The pressure gauge of a nitrous oxide cylinder in use on the anaesthesia machine has shown an unchanged reading of 5200 kPa throughout a long operating list. Which statement best explains this observation?

• A The gauge is faulty and should be replaced
• B The regulator is maintaining a constant downstream pressure irrespective of cylinder content
• C Nitrous oxide cylinders are always overfilled above their service pressure
• D Nitrous oxide is stored as a liquefied compressed gas, so pressure reflects vapour pressure until all liquid is exhausted ✓

Explanation

Nitrous oxide is stored below its critical temperature as a liquefied compressed gas, filling about 90 percent of the cylinder volume. The gauge registers the saturated vapour pressure of the liquid, which stays constant at about 5200 kPa at room temperature regardless of how much liquid remains. Only when all liquid has evaporated does the pressure begin to fall, by which point very little gas is left. Weighing the cylinder, not reading the gauge, estimates remaining content.

Reference: Miller's Anesthesia, 9th ed.

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Written and medically reviewed by the StethoPrep medical team.