Oxygen Flush Valve — FRCA Primary MCQ

Instant feedback + full explanation. One question, done properly.

ModeratePhysics & Clinical MeasurementOxygen Flush ValveFRCA Primary

During anaesthesia the emergency oxygen bypass ('oxygen flush') control on a conventional continuous-flow anaesthetic machine is pressed. It delivers 100% oxygen from the pipeline supply (approximately 400 kPa) directly to the common gas outlet, bypassing the flowmeters and the vaporiser back bar. Which range best describes the oxygen flow delivered while the control is held down?

Educational content. Not a substitute for clinical judgement or local policy.

Reveal the answer and explanation

Correct answer: E — 30–75 L/min

Explanation lettering: D = shown as A · A = shown as B · B = shown as C · E = shown as D · C = shown as E

C is correct. Machine construction standards require the emergency oxygen bypass to deliver at least about 500 mL of oxygen per second (30 L/min); measured flush flows on modern machines lie roughly between 30 and 75 L/min, driven by pipeline pressure of about 400 kPa through a wide-bore path that bypasses flowmeters and vaporisers. D and E fail because 4–15 L/min are ordinary metered fresh-gas flows, far below the bypass requirement and inadequate for rapid circuit filling. A and B fail because flush regulators and machine outlet tubing cannot sustain such flows; these figures are more typical of the peak inspiratory flows of high-pressure jet or demand systems. Clinically the high flush flow can generate dangerous airway pressures if delivered against a closed APL valve or during mechanical inspiration, and it dilutes delivered volatile concentration.

Reference: Otteni JC et al. Anesthesia equipment: fresh gas delivery systems. I. Mechanical systems with rotameters and calibrated vaporizers. Ann Fr Anesth Reanim, 1999 (O2-flush valve delivers ≥500 mL/s, i.e. ≥30 L/min; barotrauma risk). https://pubmed.ncbi.nlm.nih.gov/10615544/