Maximum safe doses
Lidocaine
- Plain: 3 mg/kg.
- With adrenaline: 7 mg/kg — vasoconstriction slows systemic absorption, allowing a higher safe dose.
Bupivacaine / levobupivacaine
- Maximum 2 mg/kg (BNF) regardless of adrenaline — the safety margin from adrenaline is less well established than for lidocaine, so do not extrapolate the lidocaine "with adrenaline" allowance.
- Longer duration of action but slower onset than lidocaine.
Dose maths — worked example
- Percentage concentration converts directly to mg/mL: 1% = 10 mg/mL; 2% = 20 mg/mL; 0.5% = 5 mg/mL.
- Worked example: a 70 kg adult, lidocaine 1% plain, max dose 3 mg/kg = 210 mg. At 10 mg/mL, that is a maximum volume of 21 mL of 1% lidocaine.
- With adrenaline, the same patient's maximum rises to 7 mg/kg = 490 mg = 49 mL of 1% lidocaine with adrenaline.
- Always use the patient's actual or estimated lean weight, and remember doses from multiple sites/procedures on the same patient are cumulative.
Reducing injection pain
- Buffering — adding sodium bicarbonate raises pH toward physiological, reducing the sting of infiltration.
- Warming the anaesthetic to body temperature before injecting.
- Slow, steady injection rather than a rapid bolus — speed of injection is a major determinant of pain.
- Use the smallest-gauge needle practical, and inject through already-anaesthetised skin where possible when extending a field block.
Adrenaline in digits
- Traditional teaching prohibited adrenaline in finger, toe, ear, nose and penile blocks, for fear of vasospasm and digital ischaemia.
- Modern evidence — multiple large case series report adrenaline-containing local anaesthetic used safely in digital blocks at standard concentrations (e.g. 1:100,000 or 1:200,000), without reported cases of resulting digital necrosis.
- Despite this evidence, many UK departments still avoid adrenaline in digital blocks as routine practice — check your local policy, and balance the benefit (prolonged block, reduced bleeding, a bloodless field) against departmental convention.
Digital nerve block technique
Dorsal two-injection ring block
Insert the needle at the dorsolateral base of the digit on each side, angling volarwards to anaesthetise both the dorsal and volar (palmar/plantar) digital nerves on that side. Repeat on the opposite side. This blocks all four digital nerves supplying the digit.
Single-injection volar subcutaneous alternative
A single volar injection at the base of the digit, raising a subcutaneous wheal across the flexor aspect, can anaesthetise the digit with one needle pass and is often less painful — a reasonable alternative, particularly in children.
Confirm the block
Test sensation before starting the procedure; allow adequate time (several minutes) for onset before assuming failure.
Digital nerve anatomy
Recognising LAST
- Early: perioral tingling/numbness, metallic taste, tinnitus, light-headedness, visual disturbance, agitation or confusion.
- Progressing: seizures, followed by cardiovascular collapse — arrhythmia, hypotension, and cardiac arrest.
- Toxicity can occur even from doses within the "normal" calculated range if there is inadvertent intravascular injection — always aspirate before injecting.
Managing LAST (AAGBI 2010)
Stop injecting immediately
Stop the local anaesthetic and call for help.
Manage airway, breathing, circulation
100% oxygen, secure the airway if needed, treat seizures (benzodiazepines), and start ALS if in cardiac arrest — avoid vasopressin, calcium-channel blockers, beta-blockers and local anaesthetic itself as antiarrhythmics.
Give lipid emulsion 20%
1.5 mL/kg bolus over approximately 1 minute, followed by an infusion at 15 mL/kg/h.
Repeat if needed
If there is no return of spontaneous circulation or the patient deteriorates, give up to two further boluses (5 minutes apart) and double the infusion rate to 30 mL/kg/h, up to a maximum cumulative dose of 12 mL/kg.
Continue resuscitation
CPR may need to continue for a prolonged period — prognosis with prompt lipid emulsion and good CPR is favourable. Transfer to a facility with cardiopulmonary bypass capability if recovery is not rapid.
Pitfalls & pearls
- Always calculate the weight-based maximum before drawing up — don't rely on memory for a "standard" volume.
- Aspirate before every injection to reduce the risk of intravascular injection and LAST.
- Give adequate time for a digital block to take effect before testing — premature testing leads to unnecessary "top-ups".
- Keep 20% lipid emulsion location known and accessible in any area where local anaesthetic is used in volume.
DOPS pointers
RCEM curriculum: ED Skill. What assessors look for at each entrustment level:
| 1 | Observed only — calculates maximum dose correctly, describes technique and LAST recognition. |
| 2a | Performs with supervisor present: correct dose calculation, aspiration technique, effective block. |
| 2b | Performs independently with supervisor in department, including troubleshooting an incomplete block. |
| 3 | Recognises and manages LAST, including lipid emulsion administration. |
| 4 | Independent; teaches and supervises others. |
References
- Joint Formulary Committee. British National Formulary — local anaesthetic monographs (lidocaine, bupivacaine).
- Association of Anaesthetists of Great Britain and Ireland (AAGBI). Management of Severe Local Anaesthetic Toxicity, 2010.
- Williams DJ, Walker JD. A nomogram for calculating the maximum dose of local anaesthetic. Anaesthesia, 2014.
- Literature on adrenaline safety in digital blocks (case series, e.g. Wilhelmi et al., Krunic et al.) — routine departmental practice still varies.