Local Anaesthetics

Question 1

DEFINITION of local anaesthetics

Answer 1

Reversibly block nerve conduction when applied to a restricted area of the body to enable a procedure to be carried out WITHOUT loss of consciousness

Question 2

Naming conventions

Answer 2

Grouped according to similar actions
Have the same ending
E.g. “Caine” cocaine

Question 3

How do they prevent pain

Answer 3

Block voltage gated Na+ channels of nociceptors

Question 4

Chemical Features of anaesthetic and how effects drug action

Answer 4

1. Aromatic ring - lipophillic. Drug can cross cell membrane
2. Linkage (amide/ ester) - site of metabolism. Drug can be hydrolysed doesn’t stay in body forever.
3. Basic amine group - can become neutral and positively charged

Question 5

Nociceptors?

Answer 5

Receptors that detect pain
Detect stimulus and generate AP to the brain

Question 6

Difference between ester and amide linkage

Answer 6

Ester metabolised faster (doesn’t last in body for long) so has limited clinical use

Metabolites formed from drugs with ester linkage give allergic reactions

Amide linkage used more commonly

Question 7

Amine group in anaesthetic creates eqm of ionised and unionised forms. How?

Answer 7

Amine groups are weak bases, can accept H+ ions

If alkalinity increases - eqm shifts to LHS (unionised form)

If acidity increases - eqm shifts to RHS (ionised form)

Question 8

Physiological pH?

Answer 8

Body pH - acidic
More ionised form of local anaesthetic

Question 9

How does local anaesthetic block Na+ channels in nociceptors?

Answer 9

Ionised and unionised forms present in extracellular environment
Unionised forms lipophillic so enter axon
pH inside axon causes new eqm of ionised and unionised forms
Ionised form physically blocks the voltage gated Na+

Question 10

Use dependent block?

Answer 10

Blocks open channels of active neurones only

Increase in pain - increase in open channels - increase AP - increase blockage

Blockage is to same degree of pain o changing degree of pain wont be felt

Question 11

Factors affecting effectiveness of LA

Answer 11

Inflammation/ infection - bacteria produces acidic byproducts
Lower pH mean increased proportion of ionised LA molecules
Fewer unionised molecules to diffuse across axon membrane

Poor anaesthesia. Provide antibiotics before procedure.

Question 12

Differential sensitivity of neurones to LA

Answer 12

All neurones sensitive to LA
Sensory neurones more sensitive because smaller diameter

Question 13

Routes of administration of LA

Answer 13

1. Topical
2. Infiltration
3. Nerve block
4. Epidural
5. Spinal
6. Regional

Question 14

Topical

Answer 14

Skin
Affect nerve endings

Question 15

Infiltration

Answer 15

Multiple injections around the area (ring block)
Injection avoids skin barrier

Question 16

Nerve block

Answer 16

Large area anaesthetised
Proximal to spinal cord so more axons affected

Question 17

Epidural

Answer 17

Epidural space to bathe nerve roots exiting the spinal cord
Anaesthetise pelvis supply to miniseries pain during childbirth

Question 18

Spinal

Answer 18

Even closer to spinal cord - into CSF in subarachnoid space to effect spinal nerves
Anaesthetise extensive parts of the body

Question 19

Side effects of LA

Answer 19

1. Non specific. Due to preservatives in LA. Hypersensitivity/ redness.
2. Specific. Injecting in wrong place. Too high a dose.
3. Affects other excitable tissue. E.g. cardiac muscle

Question 20

Properties of good LA

Answer 20

• Reversible
• block nerve conduction of nociceptors
• effective for time if procedures
• low toxicity

Question 21

Other drugs administered with LA

Answer 21

Vasoconstrictions as LA causes dilation. So LA can remain where injected.

E.g. adrenaline

Question 22

Importance of localising anaesthetic

Answer 22

Avoids unwanted side effects, only affects target tissue
Longer duration of action so can use lower dose