Local Anaesthesia

Agent choice, dosing and safety for infiltration and peripheral nerve block in general practice and urgent care. Epidural, intrathecal and IV regional anaesthesia are specialist procedures and are not covered here.

Mode of action

Local anaesthetics reversibly block sodium channels in peripheral nerves, interrupting impulse conduction. All are amides in routine general-practice use (lidocaine, bupivacaine, ropivacaine) — esters (cocaine, tetracaine) are mostly limited to topical use and carry a separate, non-cross-reactive allergy risk.

Choosing an agent

• Lidocaine (lignocaine) — fastest onset (5–10 minutes), medium duration. The default for most minor procedures: suturing, abscess drainage, skin lesion excision.
• Bupivacaine and ropivacaine — slower onset (10–15 minutes), long duration. Useful when the procedure, or the pain expected afterwards, will outlast lidocaine.

Ropivacaine causes less cardiotoxicity than bupivacaine at equivalent doses, and resuscitation from ropivacaine-induced cardiac arrest has a higher chance of success. A given dose of lidocaine is less toxic than an equivalent dose of ropivacaine or bupivacaine.

Adrenaline

Adding adrenaline prolongs the block and reduces bleeding by slowing vascular absorption — the effect is greatest with lidocaine, less with bupivacaine, and minimal with ropivacaine. Adrenaline is contraindicated in a penile block. Near other terminal arteries (fingers, toes, ears, nose) it may be used with caution, avoiding it in Raynaud phenomenon or peripheral vascular disease, and seeking specialist advice if unsure. Total adrenaline dose should not exceed 500 micrograms regardless of the local anaesthetic used.

Maximum single doses

Agent	Without adrenaline	With adrenaline	Onset (infiltration)	Duration
Lidocaine	3 mg/kg	7 mg/kg	5–10 min	Medium
Bupivacaine	2 mg/kg (max 400 mg/24h)	2 mg/kg	10–15 min	Long
Ropivacaine	3 mg/kg (rarely >200 mg)	—	10–15 min	Long

Concentrations are expressed as a percentage or in mg/mL — multiply the percentage by 10 to get mg/mL (0.5% = 5 mg/mL). These maximums are guides only: the actual safe dose depends on the site’s vascularity, the technique, and the patient’s age and condition. Do not repeat the lidocaine maximum dose within 1.5 hours, or the bupivacaine maximum within 3 hours.

Worked example: a 70 kg adult having a laceration repaired with 1% lidocaine (10 mg/mL) has a plain-lidocaine maximum of 3 mg/kg = 210 mg = 21 mL of 1% solution. With adrenaline added, that rises to 7 mg/kg = 490 mg = 49 mL.

Precautions and contraindications

• Allergy to a local anaesthetic of the same class (amide or ester) is a contraindication. True allergy is rare and does not cross-react between the two classes — most reported “allergies” are actually a vasoconstrictor reaction (anxiety, pallor, tachycardia, hypertension, sweating, which resolves once administration stops) or a vasovagal episode, not a drug allergy. Preservatives (methylparaben, sodium metabisulfite) and antiseptic skin preparations can also cause reactions wrongly attributed to the anaesthetic itself.
• Local infection or inflammation at the injection site is a contraindication — infected tissue is more acidic, which reduces anaesthetic efficacy and increases systemic absorption.
• Cardiac disease: local anaesthetics enhance conduction defects — use cautiously in severe bradycardia, heart block or impaired conduction, and in cardiogenic shock or myocardial ischaemia, where cardiovascular depressant effects are increased.
• Anticoagulated patients carry a haematoma risk with deep blocks; infiltration and simple nerve blocks are lower-risk than central techniques but still warrant care.
• Elderly patients and children under 6 months are more sensitive to toxicity — reduce the dose.
• Pregnancy and breastfeeding: local anaesthetics are considered safe, using the lowest effective concentration; unlikely to cause problems in breastfeeding.
• Renal or hepatic impairment increases toxicity risk with prolonged or repeated dosing, particularly bupivacaine and ropivacaine in severe renal dysfunction.

Systemic toxicity (LAST)

Systemic toxicity follows inadvertent intravascular injection or an excessive total dose. Early features are neurological — perioral tingling, metallic taste, tinnitus, light-headedness, visual disturbance — and can progress to seizures, then cardiovascular collapse and arrhythmia. Bupivacaine is markedly more cardiotoxic than lidocaine or ropivacaine, and cardiac arrest from bupivacaine toxicity can be extremely difficult to reverse, potentially needing prolonged resuscitation.

If LAST is suspected: stop the injection immediately, call an ambulance, and manage as a medical emergency with standard airway support and resuscitation while awaiting transfer. Specific treatment, including lipid emulsion therapy, follows current ANZCA-endorsed protocols.

Prevention matters more than treatment. Aspirate before injecting, inject slowly, use the lowest effective dose and concentration, and keep resuscitation equipment available whenever a block is performed.

Practical points

• Local anaesthetic procedures should only be performed by clinicians with appropriate training, with resuscitation equipment available.
• Dilute with normal saline if a lower concentration than what’s supplied is needed; avoid adding alkalinising agents (e.g. sodium bicarbonate) to most local anaesthetic solutions — the mixture has limited solubility above pH 6 and precipitation can occur.
• Warn patients not to drive or use machinery until sensation and motor function have fully returned, and advise on how to avoid injuring an anaesthetised area before normal sensation returns.
• Reassess and document the total dose given whenever more than one syringe or site is used in the same patient, to avoid inadvertently exceeding the maximum.