Local anaesthetics produce a reversible blockade of nerve impulse conduction. Their clinical efficacy, potency, and duration are dictated by lipid solubility and protein-binding characteristics.
Classification
Local anaesthetics are broadly classified into two groups based on their chemical structure:
- Esters: Generally associated with a higher incidence of allergic reactions due to their PABA metabolite.
- Amides: Generally safer and more stable; preferred for patients with documented ester allergies.
Mechanism of Action (MOA)
- LA agents bind to the intracellular portion of voltage-gated sodium (
) channels.
- This binding inhibits the influx of
- These agents demonstrate use-dependence: the blockade increases as the frequency of nerve firing increases.
Indications & Contraindications
- Indications: Used for infiltration, topical, and regional anaesthesia.
- Contraindications: Adrenaline (often added to prolong duration) is strictly contraindicated in areas with end-arterial supply (fingers, toes, nose, penis) to avoid tissue necrosis.
Specific Agent Profiles
| Agent | Key Characteristics |
|---|---|
| Lignocaine | Intermediate potency/duration; highly versatile. |
| Bupivacaine | Long-acting and potent, but carries a high risk of cardiac toxicity and arrhythmias. |
| Ropivacaine | Similar potency to Bupivacaine but with reduced cardiotoxicity and improved sensory/motor separation. |
| Cocaine | Unique vasoconstrictive properties; used in ENT procedures; high abuse potential. |
Clinical High-Yield: LAST
Local Anaesthetic Systemic Toxicity (LAST) presents with CNS symptoms—circumoral numbness, tinnitus, metallic taste, and seizures—which typically occur before cardiac events.
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