine: 2 hours
Cocaine: 1 hour

29
Q

What are 4 CNS side-effects of lidocaine? Explain why it has these effects.

A 
CNS stimulation 
Restlessness  
Confusion 
Tremor  
Paradoxical- GABA system (inhibitory effect on CNS) is very sensitive to LA's so effected 1st
30
Q

What are 3 CVS side-effects of lidocaine? Why do they arise?

A

Myocardial depression
Vasodilation
Decrease in BP
Due to sodium channel blockade

31
Q

What are the CNS side-effects of cocaine? Why do they arise?

A

Euphoria + excitation

Due to blockade of monoamine transporters

32
Q

What are the 3 CVS side effects of cocaine? Explain why it has these effects.

A

Increased CO
Vasoconstriction
Increased BP
Due to increased sympathetic drive caused by blockade of monoamine transporters

33
Q

Where does surface anaesthesia act? How is it often administered?

A

Mucosal surfaces e.g. mouth

Spray or powder

34
Q

Where is the anaesthetic inserted in epidural anaesthesia?

A

Fatty tissue of epidural space

needle doesn’t pierce dura

35
Q

What are the uses of epidurals?

A

Abdominal, pelvic + lower limb surgery

Painless childbirth

36
Q

What is the disadvantage of using epidural anaesthesia?

A

Slower onset so higher doses are required

Thus more likely to cause systemic toxicity

37
Q

What are the advantages of epidurals over spinal anaesthetics?

A

Epidurals have more restricted action as doesn’t diffuse into CSF + less likely to effect BP

38
Q

Describe the absorption across mucous membranes and plasma protein binding of lidocaine and cocaine

A

Both have good absorption

Both highly PPB

39
Q

Name a local anaesthetic with a longer duration of action (~6 hours)

A

Bupivicaine

YR2 LCRS Pharm (34 decks)

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