Local anaesthetics

Classes

  • Amides (majority in clinical use) (2 I’s)
    • Lidocaine/Lignocaine
    • Bupivacaine (longer onset and duration of action) (more commonly used in spinal anaesthesia/regional anaesthesia) / Levobupivacaine
    • Ropivacaine
  • Esters (1 I)
    • Cocaine

Mechanism of Action

  • Block sodium channels, preventing depolarisation and propagation of the action potential
    • Note that this effect is dependent on the molecule crossing the lipid membrane and blocking channels from within the cell
    • LAs can do this because they are weak bases (i.e. they can switch between molecular and ionic form- the latter of which can cross the membrane
      • Dissociation into ions is influenced by the local pH- this is important clinically because LAs will not be as effective in areas that are acidic e.g. inflammation
  • Local anaesthetics tend to block small (C-fibres and Aδ fibres- sensory (pain/light touch)) nerves before larger ones (e.g. Aβ (motor)).

Toxicity

  • Local anaesthetics can be very cardiotoxic at high doses and so minimal doses should be administered
  • The earliest signs of toxicity is that of the CNS, where initial blockade of inhibitory neurons can cause excitation which may manifest as perioral numbness/tingling
  • Cardiac toxicity is due to elongation of the cardiac myocyte potential and myocardial depression- which can lead to life-threatening arrhythmias and/or cardiac arrest
    • Cardiac toxicity may be treated with intralipid

Cautions

  • It may be more appropriate to use topical local anaesthetic preparations on areas such as the urethra (instilagel); eye; nose; throat; bronchial tree

Use of adrenaline

  • Adrenaline is co-administered with most local anaesthetics.
    • This causes local vasoconstriction, minimalising systemic absorption and thus allowing a much higher dose of anaesthetic to be administered
  • DO NOT USE adrenaline in areas with end-artery supplies e.g.
    • Digits, nose tip, ear lobe, penis etc)
    • Can cause ischaemia and necrosis

Advantages of local over general anaesthetic

  • Little, if any, systemic effects; no requirement for mechanical ventilation (less risk of pulmonary complications; gastric aspiration etc)
  • Good depth of analgesia at a local site; may be used with a reduced level of GA if required
  • Can be continued post-operatively if required

Administration- How to…

  1. Check that there are no allergies/contraindications
  2. Check the expiry date, concentration and amount (dose) of local anaesthetic (be sure not to draw up more than the maximum dose)
  3. Using an orange (or blue) needle, inject a weal under the skin along the incision line.
    1. Make sure to aspirate before you inject to avoid injecting into a vessel.
    2. Try also to minimise piercing the skin multiple times but rather re-angling the needle under the skin
  4. Once this is done, inject some anaesthetic deeper into the subcutaneous tissue, again in line with where the incision will be made.
  5. Wait several minutes prior to making an incision and make sure to test the area with a blunt instrument (e.g. forceps) prior to incision.  If needed, top up the sensitive area with remaining anaesthetic (be aware of the total dose injected and that it doesn’t exceed the maximum recommended dose).
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