During routine general anaesthesia, an anaesthetist presses the oxygen flush valve on the anaesthesia machine. Which statement best describes what happens?

• A Oxygen at approximately 5 L/min is added downstream of the vaporiser to enrich the inspired mixture
• B Oxygen at 35 to 75 L/min is delivered directly to the common gas outlet, bypassing the vaporiser ✓
• C The ventilator switches to a manual breathing mode while the flush is held open
• D Nitrous oxide delivery is temporarily interrupted to maintain a fixed oxygen concentration

Explanation

The oxygen flush valve delivers a high flow of pure oxygen, classically 35 to 75 L/min, straight from the pipeline or cylinder supply to the common gas outlet, completely bypassing the vaporisers. This rapidly refills the reservoir bag and circuit but can cause barotrauma if pressed during the inspiratory phase of ventilation, and it dilutes the volatile agent, risking awareness. Option A is wrong because flush flow vastly exceeds normal fresh gas flow and does pass through the patient circuit, just not through any vaporiser.

Reference: Miller's Anesthesia, 9th ed.

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Written and medically reviewed by the StethoPrep medical team.