Vd Calculation Propofol — FRCA Primary MCQ

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ModeratePharmacology - General PrinciplesVd Calculation PropofolFRCA Primary

In a pharmacokinetic study, a 70 kg adult receives propofol 3 mg/kg as a rapid intravenous bolus for induction of anaesthesia. Arterial sampling is used and the plasma concentration extrapolated back to time zero, fitted to a central compartment, is 10.5 mg/L. What is the initial apparent volume of distribution?

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Correct answer: A — 20 L

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

D is correct. Dose = 3 mg/kg × 70 kg = 210 mg; for a bolus fitted to a central compartment, Vd = dose/C0 = 210 ÷ 10.5 = 20 L. Vd is a proportionality constant, not a real anatomical space, but 20 L (0.29 L/kg) sits within the UK SmPC central volume range for propofol of 0.2–0.79 L/kg — blood plus vessel-rich tissue. Distractors arise from arithmetic or conceptual errors: A (10 L) misreads the concentration as the dose per kg divided value; B (15 L) approximates blood volume plus extracellular fluid; C (50 L) and E (30 L) confuse the initial volume with later distribution volumes — propofol's steady-state volume (1.8–5.3 L/kg, i.e. hundreds of litres) is far larger and cannot be derived from a single time-zero concentration.

Reference: Propofol 1% (10 mg/ml) emulsion for injection or infusion — Summary of Product Characteristics, section 5.2 Pharmacokinetic properties, eMC (UK), revised 2024: https://www.medicines.org.uk/emc/product/15152/smpc ; pharmacokinetic identity Vd = amount of drug in body / plasma concentration: Clinical Significance of Volume of Distribution in Pharmacotherapy, https://pubmed.ncbi.nlm.nih.gov/31424864/