skip to main content

Vaporiser Output at Altitude — FRCA Primary MCQ

Instant feedback + full explanation. One question, done properly.

HardPhysics & Clinical MeasurementVaporiser Output at AltitudeFRCA Primary

A conventional plenum variable-bypass isoflurane vaporiser is calibrated at a barometric pressure of 100 kPa. Its dial is set to 2.0% and it is then used at an altitude where barometric pressure is 60 kPa. Assuming stable vaporiser temperature and an unchanged splitting ratio, which pair best describes its output at altitude?

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

Reveal the answer and explanation

Correct answer: D — Approximately 3.3% isoflurane, with a partial pressure of approximately 2.0 kPa

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

Option A is correct. A conventional plenum variable-bypass vaporiser has an approximately fixed splitting ratio at a given dial setting. Because the agent's saturated vapour pressure is principally temperature-dependent, the vaporiser therefore delivers approximately the same anaesthetic partial pressure despite reduced ambient pressure. At sea level, 2.0% of 100 kPa is 2.0 kPa. At 60 kPa, that partial pressure represents 2.0/60 = 3.3% by volume. Option B incorrectly assumes that the dial continues to specify the actual volume percentage. Option C applies the fall in barometric pressure twice. Options D and E give incorrect combinations of concentration and partial pressure. This relationship should not be generalised without qualification to every vaporiser design.

Reference: Royal College of Anaesthetists. Guide to the FRCA Examination: The Primary, Fourth Edition. SBA question 82 and answer commentary, 2013. https://www.rcoa.ac.uk/media/49221