An anaesthetist working at a high-altitude centre where barometric pressure is 630 mmHg maintains a patient on sevoflurane. Compared with sea level practice, what adjustment is needed to achieve the same anaesthetic depth?
- A The vaporiser dial setting must be reduced because alveolar uptake is faster at altitude
- B No adjustment is needed because MAC is expressed as a universal percentage
- C The vaporiser dial setting must be increased because anaesthetic depth depends on partial pressure, not percentage concentration ✓
- D The vaporiser must be recalibrated to deliver a fixed partial pressure irrespective of dial setting
Correct answer: C. The vaporiser dial setting must be increased because anaesthetic depth depends on partial pressure, not percentage concentration
Explanation
Anaesthetic depth depends on the partial pressure of the agent in the brain, not the fractional concentration. At 630 mmHg barometric pressure, 2 percent sevoflurane delivers only about 12.6 mmHg versus 15.2 mmHg at sea level, so a higher dial setting, roughly 2.4 percent, is required to reach one MAC. Variable-bypass vaporisers output a percentage concentration, so the clinician must compensate manually for reduced ambient pressure.
Reference: Miller's Anesthesia, 9th ed.
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