Which device delivers the highest concentration of oxygen?

• A. Nasal cannula
• B. Simple face mask
• C. Nonrebreather mask
• D. Venturi mask

Answer: (C)

The capacity to deliver oxygen concentration depends on how the device controls mixing with room air and how well it can deliver a gas reservoir to the patient. The nonrebreather mask delivers the highest concentration because it includes a reservoir bag and one-way valves that prevent room air and exhaled gas from entering the inhaled stream. With an adequate flow (usually 10–15 L/min) to keep the bag inflated, most of what the patient inhales comes from the oxygen in the reservoir, approaching near-100% FiO2.

In contrast, a nasal cannula provides oxygen through small prongs and relies on the patient’s natural breathing; room air readily mixes in, so the FiO2 is typically around 24–40%. A simple face mask also lacks a reservoir and seals imperfectly, allowing room air to dilute the oxygen, giving about 40–60% FiO2. A Venturi mask is designed for precise, controlled oxygen concentrations by mixing with room air via calibrated adapters, usually delivering about 24–50% (depending on the adapter). It cannot reliably reach the near-100% concentrations possible with a nonrebreather mask.

So the highest oxygen concentration among these devices comes from the nonrebreather mask.