What FiO2 is typically delivered by a nonrebreather mask at 15 L/min?

• A. Approximately 0.80 to 0.95 depending on fit
• B. Approximately 0.21
• C. Exactly 1.00
• D. Approximately 0.50

Answer: (A)

Nonrebreather masks deliver a high fraction of inspired oxygen because the mask has a reservoir bag and one-way valves that limit the mixing with exhaled gas and reduce room-air entrainment. When you run the device at about 15 L/min, the bag stays filled, so the patient inhales oxygen-rich air from the reservoir with minimal dilution from ambient air. In practice, the FiO2 is typically around 0.80 to 0.95, though it can vary up or down depending on how well the mask fits and how strongly the patient inspirates; a poor seal or a collapsing reservoir will allow more room air to mix in, lowering the FiO2.

This is higher than room air, which is about 0.21, and it’s not exactly 1.00 because some ambient air invariably mixes in during inhalation. It’s also higher than about 0.50, which is more characteristic of other delivery methods at lower flows.