Local Anaesthesia in Dermatosurgery
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Direct answer
Local anaesthesia in dermatosurgery is the infiltration of an amide agent, most often lignocaine, to block pain before excision, biopsy or repair. The safe dose is set by body weight and by whether adrenaline is added, since adrenaline slows absorption and prolongs the effect. Toxicity affects the central nervous system first and the heart later, so the operator must calculate the dose, aspirate and inject slowly.
What you must remember
- Agent: lignocaine at 1 or 2 per cent is the usual choice, with onset within a few minutes and an effect lasting roughly 30 to 120 minutes when plain.
- Adrenaline ratio: 1 in 100,000 is added to prolong anaesthesia and reduce bleeding in the operative field.
- Dose ceiling: standard references quote about 3 to 4.5 mg/kg for plain lignocaine and about 7 mg/kg with adrenaline, so the product label should be checked before use.
- End-artery sites: plain lignocaine is used on fingers, toes, the nasal tip, ear lobes and penis, because adrenaline can cause ischaemia there.
- Buffering: adding sodium bicarbonate raises the pH of the solution, which reduces the sting of injection and speeds onset.
- Toxicity sequence: perioral tingling, metallic taste, tinnitus and restlessness come first, followed by seizures and cardiovascular depression.
- Tumescent technique: very dilute lignocaine with adrenaline is infiltrated in large volumes, which allows a higher calculated ceiling and less systemic absorption.
- Injection technique: aspirate before injecting, inject slowly, and warm the solution to reduce discomfort.
Where the exam tries to catch you
Stems often test the adrenaline rule and the dose limit rather than the choice of agent.
- Trap: using adrenaline for a digital block because it lasts longer. Correct: adrenaline can cause digital ischaemia and gangrene, so plain lignocaine is used for fingers and toes.
- Trap: applying the adrenaline ceiling to a plain solution. Correct: plain lignocaine has a lower ceiling, so the calculation must use the solution actually injected.
- Trap: giving cardiac drugs first in lignocaine toxicity. Correct: airway, oxygen and seizure control come first, and lipid emulsion is reserved for refractory cardiotoxicity.
Frequently asked questions
What is the maximum dose of lignocaine in dermatosurgery?
Standard references quote about 3 to 4.5 mg/kg for plain lignocaine and about 7 mg/kg for lignocaine with adrenaline, though the figure should be checked against the local product label. The dose is calculated from body weight before the first injection, and the lower plain-solution ceiling applies whenever no adrenaline is present.
Why is adrenaline avoided in fingers and toes?
Adrenaline causes vasoconstriction, and the end arteries of fingers, toes, the nasal tip, ear lobes and penis have little collateral supply. Plain lignocaine is used at these sites so that digital ischaemia, which can progress to gangrene, is not provoked.
What are the early signs of lignocaine toxicity?
Early central signs of lignocaine toxicity include perioral tingling, a metallic taste, tinnitus, dizziness and restlessness. Seizures and cardiovascular depression can follow if the injected dose exceeds the safe limit or the drug enters a vein.
Why is sodium bicarbonate added to lignocaine before injection?
Sodium bicarbonate is added to lignocaine to raise the pH of the solution, which reduces the sting of injection and speeds the onset of anaesthesia. Buffering is a standard step for painful infiltration, and the mixture should be used promptly after it is prepared.
What is tumescent anaesthesia?
Tumescent anaesthesia is the infiltration of very large volumes of dilute lignocaine with adrenaline into the subcutaneous fat, which firms the tissue and allows surgery with less systemic absorption. It is used mainly for liposuction and extensive excisions, and it has a higher calculated dose ceiling than conventional injection.