46. Local anaesthetics

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Local anaesthetics are sodium channel blockers which block pain sensation in a smaller area (inducing regional anaesthesia) by preventing transmission of pain signals from pain receptors to the CNS. They are administered intramuscularly, subcutaneously, intravenously, intraarticularly, topically, but not orally. Local anaesthetics are used to prevent or eliminate pain during surgical procedures.

Indications

  • Surface anaesthesia – by applying directly to mucous membrane or skin
    • As a spray, gel, ointment, etc.
  • Infiltration anaesthesia – by giving multiple injections targeting small nerve branches, as done at the dentist
  • Nerve block anaesthesia – giving injections close to major nerve trunks, which anaesthetizes the entire are innervated by this trunk
  • Spinal anaesthesia – by administering directly into the subarachnoid space, into the CSF
  • Epidural anaesthesia – by administering directly into the epidural space
  • IV regional anaesthesia – giving IV and applying a tourniquet to an extremity

Lidocaine is the most used local anaesthetic for the skin as it has the highest penetration to the skin. It can also be given IV as an antiarrhythmic, although this is rare. As a gel lidocaine can be used prior to catherization of the bladder or in the form of a spray prior to performing laryngoscopy.

Mechanism of action

Local anaesthetics work by reversibly blocking voltage-gated Na+ channels. These channels are responsible for the upstroke phase of the action potential in nerve fibres. By blocking them will there be no depolarization, so nociceptive nerve signals won’t be carried to the brain. These drugs physically “plug” and therefore block the channels. At high concentrations they may block other ion channels as well.

Side effects

Voltage-gated Na+ channels are found everywhere, so when the local anaesthetics are absorbed into the plasma channels all over the body can be affected.

  • CNS effects:
    • Tremor
    • Confusion, agitation
    • Paralysis
    • Seizures
  • Cardiovascular effects:
    • Bradycardia (negative chronotropic effect)
    • AV block (negative dromotropic effect)
    • Decreased contractility (negative inotropic effect)
    • Arrhythmias
    • Vasodilation
  • Allergic reactions
    • Mostly to ester local anaesthetics

Spinal anaesthesia carries some extra risk. Not only sensory nerves travel in the spinal cord, but autonomic and motor nerves as well. The following effects may occur:

  • Sympathetic block -> Bradycardia, hypotension
  • Parasympathetic block -> urinary retention
  • Paralysis of phrenic nerve