LA Onset Order — FRCA Primary MCQ

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ModeratePharmacology - AnalgesicsLA Onset OrderFRCA Primary

Regarding the pharmacology of local anaesthetics the order of nerve block onset from fastest to slowest is:

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Correct answer: C — Chloroprocaine > lidocaine > prilocaine > bupivacaine

Onset speed correlates inversely with pKa (lower pKa = more unionised at pH 7.4 = faster onset). Chloroprocaine pKa 8.7 has fast onset due to high concentration used (3%). Lidocaine pKa 7.9 (25% unionised) is faster than prilocaine pKa 7.7 in practice due to concentration/dose effects. Bupivacaine pKa 8.1 (15% unionised) has slowest onset.

Reference: Peck Hill Williams 2014 Pharmacology for Anaesthesia and Intensive Care