A nitrous oxide cylinder connected to the anaesthesia machine shows a gauge pressure of 5200 kPa throughout a long operating list, even though substantial gas has been consumed. What does this observation indicate?
- A The cylinder contents are nearly full regardless of prior usage
- B N2O is stored as a liquid and vapour, so gauge pressure stays constant until all liquid has evaporated ✓
- C The regulator valve is malfunctioning and needs replacement
- D The cylinder contains oxygen contamination raising the measured pressure
Correct answer: B. N2O is stored as a liquid and vapour, so gauge pressure stays constant until all liquid has evaporated
Explanation
Nitrous oxide has a critical temperature above room temperature, so it exists as a liquid in equilibrium with vapour inside the cylinder. Gauge pressure reflects only the vapour phase pressure, which remains constant at about 5200 kPa as long as any liquid remains. Only when all liquid has evaporated does the pressure begin to fall with further use. Therefore the gauge cannot be used to estimate remaining N2O content; cylinder weight is used instead.
Reference: Morgan and Mikhail's Clinical Anesthesiology, 6th ed.
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Written and medically reviewed by the StethoPrep medical team.