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.
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