15 Local anaesthetics

Question 1

What are the three types of local anaesthesia?

Answer 1

Regional anaesthesia.
Local infiltration.
Topical.

Question 2

What are the non pharmacological methods of local anaesthesia? (3).

Answer 2

Cold.
Pressure.
Hypoxia.

Question 3

What are the non reversible pharmacological methods of anaesthesia? (4).

Answer 3

Phenol.
Ethanol.
Radio frequency.
Surgical.

Question 4

Define local anaesthetic:

Answer 4

A drug that reversible prevents transmission of nerve impulses to the region to which it is supplied, without affecting consciousness.

Question 5

Which layers does a local anaesthetic have to pass though? (3).

Answer 5

Epineurium.
Perineum.
Endoneurium.

Question 6

How do local anaesthetics work?

Answer 6

Cross membrane as non-ionised form. Bind to intracellular surface of voltage gated sodium channels. Act as ionised form.

Question 7

What is the structure of local anaesthetics?

Answer 7

Lipophilic aromatic residue.
Intermediate chain.
Hydrophilic amino group.

Question 8

Differentiate between esters and amides.

Answer 8

Esters: COO group.
Amides: NHCO group.

Question 9

How do you tell the difference between an ester and an amide?

Answer 9

Amides have an ‘i’ before the suffix ‘caine’.

Question 10

What is pKa?

Clinical relevance?

Answer 10

pH at which ionised and non-ionised forms of LA are equal.

If pKa&raquo_space; pH then onset of action is slower.

Question 11

What is the pH of the body? Pus?

Answer 11

Body: 7.4
Pus: 6.9

Question 12

What controls the duration of action of a local anaesthetic?

Answer 12

Protein binding. More binding = longer duration.

Dependant on intermediate chain length.

Question 13

What does the potency of a local anaesthetic depend on?

Answer 13

Lipid solubility.

More soluble - more penetrates the membrane, less required for same effect.

Question 14

What does the ability of a local anaesthetic to block nerve conduction depend on?

Answer 14

Type of nerve fibre (larger=slower).

Location - inside or outside the mantle.

Question 15

In what order is sensory function lost with a local anaesthetic?

Answer 15

Col, warmth.
First pain.
Second pain.
Touch, pressure.
Motor function.

Question 16

Why are vasoconstrictors used with local anaesthetics? (4).

Answer 16

Prolong action.
Reduced plasma levels: reduce CNS effects.
Reduce dose needed.
Reduce operative haemorrhage risk.

Question 17

When are vast-constrictors not used with local anaesthetics?

Answer 17

In areas supplied by end arteries.

Penis, fingers and toes, ear lobules, ala of nose.

Question 18

Which vasoconstrictors are used with local anaesthetics? (2).

Answer 18

Adrenaline.

Felypressin (no effect on heart).

Question 19

In which type of local anaesthetic is hypersensitivity more of a problem?

Answer 19

Esters.

Question 20

What is the main toxic side effect of prilocaine?

How does this occur?

Answer 20

Methaemoglobinaemia.

Metabolite 0- toluidine oxidises ferrous to ferric ions.

Question 21

What are the symptoms of methaemoglobinaemia?

Treatment?

Answer 21

Cyanosis, lethargy, respiratory distress which doesn’t respond to oxygen.
IV methylene blue.

Question 22

What order do the symptoms of local anaesthetic toxicity occur? (6).

Answer 22

Lightheadedness.
Visual disturbances.
Muscular twitching then convulsions.
Unconsciousness then coma.
Respiratory arrest.
CVS depression.

Question 23

How is local anaesthetic toxicity treated? (5).

Answer 23

Maintain airway. 
100% oxygen. 
Control seizures. 
Cardiopulmonary resuscitation.
Intravenous lipid emulsion.

Question 24

Why is intravenous lipid emulsion the last line treatment for reversal of local anaesthetic toxicity?

Answer 24

May cause pancreatitis.