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LA Tachyphylaxis — MFDS Part 1 MCQ

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HardPharmacologyLA TachyphylaxisMFDS Part 1

Repeated equal doses of a local anaesthetic are used to maintain a regional nerve block. Later doses produce a block of shorter duration and reduced intensity. Which statement best reflects current evidence concerning the mechanism of this tachyphylaxis?

Educational content. Not a substitute for clinical judgement or local policy.

Reveal the answer and explanation

Correct answer: D — No single mechanism is established; altered local pharmacokinetics after repeated dosing is one experimentally supported explanation

Explanation lettering: B = shown as A · D = shown as B · E = shown as C · A = shown as D · C = shown as E

A is correct. Local anaesthetic tachyphylaxis describes reduced block duration, spread or intensity after repeated comparable doses, but no single human mechanism has been conclusively established. Experimental work demonstrates faster loss of lidocaine from nerve and infiltrated tissue after repeated administration, supporting altered local pharmacokinetics. Permanent intrinsic resistance of the nerve is not supported experimentally. Hepatic enzyme induction and antibody formation cannot develop over the short timescale involved, while adipose sequestration is not an established explanation. Low pH in inflamed tissue can reduce the unionised fraction and impair local anaesthesia, but this should not be presented as the proven cause of tachyphylaxis. The original item therefore retained the correct label but overstated an uncertain pH hypothesis.

Reference: Kongsgaard UE, Werner MU. Tachyphylaxis to local anaesthetics: what is the clinical evidence? A systematic review. Acta Anaesthesiologica Scandinavica. 2016;60:6–14. https://pubmed.ncbi.nlm.nih.gov/26358179/; supported mechanistically by Choi RH et al. Pharmacokinetic nature of tachyphylaxis to lidocaine. 1997. https://pubmed.ncbi.nlm.nih.gov/9315513/