What is the maximum safe dose of plain lignocaine (lidocaine) for infiltration anaesthesia in adults, and how does the addition of adrenaline (epinephrine) 1:200,000 affect this limit?
- A Plain: 3 mg/kg; with adrenaline: 5 mg/kg
- B Plain: 6 mg/kg; with adrenaline: 10 mg/kg
- C Plain: 2 mg/kg; with adrenaline: 4 mg/kg
- D Plain: 4.5 mg/kg; with adrenaline: 7 mg/kg ✓
Explanation
The maximum safe dose of plain lignocaine is 4.5 mg/kg (some texts cite 3–4 mg/kg, but 4.5 mg/kg is the standard figure in most anaesthetic texts including Morgan & Mikhail). Adding adrenaline 1:200,000 causes vasoconstriction at the injection site, slowing systemic absorption and reducing peak plasma levels, allowing the dose to be increased to 7 mg/kg. The vasoconstrictive effect both prolongs the duration of action and raises the safety ceiling. Adrenaline-containing solutions are contraindicated for ring blocks of extremities (digits, penis, ear) due to risk of ischaemic necrosis.
Reference: Morgan & Mikhail's Clinical Anesthesiology, 6th ed.
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