During induction, the Ritchie whistle attached to the anaesthesia machine begins to sound continuously. What does this indicate?
- A The oxygen analyser reads below 21 percent
- B Nitrous oxide supply pressure has exceeded the oxygen supply pressure
- C Pipeline oxygen pressure has fallen below approximately 200 kPa (30 psi) ✓
- D The ventilator driving gas pressure is excessive
Correct answer: C. Pipeline oxygen pressure has fallen below approximately 200 kPa (30 psi)
Explanation
The Ritchie whistle is a gas powered pneumatic alarm on older machines that activates when the oxygen supply pressure falls to roughly 200 kPa (about 30 psi), well above the minimum safe working pressure of around 400 kPa pipelines. It warns before a hypoxic mixture can be delivered. It responds to supply pressure alone, so it does not detect a hypoxic mixture caused by vaporizer malfunction or a disconnected inspired line.
Reference: Understanding Anesthesia Equipment (Dorsch and Dorsch), 6th ed.
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Written and medically reviewed by the StethoPrep medical team.