Which device generally delivers the highest oxygen concentration to a spontaneously breathing patient?

• A. Nonrebreather mask
• B. Nasal cannula
• C. Simple face mask
• D. Bag-valve mask

Answer: (A)

When comparing how much oxygen a patient who is breathing on their own can receive, the equipment that limits mixing with room air the most delivers the highest FiO2. A nonrebreather mask uses a tight seal, a reservoir bag that fills with oxygen, and one-way valves, which prevent exhaled gas from entering the reservoir. This setup minimizes dilution by room air, so the patient often inhales nearly pure oxygen—roughly 80-100% when flow and fit are correct. In contrast, a nasal cannula delivers oxygen through prongs with significant mixing with room air, typically up to about 44% at higher flows. A simple face mask also dilutes with room air because it lacks a reservoir, usually delivering around 35-60%. A bag-valve mask is designed mainly for ventilation and, while it can deliver high oxygen concentrations with a reservoir, it is used for assisted breathing rather than a spontaneously breathing patient. Therefore, the nonrebreather mask generally provides the highest oxygen concentration for someone who is breathing on their own.