A full size E-type nitrous oxide cylinder connected to the machine shows a gauge reading of 5200 kPa. As the case proceeds, this gauge pressure remains essentially unchanged for hours and then falls abruptly. The reason for this behaviour is:
- A Nitrous oxide gas is far more compressible than oxygen, buffering the gauge pressure
- B The pressure reducing valve continuously refills the cylinder to maintain a fixed reading
- C Nitrous oxide is stored as a liquefied compressed gas and the gauge records its saturated vapour pressure, which stays constant while any liquid remains ✓
- D The pin index safety system mechanically locks the cylinder pressure at a fixed value
Correct answer: C. Nitrous oxide is stored as a liquefied compressed gas and the gauge records its saturated vapour pressure, which stays constant while any liquid remains
Explanation
Because the cylinder contains liquid nitrous oxide in equilibrium with its vapour, the gauge reflects only the saturated vapour pressure, which depends on temperature and not on remaining volume. The reading stays steady until all liquid has evaporated, after which pressure drops rapidly. Contents must therefore be estimated by weighing the cylinder and subtracting the tare weight, not by reading the gauge.
Reference: Morgan and Mikhail's Clinical Anesthesiology, 7th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.