Local Anaesthetic Onset — FRCA Primary MCQ

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ModeratePharmacology - AnalgesicsLocal Anaesthetic OnsetFRCA Primary

Which local anaesthetic is characterised by a fast onset, medium duration of action and a dose-related risk of methaemoglobinaemia that limits its use in continuous techniques?

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Correct answer: A — Prilocaine

Prilocaine is the correct answer. It is an amide local anaesthetic with a fast onset and medium duration, but its metabolite o-toluidine can oxidise haemoglobin to methaemoglobin; this dose-related toxicity makes prilocaine unsuitable for continuous techniques. Local-anaesthetic onset is influenced by pKa and tissue pH because the unionised base crosses lipid membranes, but also by concentration, dose, site and technique. Lidocaine also has a rapid onset and may have a clinically similar onset to prilocaine, so it cannot be called uniquely fastest without specifying experimental conditions. Bupivacaine, ropivacaine and levobupivacaine are principally long-acting agents and do not have prilocaine's characteristic methaemoglobinaemia liability.

Reference: electronic Medicines Compendium. Prilocaine hydrochloride 1% Solution for Injection, Summary of Product Characteristics, sections 4.8 and 5.1. Updated 14 July 2022. https://www.medicines.org.uk/emc/product/870/smpc