Alfentanil Onset — FRCA Primary MCQ
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Correct answer: A — A greater fraction is unionised at physiological pH, giving a faster onset
Explanation lettering: B = shown as A · C = shown as B · D = shown as C · E = shown as D · A = shown as E
B is correct. The alfentanil SmPC reports only 11% ionisation at pH 7.4, meaning approximately 89% is unionised. The unionised form of this weak base crosses the blood–brain barrier more readily, contributing to rapid blood–brain equilibration and an almost immediate onset after intravenous administration. Alfentanil has a shorter, not longer, elimination half-life than fentanyl. It is more highly protein bound: approximately 92%, compared with about 84% for fentanyl. Its limited tissue distribution produces a smaller volume of distribution, approximately 0.4–1.0 L/kg. Finally, alfentanil is metabolised by CYP3A4, so inhibitors of this enzyme can reduce its clearance and prolong its effects.
Reference: Alfentanil Kalceks, Summary of Product Characteristics, sections 5.1–5.2, revised 17 September 2024. https://www.medicines.org.uk/emc/product/12621/smpc