Venturi mask delivers a fixed FiO2 regardless of patient respiratory rate or tidal volume. The mechanism ensuring constant FiO2 is:

• A A pressure-regulated valve that increases oxygen flow proportionally to patient inspiratory effort
• B A CO2-absorbing canister maintaining a controlled atmospheric mixture
• C A large reservoir bag that pre-mixes oxygen and room air at a set concentration
• D The Bernoulli principle — high-velocity oxygen jet creates entrainment of room air at a fixed ratio determined by jet orifice diameter ✓

Explanation

The Venturi mask operates via the Bernoulli effect/Venturi principle: a high-velocity jet of oxygen through a narrow orifice creates a region of low lateral pressure, which entrains room air through side ports at a fixed ratio determined by the jet orifice size. For example, a blue Venturi adapter entrains 12 parts air to 1 part O2 to deliver 28% FiO2. Because the total flow delivered (oxygen + entrained air) exceeds the patient's peak inspiratory flow, the delivered FiO2 remains constant regardless of breathing pattern — making it ideal for COPD patients requiring accurate low-FiO2 therapy.

Reference: Morgan & Mikhail's Clinical Anesthesiology, 6th ed.

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Written and medically reviewed by the StethoPrep medical team.