Lignocaine (Lidocaine)

Quick Answer

Lignocaine (lidocaine) is the prototypical amide local anaesthetic, widely used for infiltration, nerve blocks, spinal and epidural anaesthesia, intravenous regional anaesthesia (Bier's block), and as an antiarrhythmic agent. Structure: Amide local anaesthetic (xylidine derivative), intermediate chain via amide bond, tertiary amine, molecular weight 234 Da. Mechanism: Blockade of voltage-gated sodium channels (VGSC) in nerve axons, preventing Na+ influx and action potential propagation; preferentially blocks small unmyelinated C fibres (pain) before large myelinated A fibres (motor, proprioception); frequency-dependent block (more effective at higher firing rates—use-dependent block). Potency: Intermediate (2× procaine, 1/4 bupivacaine); lipid solubility (partition coefficient 2.9) intermediate between low (procaine 0.02) and high (bupivacaine 28). Onset: Rapid (1-3 minutes)—low pKa 7.7 (35% unionized at pH 7.4, crosses nerve membrane easily). Duration: Intermediate (60-120 minutes without adrenaline, 90-180 minutes with adrenaline 1:200,000). Dosing: Infiltration 1% (10 mg/mL), nerve block 1-2%, spinal 5% hyperbaric, epidural 1-2%, Bier's block 0.5% (prilocaine preferred to reduce toxicity), antiarrhythmic 1-1.5 mg/kg IV bolus then 1-4 mg/min infusion. Maximum dose: 3 mg/kg plain (210 mg/70 kg), 7 mg/kg with adrenaline (500 mg/70 kg). Toxicity (LAST): CNS effects first (circumoral numbness, metallic taste, tinnitus, visual disturbances, agitation, tremors, seizures, coma, respiratory arrest), then cardiovascular (hypotension, arrhythmias, cardiac arrest—lignocaine least cardiotoxic amide); treatment: stop injection, lipid emulsion 20% 1.5 mL/kg bolus then infusion (lipid sink mechanism), airway support, benzodiazepines for seizures, prolonged resuscitation. Antiarrhythmic: Class IB agent (blocks inactivated Na+ channels, shortens refractory period, minimal effect on QT interval); used for ventricular arrhythmias (VT, VF), not for atrial arrhythmias. Additives: Adrenaline 1:200,000 (5 μg/mL) reduces systemic absorption by 30-50%, prolongs block, allows intravascular injection detection (tachycardia); bicarbonate (1 mEq/10 mL LA) raises pH, increases unionized fraction, speeds onset, reduces pain on injection; opioids (morphine, fentanyl) for neuraxial use. Clinical pearls: Most versatile local anaesthetic—usable for all routes except high concentrations intrathecally (neurotoxicity—5% hyperbaric safe, higher concentrations not); prilocaine preferred for Bier's block (less toxicity, no longer commercially available in Australia); lignocaine metabolized by CYP1A2 (inhibited by ciprofloxacin, smoking induces). Indigenous considerations: No specific alterations; standard dosing applies; higher obesity rates may require weight-based dose adjustments. [1-10]