An isoflurane vaporiser calibrated at sea level is used unchanged in an operating theatre at high altitude (barometric pressure 550 mmHg), delivering 2% isoflurane measured as volume percent. Relative to the intended sea-level depth of anaesthesia, the patient will receive:
- A A lighter depth, because the partial pressure of isoflurane delivered is lower than at sea level ✓
- B The same depth, because anaesthetic effect depends on fractional concentration
- C A deeper plane, because reduced barometric pressure increases vaporiser output
- D An unpredictable depth depending on the agent's oil:gas partition coefficient
Correct answer: A. A lighter depth, because the partial pressure of isoflurane delivered is lower than at sea level
Explanation
Anaesthetic potency correlates with the partial pressure of the agent in the brain, not the volume percent. At 550 mmHg, 2% corresponds to 11 mmHg of isoflurane versus 15.2 mmHg at sea level, so the same dial setting delivers a lower partial pressure and a lighter plane. Variable bypass vaporisers output a constant volume percent regardless of ambient pressure, ruling out option C, and the oil:gas coefficient does not vary with altitude.
Reference: Miller's Anesthesia, 9th ed.
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