Local Anaesthetics

Local anaesthetics reversibly block voltage-gated sodium channels in nerve fibres, abolishing sensation without loss of consciousness. They are used for infiltration, topical, peripheral nerve, neuraxial and intravenous regional anaesthesia. Safety depends on total dose, site and speed of injection, because intravascular injection or overdose causes seizures and cardiac arrest.

Classification. Amides (lidocaine, bupivacaine, levobupivacaine, ropivacaine, prilocaine, mepivacaine, articaine) and esters (procaine, chloroprocaine, tetracaine, benzocaine, cocaine). The amide or ester link decides metabolism and allergy risk.

Mechanism. The uncharged base crosses the nerve membrane; the ionised form blocks the sodium channel from inside. Small myelinated fibres block first: sympathetic, then temperature, pain, touch and finally motor function.

Determinants of action. Lower pKa gives faster onset (more base at tissue pH); greater lipid solubility gives greater potency; higher protein binding gives longer duration. Infected or acidic tissue reduces effect because less base is available.

Adjuncts. Adrenaline 1 in 200 000 (5 micrograms/mL) prolongs block and lowers peak plasma level; avoid it in digits, penis, nasal tip, ears and intravenous regional anaesthesia. Sodium bicarbonate (1 mL of 8.4% to 10 mL of lidocaine) speeds onset and reduces injection pain.

Concentration conversion: 1% equals 10 mg/mL. Doses in the table are for healthy adults and are reduced in the frail, in pregnancy, in hepatic and cardiac disease and with vascular sites.

Metabolism. Amides are metabolised in the liver (CYP1A2 and CYP3A4), so hepatic failure, low hepatic blood flow and enzyme inhibitors such as fluvoxamine prolong effect. Esters are hydrolysed rapidly by plasma cholinesterase.

Toxicity ranking. Bupivacaine is the most cardiotoxic because of avid sodium channel binding; levobupivacaine and ropivacaine are less cardiotoxic at equipotent doses.

• First-line. Lidocaine 0.5 to 2% for infiltration, wound closure and short blocks; bupivacaine, levobupivacaine or ropivacaine for longer surgical blocks, spinal and epidural analgesia.
• Alternative. Ropivacaine where lower motor block and lower cardiotoxicity are wanted (labour, continuous infusions); prilocaine for intravenous regional anaesthesia where available; chloroprocaine for short spinal anaesthesia.
• Specialist. Neuraxial and plexus techniques, continuous catheter infusions, topical preparations, lidocaine 5% plaster.
• Emergency. Lidocaine 1 to 1.5 mg/kg intravenously for ventricular arrhythmia when amiodarone is unavailable; lidocaine 0.5% around fracture sites and for reduction of dislocations.

Selection. Match duration to the procedure; calculate the maximum dose in mg before drawing up; use the lowest effective volume; aspirate and inject slowly in divided doses; use ultrasound-directed technique where available.

Local anaesthetic systemic toxicity. Early signs are perioral numbness, tinnitus, metallic taste, agitation, then seizures, coma, arrhythmia and cardiac arrest.

• Stop injection, call for help, give 100% oxygen and secure the airway.
• Control seizures with small intravenous doses of a benzodiazepine (midazolam 1 to 2 mg, repeated).
• Give 20% lipid emulsion 1.5 mL/kg intravenously over 1 minute, then 0.25 mL/kg/min (15 mL/kg/h).
• If instability persists, repeat the bolus up to twice at 5-minute intervals and double the infusion rate; stop at a cumulative 12 mL/kg.
• In cardiac arrest use adrenaline in doses of 1 microgram/kg or less, avoid vasopressin, calcium channel blockers, beta-blockers and further local anaesthetic, and continue resuscitation for a prolonged period.

Contraindications. Allergy to the agent (rare with amides; more frequent with esters); infection at the injection site; coagulopathy or therapeutic anticoagulation for neuraxial and deep blocks; bupivacaine 0.75% in obstetric epidural anaesthesia; adrenaline-containing solutions at end-arterial sites; complete heart block (relative, lidocaine and bupivacaine).

Serious adverse effects. Systemic toxicity; high or total spinal block; nerve injury; haematoma; methaemoglobinaemia (prilocaine, benzocaine, large doses of lidocaine); transient neurological symptoms (lidocaine spinal); cardiac arrest with bupivacaine.

Major interactions. Amiodarone, beta-blockers and cimetidine raise lidocaine levels; fluvoxamine and ciprofloxacin raise ropivacaine; sulphonamides are antagonised by the para-aminobenzoic acid released from esters; other sodium channel blockers add to cardiac effects.

Common prescribing errors.

• Dose calculated in percent rather than mg.
• Maximum dose not reduced for adrenaline-free injection into vascular sites.
• Cumulative dose ignored when topical, infiltration and block doses are combined.
• Bupivacaine 0.75% used in obstetrics.
• Adrenaline-containing solutions at digits.

Special groups.

• Pregnancy: use the lowest effective dose; bupivacaine 0.75% is contraindicated in obstetric epidural.
• Breastfeeding: compatible.
• Children: calculate every dose by weight; use maximum mg/kg limits strictly (neonates have reduced binding and clearance).
• Older adults: reduce doses by 20 to 30% for spinal and epidural block.
• Kidney: little change except for accumulation of metabolites with prolonged infusion.
• Liver: reduce amide doses and avoid prolonged infusions.
• East Africa: lipid emulsion may not be stocked; confirm availability before regional blocks, and keep airway equipment and benzodiazepine at hand.

Monitoring. Continuous ECG, pulse oximetry and blood pressure during block; verbal contact during injection; observation for at least 30 minutes after block and prolonged monitoring after lipid emulsion; check for pancreatitis after lipid use.

Referral and escalation. Seizure, arrhythmia or hypotension after injection; persistent neurological deficit or severe back pain after neuraxial block (urgent MRI and neurosurgical review); methaemoglobin level over 20% or saturation not responding to oxygen.

Comparison with alternative classes. Local anaesthetics give targeted analgesia without sedation or respiratory depression and lower opioid requirement, but add procedural risk and a narrow toxic margin compared with systemic analgesics.