Mepivacaine

Mepivacaine is an amide local anaesthetic of intermediate duration with rapid onset and little vasodilatation, which allows plain solutions to be used without adrenaline. It is used mainly for dental anaesthesia, infiltration and nerve blocks. Its main limitation is slow clearance in neonates.

Class. Amide local anaesthetic (intermediate duration).

Mechanism. Reversible blockade of voltage-gated sodium channels.

Pharmacokinetics. Onset 3 to 5 minutes (infiltration), 10 to 20 minutes (blocks). Duration 1.5 to 3 hours plain, longer with adrenaline. Protein binding about 75%. Half-life about 2 to 3 hours in adults and up to 9 hours in neonates. Hepatic metabolism (hydroxylation, N-demethylation) with renal excretion.

Indications are dental anaesthesia (infiltration and block), peripheral nerve block, epidural anaesthesia in some settings, and infiltration for minor surgery.

Maximum doses (adults)

• 4.4 mg/kg, not above 300 mg, in a single dose. Some labels allow 7 mg/kg, up to 400 mg, and 1000 mg in 24 hours.
• Do not repeat doses more often than every 90 minutes.

Techniques

• Dental: 3% plain or 2% with adrenaline (1 in 100,000), 1.8 mL cartridges (54 mg for 3%, 36 mg for 2%). Usual dose is 1 to 2 cartridges; maximum 6.6 mg/kg and 400 mg in 24 hours in most dental labels.
• Infiltration: 0.5 to 1%, up to the maximum dose.
• Peripheral nerve block: 1 to 1.5% in 10 to 20 mL (within the maximum single dose).
• Epidural: 1.5 to 2% in 15 to 25 mL, in incremental doses; not used in obstetrics.
• Spinal: not advised.

Children

• Dental: maximum 5 to 6 mg/kg, using the smallest effective volume.
• Not for neonates because of prolonged half-life.
• Adjust dose by weight, using the lowest effective volume.

Contraindications. Hypersensitivity to amide anaesthetics; obstetric paracervical block (foetal bradycardia and death); neonatal use; complete heart block; infection at the site; adrenaline-containing preparations in end-arterial sites.

Boxed-warning-level risks. Systemic toxicity with seizures and cardiovascular collapse after overdose or intravascular injection; foetal bradycardia after paracervical block.

Adverse effects. Common: injection pain, transient dizziness, nausea. Serious: local anaesthetic systemic toxicity, hypotension, bradycardia, methaemoglobinaemia (rare), nerve injury, anaphylaxis (rare).

Interactions. Other amide anaesthetics add systemic toxicity. Beta blockers and cimetidine raise levels. Adrenaline-containing preparations interact with tricyclic antidepressants, monoamine oxidase inhibitors and non-selective beta blockers (hypertension).

Common prescribing errors. Using it in neonates; paracervical block; repeating injections too soon; failing to count dental cartridges against the weight-based limit; adding adrenaline in a digital block.

Special groups

• Pregnancy: acceptable for dental and minor procedures; avoid paracervical use and epidural for labour.
• Breastfeeding: compatible.
• Older adults: reduce dose by 20 to 30%.
• Kidney impairment: no dose change for single doses.
• Liver impairment: reduce repeat dosing.
• Regional: check cartridge concentration, since 2% and 3% look alike.

Formulations. Dental cartridges 3% plain and 2% with adrenaline (1 in 100,000); vials 1, 1.5 and 2% for infiltration, blocks and epidural use.

Administration. Aspirate before injection and inject slowly. Keep the total dose within the weight-based ceiling when blocks are combined.

Baseline. Allergy history, age, weight, cardiac and liver status, pregnancy, and interacting drugs.

Monitoring. Observe for 30 minutes after large doses for signs of toxicity. Monitor electrocardiogram and oxygen saturation for blocks of larger volume. Check sensory and motor recovery.

Therapeutic drug monitoring. Not used.

Dose reduction and stopping thresholds. Stop injection at the first sign of toxicity (perioral numbness, tinnitus, confusion, twitching). Reduce doses in older adults and in impaired liver function.

Referral and escalation. Seizures or collapse: stop the injection, give oxygen and airway support, benzodiazepine for seizures, and 20% lipid emulsion 1.5 mL/kg over 1 minute then 15 mL/kg/hour (maximum cumulative 12 mL/kg). Neonates exposed through maternal blocks need respiratory observation.