What prevents nitrous oxide from flowing if the oxygen cylinder runs out?

• A. Hypoxic guard
• B. Oxygen fail-safe
• C. Pressure relief valve
• D. Flow sensor

Answer: (A)

The main idea is a safety interlock that stops nitrous oxide if there isn’t enough oxygen available. The hypoxic guard is a mechanical interlock on the anesthesia machine that links the oxygen and nitrous oxide flow controls. When oxygen pressure falls (for example, if the oxygen cylinder runs out), the guard automatically reduces or blocks nitrous oxide flow to prevent delivering a hypoxic gas mixture. This direct coupling protects the patient by ensuring a minimum oxygen fraction is maintained whenever nitrous oxide is used.

The oxygen fail-safe is a related monitoring function that detects low oxygen pressure and can alarm and reduce nitrous oxide flow, but the device that directly prevents flow when oxygen is gone is the hypoxic guard. The other options—pressure relief valve and flow sensor—do not prevent nitrous oxide flow based on oxygen availability.