Local Anaesthesia

• LAs are mostly weak bases
  • preparations without adrenaline usually come at pH 5-6
  • preparations with adrenaline usually come at pH 2-3 (stings more)
• Mechanism of action
  • Prevent membrane depolarisation and block action potential generation through binding to sodium channels.
• Adrenaline with LAs
  • Adrenaline counters the vasodilatory effects of most LAs and thus reduces bleeding.
    • Exception: cocaine-based preparations cause vasoconstriction
  • Allows faster onset of and longer duration of action.
  • Half-life is 2 minutes.
• Types:
  • Lignocaine/Lidocaine - Most commonly used LA.
  • Bupivacaine - Slower but longer acting and can be used in combination with lignocaine.
• Issues:
  • Severe pain - the needle may pierce the nerve directly. Withdraw and inject somewhere else.
  • Ischaemia - may cause blanching and pain. Due to the short half-life of adrenaline permanent damage is rare.
  • Allergy - this is rare, more likely the patient is reacting to metylparaben (bacteriostatic preservative found in vials).
  • Toxicity - most common cause for this is injecting directly into a vessel.
    • In relation to adrenaline - hypertension, tachycardia, arrhythmias.
    • In relation to LA - CNS symptoms e.g. dizziness, tinnitus, numbness/tingling.
    • In relation to the cardiovascular system - can cause arrhythmias.
• To reduce discomfort during injection:
  • use a small needle
  • inject slowly
  • minimize number of stabs

Local Anaesthetic Doses with and without Adrenaline: