Local Anaesthetic & Digital Block

Maximum safe doses

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

1. 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.

2. 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.

3. 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 (AoA QRH 3-10, 2023)

1. Stop injecting immediately

   Stop the local anaesthetic (remember infusion pumps) and call for help, the cardiac arrest trolley and the lipid rescue pack.

2. Manage airway, breathing, circulation

   100% oxygen and adequate ventilation, securing the airway if needed. Avoid hypercarbia — consider mild hyperventilation. Treat seizures with small incremental doses of a benzodiazepine, and consider neuromuscular blockade if seizures cannot be controlled. Avoid vasopressin, calcium-channel blockers, beta-blockers and local anaesthetic itself as antiarrhythmics.

3. Give lipid emulsion 20%

   1.5 mL/kg over 2–3 minutes (~100 mL at 70 kg), followed immediately by an infusion at 15 mL/kg/h. Use 20% Intralipid — propofol is not a substitute.

4. Repeat if needed

   If cardiovascular stability is not restored or an adequate circulation deteriorates, give a repeat bolus at 5 and at 10 minutes (up to two further boluses) and, at any time after 5 minutes, double the infusion rate to 30 mL/kg/h — up to a maximum cumulative dose of 12 mL/kg (840 mL at 70 kg).

5. In cardiac arrest, cut the adrenaline dose

   Use a reduced adrenaline dose of ≤1 microgram/kg — not the standard 1 mg. This instruction is new in the 2023 handbook and is absent from the withdrawn 2010 poster.

6. 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.

References