Infiltrating local anaesthetic and digital blocks

What it is

Local anaesthetics block sodium channels in nerve fibres so that pain signals stop for as long as the drug stays in the tissue. For a laceration the agent is infiltrated directly into the wound edges, through the open wound rather than through intact skin, which hurts less. Amides such as lidocaine (fast onset, intermediate duration) and bupivacaine (slower onset, long duration) are the usual choices. Adding adrenaline (epinephrine) narrows the vessels, so the anaesthetic lasts longer, bleeding is less and a larger total amount can be used; current evidence supports adrenaline-containing solutions even in the fingers of healthy patients, but local protocol and the circulation of the part decide. Every agent has a maximum safe dose by weight, lower for the plain solution than for the solution with adrenaline; it is calculated before infiltrating, especially in children and for large wounds. A digital block numbs a whole finger or toe by placing anaesthetic beside the two dorsal and two palmar digital nerves at the base of the digit, and is used for nail-bed injuries, fingertip wounds and dislocations. Local anaesthetic systemic toxicity (LAST) follows accidental intravascular injection or overdose and starts with neurological symptoms before cardiac ones.

Indications

• Repair of lacerations and other minor skin procedures (foreign body removal, skin biopsy, abscess drainage).
• Digital block: nail-bed repair, nail removal, fingertip and toe wounds, reduction of a digital dislocation.
• Anaesthetic with adrenaline: vascular areas such as the scalp and face, to reduce bleeding and lengthen the block.
• Long-acting agents such as bupivacaine: procedures that take long or when prolonged pain relief afterwards helps.
• Regional blocks (for example of the face or wrist) for large wounds where infiltration would need too much drug.

Contraindications and precautions

• True allergy to amide anaesthetics is rare; ester allergy is more common. Ask what happened before labelling a patient allergic.
• Avoid injecting through infected skin: it works poorly in acidic tissue and can spread infection.
• Use adrenaline with caution or not at all in parts with poor circulation (peripheral vascular disease, vasospasm, crush).
• Lower the maximum dose in the elderly, in liver or heart failure and in small children; add up every injection.
• Never leave a finger tourniquet on after the procedure, and avoid large circumferential volumes in a digit.

Equipment

• Chosen agent (plain or with adrenaline), checked for name, concentration and expiry.
• Syringe, a drawing-up needle and a fine injecting needle (27 or 30 gauge).
• Antiseptic, gloves and gauze.
• Resuscitation equipment, oxygen, suction and lipid emulsion available where local anaesthetic is used.
• Monitoring for large volumes or regional blocks: pulse oximetry, ECG and blood pressure.

Step by step (and why)

1. Check allergies, weight and the maximum safe dose; examine sensation and movement — doses add up and deficits must be recorded first.
2. Explain that it will sting briefly — warming the solution and injecting slowly reduce the pain.
3. Insert a fine needle through the wound edge, not intact skin — the open wound is less sensitive.
4. Aspirate before each injection — blood in the syringe means a vessel; move the needle before injecting.
5. Inject slowly while advancing, re-entering through skin already numb — fast injection stretches tissue and hurts.
6. Digital block: inject on each side at the base of the digit, dorsal then palmar — this reaches all four digital nerves.
7. Wait for the block and test with a pin or forceps before starting — onset takes minutes, longer with long-acting agents.
8. Watch the patient and ask about tingling lips, tinnitus or odd taste — these are the first signs of toxicity.
9. Record agent, concentration, total amount and time — needed if more anaesthetic is required later.

Complications

• Local anaesthetic systemic toxicity: neurological then cardiovascular; lipid emulsion is the specific treatment.
• Vasovagal faint during injection, common in anxious patients sitting up; lie them down.
• Allergic reaction, more often to esters or to a preservative than to amides.
• Haematoma, nerve injury from direct needle trauma, and infection at the injection site.
• Methaemoglobinaemia with some agents (prilocaine, benzocaine): cyanosis with low oxygen saturation that does not improve with oxygen.

Next step

• Stop, call for help and start the toxicity protocol at the first neurological or cardiac warning sign.
• Call a senior when the anaesthetic needed would approach the maximum dose; consider a regional block or sedation.
• Explain that numbness lasts some hours: protect the area from burns and biting, especially lips and tongue.
• Record any adverse reaction in the clinical record and tell the patient so it is not repeated.
• Review the digit before discharge after a digital block: colour and capillary refill must be normal.

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Educational content; it does not replace supervised hands-on training, your institution's protocols or a prescriber's order. A procedure is learnt with a teacher and done only once you are trained to do it.

Sources: Digital Nerve Block (StatPearls (NCBI Bookshelf), 2024); Local Anesthetic Toxicity (StatPearls (NCBI Bookshelf), 2026).

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Representative model, for educational purposes only.