Vasoconstrictor Rationale — MFDS Part 1 MCQ
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Correct answer: E — It reduces local blood flow, slowing systemic absorption and prolonging the local anaesthetic effect
Explanation lettering: E = shown as A · D = shown as B · B = shown as C · C = shown as D · A = shown as E
A is correct. A vasoconstrictor such as adrenaline constricts local vessels, reducing perfusion at the injection site. The local anaesthetic is therefore removed into the systemic circulation more slowly, remains near the nerve for longer and has a prolonged clinical effect; reduced local bleeding may provide an additional procedural benefit. Vasoconstrictors do not produce their principal benefit by directly increasing sodium-channel affinity: sodium-channel blockade is the intrinsic action of the local anaesthetic itself. Increased local blood flow would promote systemic removal rather than retention. Adrenaline does not act through opioid receptors, and adding a vasoconstrictor does not increase the local anaesthetic's pKa.
Reference: Aspen Pharma Trading Ltd. Lidocaine 2% with adrenaline (epinephrine) 1:200,000 solution for injection, Summary of Product Characteristics, section 5.2 Pharmacokinetic properties, revised July 2026. https://www.medicines.org.uk/emc/product/2396/smpc