Local Anaesthetics

Question 1

What is a local anaesthetic?

Answer 1

local anaesthetics reversibly block nerve conduction when applied to a restricted area of the body without loss of consciousness.

Question 2

Why would we want to use local anaesthetics?

Answer 2

Reduce pain that individuals would or are feeling.

an - without
aesthesia - feeling

Question 3

Why do anaesthetics block nerve conduction?

Answer 3

They prevent the message of pain from reaching the brain so the individual has no feeling of nociception.

Question 4

How do anaesthetics block nerve conduction?

Answer 4

They block the voltage gated Na channels preventing action potentials from being transmitted and therefore no impulse sent to the brain perceiving nociception.

Question 5

What was the first local anaesthetic?

Answer 5

Using cocaine by squirting it into eyes to anesthetise the eye without causing the hyperactive functions

Question 6

What is the prefix for most local anaesthetics?

Answer 6

Caine.
eg. cocaine
procaine

Question 7

What is the basic structure of a local anaesthetic?

Answer 7

Aromatic ring - linkage - basic amine group

Question 8

Why is the linkage group important in the structure of the anaesthetic?

Answer 8

The linkage group is the site of metabolism.
Often an amide or an ester. (ester faster than amide)
Also ensures that the local anaesthetic is reversible.

Question 9

give some examples of anaesthetics and their linkage and duration.

Answer 9

Procaine/Cocaine - E - short duration

Lidocaine - A - medium duration

Prilocaine - A - Medium duration

Bupivacaine - A - long duration.

Question 10

What is the importance of the aromatic ring in the basic structure of local anaesthetics?

Answer 10

Aromatic rings show lipophilicity and hydrophobicity.
This means that the local anaesthetic is a weak base because it can be charged or neutral (ionised or unionised)
Therefore the drug is dependent on the bodies pH

Question 11

What happens to the ionisation of a local anaesthetic when the environment gets more alkaline or acidic?

Answer 11

Increase alkalinity - more unionised

Increase acidity - More ionised.

Question 12

Which type of form of anaesthetic is more prominent at physiological pH?

Answer 12

Both ionised and unionised forms of the anaesthetic are present at physiological pH however there will be a greater proportion of ionised anaesthetic.

Question 13

How can anaesthetic move into axons?

Answer 13

At physiological pH there will be both unionised and ionised forms of the anaesthetic.
The unionised form is lipophilic and therefore can diffuse through the membrane where they can then become ionised inside the axon to begin interacting within the axon prevent the sodium channels from functioning and action potentials form being produced.

Question 14

What is use-dependent block?

Answer 14

Local anaesthetics only block open or recently opened channels and therefore target the channels active and thus blocking the channels that are transmitting nociception.

Question 15

What factors affect the effectiveness of local anaesthetics?

Answer 15

Tissue pH:

• inflammation and infection –> causes acidic conditions
• therefore acidity is increasing and pH drops meaning that more of the anaesthetic becomes ionised (on top of the already greater proportion).
• This means that there is a less effective anaesthesia as a greater proportion of the drug will be unable to interact within the axon due to its ionised state.

Question 16

What is the differential sensitivity of local anaesthetics for different sizes of axon diameters?

Answer 16

Local anaesthetics find it easier to block smaller diameter neurones such as the Adelta or C fibres.
These fibres have a higher sensitivity compared to the large motor neurones which are less sensitive.

The benefit to this is that the smaller fibres are the ones that convey nociception.

Question 17

What are the various routes of administration of local anaesthetics?

Answer 17

Topical
Infiltration
Nerve block
Spinal 
Epidural

Question 18

What are the unwanted effects of local anaesthetics?

Answer 18

Non-specific - Hypersensitivity reactions
Specifc - High doses / injected into the wrong vessels.

Leads to other excitable tissues being anaesthetised where it finds Na channels.

Question 19

What could be caused if the CNS is accidentally anaesthetised?

Answer 19

Cause convulsions, tremors, respiratory failure.

Question 20

What could be caused if the cardiovascular system is accidentally anaesthetised?

Answer 20

Cardiac muscle loses contractility

blood vessels dilate

Question 21

what type and why are other types of drugs administered with Local anaesthetics?

Answer 21

Vasoconstrictors (eg. adrenaline) enable us to localise the anaesthetics to areas we want to target it and also prevents the drug from leaving the tissues leading to its own breakdown or causing unwanted effects.

Question 22

What are the properties of a good local anaesthetic?

Answer 22

Reversible
Block nerve conduction in nociceptive neurones
effective for time of procedure
low toxicity.

Question 23

What is topical administration?

Answer 23

application to body surfaces (isnt very effective)

Question 24

What is infiltration administration?

Answer 24

Injection of LA that surround the nerve terminals of the nociceptive neurones.

Question 25

What is nerve block administration?

Answer 25

Nerve block - Injecting at a point where there are more axons to anaesthetise a larger area.

Question 26

What is epidural administration?

Answer 26

Epidural - Injecting the LA into the epidural space on the outside of the spinal cord which then baths the nerves leaving the spinal cord reducing the pain that they feel.

Question 27

What is spinal administration?

Answer 27

Spinal - injecting the LA into fluid that surrounds the spinal cord leading to complete anaesthetic affect from the point of injection downwards.