Local anaesthetic systemic toxicity (LAST) is tested across multiple exam banks — FRCA Primary (pharmacology), FRCA Final (clinical management), ORE Part 1 (dental emergencies), and MFDS Part 1 (pharmacology). The pharmacological principles are identical across all exams; the clinical context differs.
Maximum doses
You must memorise the maximum doses for the commonly used local anaesthetics. All doses assume a healthy 70 kg adult without adrenaline co-administration unless stated.
Lidocaine: 3 mg/kg without adrenaline, 7 mg/kg with adrenaline. Bupivacaine: 2 mg/kg. Levobupivacaine: 2 mg/kg. Ropivacaine: 3 mg/kg. Prilocaine: 6 mg/kg without adrenaline, 9 mg/kg with adrenaline. Articaine (dental): 7 mg/kg with adrenaline.
For dental practice specifically, the maximum dose of lidocaine 2% with 1:80,000 adrenaline is 7 mg/kg. The standard 2.2 mL dental cartridge of lidocaine 2% contains 44 mg of lidocaine. For a 70 kg patient, the maximum number of cartridges is approximately 11 (490 mg ÷ 44 mg). In practice, this limit is rarely approached in general dentistry, but the calculation is commonly examined.
Mechanism of toxicity
Local anaesthetics block sodium channels. At therapeutic doses, this occurs selectively in peripheral nerves. At toxic plasma concentrations, sodium channel blockade extends to the central nervous system and the heart.
CNS toxicity occurs first (lower threshold) and manifests as circumoral tingling, metallic taste, tinnitus, visual disturbances, tremor, and seizures. Cardiovascular toxicity occurs at higher plasma concentrations and manifests as hypotension, bradycardia, arrhythmias (including VT and VF), and cardiac arrest. Bupivacaine is particularly cardiotoxic because it binds tightly to cardiac sodium channels with slow dissociation — the "fast in, slow out" kinetics make bupivacaine-induced cardiac arrest notoriously difficult to treat.
Risk factors for LAST
Inadvertent intravascular injection is the most common cause. Other risk factors include exceeding the maximum dose, injection into highly vascular tissue, extremes of age (reduced clearance in neonates and the elderly), hepatic impairment (reduced metabolism of amide LAs), cardiac failure (reduced hepatic blood flow), acidosis (increases the unbound fraction), and low plasma protein levels.
The AAGBI lipid rescue protocol
The Association of Anaesthetists published guidelines for the management of severe local anaesthetic toxicity. The key steps are as follows.
Stop injecting the local anaesthetic. Call for help. Manage the airway and ventilate with 100 per cent oxygen. If seizures occur, treat with small doses of midazolam, thiopental, or propofol.
If cardiovascular collapse occurs, start CPR and give intravenous lipid emulsion (Intralipid 20 per cent). The dosing regimen is an initial IV bolus of 1.5 mL/kg over one minute, followed by an IV infusion of 15 mL/kg/hour. If cardiovascular stability is not restored after 5 minutes, give a further bolus of 1.5 mL/kg and increase the infusion to 30 mL/kg/hour. The maximum cumulative dose is 12 mL/kg.
Do not use adrenaline doses greater than 1 microgram/kg. Avoid vasopressin, calcium channel blockers, and beta-blockers. Avoid lidocaine as an antiarrhythmic (it will worsen the toxicity). Consider cardiopulmonary bypass as a rescue strategy if standard measures fail.
The lipid rescue mechanism is thought to involve sequestration of the lipophilic local anaesthetic molecules into the lipid phase, effectively reducing the free plasma concentration available to bind cardiac sodium channels.
What each exam tests
FRCA Primary tests the pharmacology — mechanism, pKa, protein binding, maximum doses, and metabolism. FRCA Final tests clinical management — recognition, AAGBI protocol, and the lipid rescue dosing. ORE Part 1 and MFDS Part 1 test both the pharmacology and the emergency management within the dental practice context — recognition of early signs, stopping the procedure, managing seizures, and initiating lipid rescue.
iatroX covers LAST across all four exam banks — FRCA Primary, FRCA Final, ORE Part 1, and MFDS Part 1. Questions are tagged by exam and by topic for focused practice. All included at £29 per month or £99 per year.