Pharmacology
- Block voltage gated sodium channels from inside the nerve, preventing depolarisation
- Unionised form crosses the membrane. Acidic infected tissue reduces effect
- Small unmyelinated fibres are blocked first, so pain and temperature go before motor function
- Potency relates to lipid solubility, duration to protein binding
- Adrenaline reduces absorption and prolongs action
- Local anaesthetic with adrenaline is safe in the fingers
Classification
| Group | Examples | Metabolism |
|---|
| Amides | Lignocaine, bupivacaine, levobupivacaine, ropivacaine | Liver |
| Esters | Procaine, tetracaine | Plasma cholinesterase. Higher allergy risk from PABA |
Maximum Doses
| Agent | Plain | With adrenaline |
|---|
| Lignocaine | 3 mg/kg | 7 mg/kg |
| Bupivacaine | 2 mg/kg | 2 mg/kg |
| Ropivacaine | 3 mg/kg | 3 mg/kg |
Toxicity
- Early signs are perioral numbness, tinnitus and metallic taste
- Then seizures, arrhythmia and cardiovascular collapse
- Bupivacaine is the most cardiotoxic
- Treat with airway support, seizure control and 20% lipid emulsion 1.5 mL/kg bolus
- Continuous intra-articular bupivacaine infusion is linked to chondrolysis
- Toxicity occurs with intravascular injection or overdose
- Aspirate before injecting and inject slowly in small aliquots
- Monitor for 30 minutes after large volume blocks
- Methaemoglobinaemia with prilocaine
Clinical Use
- Ropivacaine for long acting nerve blocks and periarticular infiltration
- Intra-articular bupivacaine infusions can cause chondrolysis and are avoided
- Lignocaine for short procedures and haematoma blocks
- Bier block uses prilocaine or lignocaine without adrenaline, keeping the cuff inflated for at least 20 minutes
Reviewed by Professor Phong Tran, Head of Orthopaedic Surgery, Western Health. Last updated 10 October 2026.
