Local Anaesthesia

Local Anaesthesia

Local anaesthetics suppress pain by blocking the conduction of an impulse along axons located near the site of administration.

2 main types – amides (lidocaine, Bupivicaine, mepivacaine, etidocaine) and esters (procaine, tetracaine, chloroprocaine, hexylcaine (

Mechanism of Action:
Block sodium channels in axon membrane  prevent influx of sodium  no conduction of impulse

Characteristics of Local Anaesthetics:
Desirable properties: – water soluble, heat sterilisable, rapid onset of action, have appropriate duration, no toxic effects locally or systemically, no after effects.

All act on nerves to prevent nerve impulses – by reducing passage of sodium through the volt gated Na ion channels  raises threshold of excitability.

Pharmacokinetics:
Absorption:
• even though administered locally, they are absorbed into the systemic circulation  distributed to all parts of body
• rate of absorption depends on blood flow

Metabolism:
• ester-type local anaesthetics are metabolised in the blood by esterases
• amide-type local anaesthetics are metabolised in the liver

Adverse effects:
Local effects:

Allergic reactions:

Central nervous system:
• if sufficient quantities  excitation (may get convulsions) then depression (vary from drowsiness to unconsciousness.
• if excitation  IV benzodiazepine or thiopental
• death can occur secondary to respiratory depression ( mechanical ventilation and oxygen if indicated)

Cardiovascular system:
• if large doses  suppress excitability in myocardium and conducting system  bradycardia, heart block, cardiac arrest; in blood vessels  relax smooth muscles  vasodilation  hypotension

Specific Local Anaesthetics:
Lignocaine (Xylocaine®):
• amino amide – detoxified in liver
• rapid onset – intermediate duration of action
• maximum single adult dose of 300mg
• vasoconstriction additives can be used to prolong action

Bupivicaine (Marcain®):
• amino amide – detoxified in liver
• slow onset – long duration of action (up to 6 hours)
• 4 times as toxic as lignocaine
• maximum safe dose for adults is about 150mg

Procaine:
• ester type substance
• not used much, as newer anaesthetics have supplanted it

Mepivacaine:
• similar to lignocaine, but has higher maximum safe dose (400mg) and rate of onset is faster

Prilocaine (Citanest®):
• similar to lignocaine, but half as toxic

Adverse Reactions:
Toxic dose of 1% lidocaine – 300mg/30ml; with 1% adrenaline – 500mg/50ml
0.25% Bupivicaine – 175mg (70ml); with 1% adrenaline – 225mg (90ml)

1) Allergy:
• extremely rare

2) Systemic toxicity:
• CNS toxicity  light head feeling and dizziness initially, then difficulty focusing eyes and tinnitus.

Administration of local anaesthetics:
Two common techniques:
1) Local infiltration
2) Nerve block

Needle size – in foot usually use 25 or 27 gauge needle.