Local Anesthetics

Question 1

Anaesthetic Classification?

Answer 1

1. Natural : Cocaine
2. PABA derivatives : Benzocaine/tetracaine (topical use)
3. Amines: “Lidocaine spray” (10%)

Question 2

Types of Local anaesthetics?

Answer 2

1. Topical : Applied on skin and mucosal surfaces.
2. Field block : Subcutaneous, involvement of the distal nerve endings.
3. Peripheral block : injection of anaesthetic near the trunk.
4. Central “neuraxial” blockade
   ( spinal ) - CSF injection
   ( epidural ) - injection into the epidural space.

Question 3

Pharmacokinetics?

Answer 3

Good oral absorption, which can be increased with vasodilation (rapid diffusion) which causes a shorter duration of quick action. For a more prolonged action the local anaesthetics can be administered with adrenaline/noradrenaline to prevent the effects of vasodilation and reduce diffusion rate. Hepatic metabolism and urinary excretion.

The onset and duration of action of local anaesthetics is mediated by the following : Tissue PH, PKa and lipophilicity

• Tissue Ph: Non-ionised form allows for greater penetration, low tissue ph favours non-ionised form. At physiological PH anaesthetics are ionised.
• Pka : Low pka = greater ratio of non-ionised to ionised then has greater penetration into nerves.
• Lipid solubility: Increased lipid solubility = faster onset.

Question 4

Adverse drug reactions of local anaesthetics?

Answer 4

Overdose can lead to depression of the cardiovascular and respiratory centres. The may also present pro-arrhythmic effects.