Venturi Mask Principle — FRCA Primary MCQ
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Correct answer: C — A high-velocity oxygen jet entrains room air in a predetermined ratio to deliver the selected FiO2.
Explanation lettering: E = shown as A · D = shown as B · B = shown as C · C = shown as D · A = shown as E
B is correct. Oxygen passes through a narrow orifice as a high-velocity jet, producing a pressure reduction that entrains room air through side ports. The adapter geometry determines the air-to-oxygen entrainment ratio and therefore the selected FiO2. Within the specified operating range, increasing oxygen flow through the same adapter increases both oxygen and entrained-air flow rather than proportionally increasing FiO2, so C is false. A fixed-performance device is intended to provide sufficient total flow to meet inspiratory demand, making A and D incorrect, although very high inspiratory flow or downstream obstruction can impair performance. The Coanda effect describes attachment of a jet to a nearby surface and is not the operating principle here, excluding E.
Reference: Royal Cornwall Hospitals NHS Trust. Prescription, Administration and Monitoring of Oxygen in Adults Policy V5.0, Appendix 6: Venturi Masks. June 2025. https://doclibrary-rcht.cornwall.nhs.uk/DocumentsLibrary/RoyalCornwallHospitalsTrust/Clinical/Respiratory/PrescriptionAdministrationAndMonitoringOfOxygenInAdultsPolicy.pdf