Local Anaesthetic

Contraindications:

• Do not use lignocaine with adrenaline to infiltrate areas with an end-arterial supply, such as fingers, toes, pinna, penis and nose.
• Local anaesthetic allergy (uncommon).

Use of adrenaline:

• Use local anaesthetic with adrenaline for vascular sites or for sites where the skin needs to be incised, because adrenaline causes vasoconstriction.
• Adrenaline will reduce systemic absorption, maintain a higher anaesthetic concentration near nerve fibres and prolong local anaesthetic conduction blockade.
• Maximal effect with the less lipophylic anaesthetic agents (e.g. lignocaine). Note: The more lipophilic local anesthetics will tend on their own to associate strongly with tissues

Calculating the maximum dose of local anaesthetic agent

• Maximum dose of lignocaine without adrenaline is 3 mg/kg.
• Maximum dose of lignocaine with adrenaline is 7 mg/kg.

• Lignocaine concentration in solution:
  • 0.25% =  0.25 g/100mL or 2.5mg/mL
  • 0.5%  = 0.5 g/100mL or 5mg/mL
  • 1%     = 1 g/100mL or 10mg/mL
  • 2%     = 2 g/100mL or 20mg/mL
• Thus, in a 70-kg patient do not use more than:
  • 20 mL 1% plain lignocaine or 10 mL 2% plain lignocaine
  • 48 mL 1% lignocaine with adrenaline or 24 mL 2% lignocaine with adrenaline

Handy Hints:

• The sting of lignocaine infiltration may be reduced by warming the solution to T37 deg C prior to administration and by the addition of HCO3.
• Placing the needle “up to the hub” and injecting while withdrawing along the just-created sub-dermal tunnel minimizes tissue distention and pain.
• After an initial injection, instead of totally withdrawing the needle from the tissue, redirect it along another path to lessen the number of punctures.
• If multiple injections are required, enter the skin through an area that has already been infiltrated.
• Slowly injecting the smallest volume necessary reduces pain
• Make all wound injections through the wound edge and not through the skin

Additional Reading:

• Local anaesthetic questions you will be asked in exams
• Local anaesthetic pharmacology (FRCA)