The fail-safe valve (hypoxic guard/pressure sensor shut-off) on the anaesthesia machine is designed to prevent delivery of a hypoxic mixture. What is the key limitation of this device?
- A It does not prevent erroneous cylinder connections
- B It cannot prevent backflow into the flowmeter tubes
- C It responds to oxygen pipeline pressure, not oxygen flow — it prevents gas flow without oxygen pressure but cannot detect a cross-connected pipeline supplying N2O in the oxygen port ✓
- D It only functions when N2O is used, not with air or helium
Explanation
The fail-safe (hypoxic guard) valve shuts off N2O and other gases if oxygen pressure falls below threshold (~30 psi). However, it is a pressure-sensing device only — it responds to oxygen pipeline pressure, not actual oxygen content flowing to the patient. A critical limitation is that it cannot detect a cross-connected pipeline (N2O accidentally connected to the oxygen pipeline) because N2O at adequate pressure will satisfy the hypoxic guard. The Pin Index Safety System (PISS) for cylinders and Diameter Index Safety System (DISS) for pipelines provide additional protection but are not foolproof. Paramagnetic oxygen analysers in-line are the definitive safeguard against delivery of a hypoxic mixture.
Reference: Morgan & Mikhail's Clinical Anesthesiology, 6th ed.
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Written and medically reviewed by the StethoPrep medical team.