Lidocaine Local Anaesthetic

Lidocaine is an amide local anaesthetic with rapid onset and intermediate duration, used for infiltration, topical, nerve block, epidural and intravenous regional anaesthesia, and as a class 1b antiarrhythmic. It remains the most widely used local anaesthetic worldwide.

Class. Amide local anaesthetic; class 1b antiarrhythmic.

Mechanism. Reversible blockade of voltage-gated sodium channels in the neuronal membrane, preventing propagation of action potentials. Small, unmyelinated and fast-firing fibres are blocked first, so pain, temperature and sympathetic function go before touch and motor power.

Pharmacokinetics. Onset 2 to 5 minutes for infiltration, longer for large nerves. Duration 30 to 60 minutes plain and 1 to 3 hours with adrenaline. Protein binding about 65% (alpha-1 acid glycoprotein). Half-life 1.5 to 2 hours. Hepatic metabolism (CYP1A2, CYP3A4) to monoethylglycinexylidide (active) and glycinexylidide. Clearance depends on hepatic blood flow and falls in heart failure.

Indications are infiltration and field block, peripheral nerve block, topical anaesthesia of skin and mucosa, epidural anaesthesia, intravenous regional anaesthesia, and ventricular arrhythmia when amiodarone is unavailable. It is also used in perioperative intravenous infusion by specialists.

Maximum doses (adults)

• Plain: 4.5 mg/kg, not above 300 mg.
• With adrenaline: 7 mg/kg, not above 500 mg.
• Concentrations: 1% is 10 mg/mL and 2% is 20 mg/mL; 1 mL of 1% is 10 mg.
• Lower the maximum for debilitated, older, hepatic impairment or heart failure patients.

Techniques

• Skin infiltration: 0.5 to 1% in the volume needed. Onset in 2 to 5 minutes.
• Digital (ring) block: 1 to 2% plain, 2 to 4 mL each side. Never with adrenaline.
• Peripheral nerve block: 1 to 2% in 10 to 20 mL, with adrenaline to prolong effect where appropriate.
• Epidural: 1.5 to 2% in 15 to 25 mL for surgical anaesthesia, titrated in increments, after a test dose.
• Intravenous regional anaesthesia (Bier block, arm): 0.5% plain, 3 mg/kg (about 40 mL for a 70 kg adult), with a tourniquet correctly applied and tested. Never release the cuff within 20 to 30 minutes.
• Topical: 2 to 4% solutions, gels (2%), sprays (10%) and patches. Limit total mucosal dose to avoid systemic toxicity.
• Spinal: not advised; transient neurological symptoms occur.

Ventricular arrhythmia (second-line)

• Intravenous: 1 to 1.5 mg/kg bolus, then repeat 0.5 to 0.75 mg/kg every 5 to 10 minutes to a total of 3 mg/kg.
• Maintenance infusion: 1 to 4 mg/minute (reduce in heart failure, liver disease and older adults).

Perioperative intravenous infusion (specialist, monitored). 1.5 mg/kg bolus before induction, then 1.5 to 2 mg/kg/hour for the duration of surgery, using ideal body weight. Total daily dose is limited by the mg/kg ceilings for local anaesthetic doses.

Children. Maximum 4 mg/kg plain and 7 mg/kg with adrenaline. Neonates: maximum 3 mg/kg. Use the lowest effective concentration and volume.

Contraindications. Hypersensitivity to amide anaesthetics; heart block, sinoatrial disease, Wolff-Parkinson-White syndrome and severe bradycardia (intravenous use); adrenaline-containing preparations in end-arterial sites (fingers, toes, penis, nose tip, ear) or with uncontrolled hypertension or tachyarrhythmia; infection at the injection site.

Boxed-warning-level risks. Systemic toxicity (local anaesthetic systemic toxicity) with seizures and cardiovascular collapse after intravascular injection or overdose; cardiac arrest with inadvertent intravascular injection; methaemoglobinaemia at high doses.

Adverse effects. Common: injection pain, transient dizziness and perioral numbness at low plasma levels. Serious: tinnitus, metallic taste, visual disturbance, muscle twitching, convulsions, coma, hypotension, arrhythmias, cardiac arrest, high spinal block, nerve injury, anaphylaxis (rare, mostly from preservatives).

Interactions. Beta blockers, cimetidine and other CYP1A2 or CYP3A4 inhibitors (fluvoxamine, ciprofloxacin, erythromycin) raise levels. Other antiarrhythmics add cardiac depression. Combined local anaesthetics have additive toxicity. Prilocaine and other methaemoglobin-forming drugs add risk of methaemoglobinaemia. Antipsychotics and tricyclic antidepressants can interact with adrenaline.

Common prescribing errors. Mixing up percent and mg/mL; exceeding the maximum when several blocks are used; adding adrenaline to a digital block; injecting without aspiration; using a test dose of adrenaline in beta-blocked patients.

Special groups

• Pregnancy: acceptable; the safest amide for labour and obstetric procedures.
• Breastfeeding: compatible at usual doses.
• Older adults: reduce dose by 20 to 30% for epidural; reduce infusion rates.
• Kidney impairment: no dose change for single doses; active metabolites may accumulate with infusions.
• Liver impairment: reduce infusion and repeat doses; avoid prolonged infusion in severe disease.
• Regional: confirm the percentage and the presence of adrenaline on vials before drawing up, since the same vial colour or shape may carry different strengths.

Formulations. Solutions 0.5, 1, 1.5, 2 and 4%, with or without adrenaline (1 in 200,000 is 5 micrograms/mL); gel 2%; ointment 5%; spray 10%; patches 5%; ampoules for intravenous arrhythmia use (100 mg in 5 mL).

Administration. Aspirate before each injection and inject in small increments, watching for signs of toxicity. Use the lowest effective concentration. Preparations with adrenaline are used in vascular areas to limit absorption. Use sterile technique.

Baseline. Allergy history, cardiac conduction disease, liver function and drug interactions. Ensure resuscitation equipment, intravenous access and a lipid emulsion are available before nerve block or infusion.

Monitoring. Observe for 30 minutes after large doses or blocks for early toxicity. Continuous electrocardiogram and blood pressure for intravenous infusion. Plasma levels (target 1.5 to 5 micrograms/mL for arrhythmia; toxic above 5 micrograms/mL) in prolonged infusion.

Therapeutic drug monitoring. Plasma lidocaine may be measured during prolonged antiarrhythmic infusions.

Dose reduction and stopping thresholds. Stop any injection or infusion at the first sign of toxicity (perioral numbness, tinnitus, confusion, twitching, bradycardia). Reduce infusion by 50% in heart failure, shock or liver disease.

Referral and escalation. Seizures or cardiovascular collapse after local anaesthetic: stop injection, call for help, give oxygen and airway support, treat seizures with a benzodiazepine (avoid propofol in large doses), and give 20% lipid emulsion 1.5 mL/kg intravenously over 1 minute, followed by 15 mL/kg/hour (maximum cumulative 12 mL/kg). Use reduced adrenaline doses (1 microgram/kg or less) in arrest and avoid vasopressin, calcium channel blockers and beta blockers.