Obesity Pharmacokinetics — FRCA Final MCQ

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HardGeneral AnaesthesiaObesity PharmacokineticsFRCA Final

A 42-year-old woman with a BMI of 52 kg.m−2 is scheduled for laparoscopic bariatric surgery. Propofol is given as a manually controlled infusion and stopped at the point she becomes unresponsive to verbal command. Which body-size scalar most closely normalises the propofol dose required to reach this endpoint to that of non-obese adults?

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Correct answer: C — Lean body weight

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

C is correct. Propofol's initial distribution and early clearance track lean tissue and cardiac output rather than fat mass, so lean body weight normalises the induction requirement: in the randomised comparison of scalars in morbid obesity, LBW-scaled propofol produced induction doses and times to unresponsiveness similar to lean subjects, while total-body-weight scaling gave a significantly larger dose with attendant hypotension. B (actual body weight) therefore overdoses. A (ideal body weight) ignores the real increase in lean mass with obesity and is associated with more signs of inadequate anaesthesia. D (adjusted body weight) is a sound scalar for maintenance infusions and a pragmatic clinical compromise to reduce awareness risk, but it is not the scalar shown to normalise this loss-of-responsiveness endpoint. E is not a validated propofol scalar. Whatever scalar is chosen, titrate to effect and confirm adequate depth before neuromuscular blockade and airway instrumentation.

Reference: Ingrande J, Brodsky JB, Lemmens HJ. Lean body weight scalar for the anesthetic induction dose of propofol in morbidly obese subjects. Anesth Analg 2011;113:57–62 (conclusion: LBW is the more appropriate scalar for induction). https://pubmed.ncbi.nlm.nih.gov/20861415/ — read alongside UK guidance: Peri-operative management of the obese surgical patient 2015 (AAGBI/SOBA), https://pubmed.ncbi.nlm.nih.gov/25950621/