The fail-safe mechanism on an anaesthesia machine is designed to prevent delivery of a hypoxic mixture. This device:
- A Shuts off all gas flows if nitrous oxide pipeline pressure drops below atmospheric
- B Activates an alarm and terminates N2O flow when O2 flow drops below 200 mL/min
- C Ensures O2 flowmeter is always positioned downstream of N2O flowmeter on the back bar
- D Proportionally reduces N2O flow when O2 supply pressure falls, maintaining at least 25% O2 at the common gas outlet ✓
Explanation
The fail-safe (hypoxic guard/proportioning system, e.g., Link-25 or ORMC) monitors oxygen supply pressure and proportionally reduces or shuts off nitrous oxide flow if the O2 pressure falls below a threshold, ensuring the minimum O2 concentration at the common gas outlet does not fall below 25% (some systems 21%). This prevents delivery of a hypoxic mixture caused by O2 pipeline depletion while N2O continues flowing. It does NOT guarantee a minimum O2 concentration at the patient — other factors (leaks, wrong gas in pipeline) can still produce hypoxia. The O2 flowmeter is always placed last (most downstream) on the back bar as an additional safety feature.
Reference: Morgan & Mikhail's Clinical Anesthesiology, 6th ed.
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Written and medically reviewed by the StethoPrep medical team.