Wk4 Uses And Actions Of 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 to enable a procedure to be carried out without loss of consciousness

Question 2

How do they prevent pain?

Answer 2

Nociception - nocireceptors perceive pain/damage to body. Sensory receptors, block pain, voltage gated Na+ channels blocked

Question 3

What is the chemical nature of local anaesthetics?

Answer 3

Aromatic ring – linkage – basic amine group

Linkage = amide or ester - variable - linkage is site of metabolism - chemical nature of linkage = how fast anaesthetics are broken down (cant be too fast, but also cant produce toxic metabolites)

Weak bases - can accept H+ - ionisation state determined by pH

Question 4

Nociceptive neurone

Answer 4

Not lipid soluble

Can freely pass lipid balayer due to being neutral

Question 5

Use-Dependent block

Answer 5

Degree of block depends on whether channels are being used

Blocks open channels

Increase pain = increase block

Tissue pH = inflammation/infection = acidic condition

Question 6

What are the differential sensitivity of neurones?

Answer 6

Smallest axons carry the most pain

Thickest walls = myelinated = low sensitivity

Question 7

What is the general principle?

Answer 7

The more proximal the site of administration, the greater the area anaesthetised

Question 8

What are the routes of administration of LA?

Answer 8

Topical anaesthesia - skin/back surface of skin - not that likely to be effective - limited use in clinical practice

Infiltration anaesthesia - injected into skin - neurones anaesthetised

Nerve block anaesthesia - injection given proximally - quite a large area e.g. all of jaw

Epidural anaesthesia - epidural space: parts of childbirth - need drug to stick around

Spinal anaesthesia - any area in spine/spinal area - fairly serious surgery

Question 9

Side effects

Answer 9

Non-specific
- hypersensitivity reactions
Specific
- high does
- injected into vessels - get into bloodstream and go round body : interfere with na+ channels in heat
Affect on other excitable tissues
- CNS : tumours, convulsions, respiratory failure
Cardiovascular system
- cardiac muscle: decreased contraction
- blood vessels: dilate - decreased BP

Question 10

What are other drugs administered with LA?

Answer 10

• vasoconstrictors : remain in one place not full body effect
  E.g. adrenaline/felypressin
  Localise LA
  Decrease effects
  Increase duration of action
  CAUTION - reducing blood flow - risks extremities

Question 11

Properties of good local anaesthetics

Answer 11

• Reversible
• Block nerve conduction in nociceptive neurones
• Effective for time of procedure
• Low toxicity