Which flow rate is commonly used with a well-sealed nonrebreathing mask to deliver up to about 90% inspired oxygen?

• A. 10 L/min
• B. 15 L/min
• C. 25 L/min
• D. 40 L/min

Answer: (B)

The key idea is that delivering a high oxygen concentration with a nonrebreathing mask relies on keeping the reservoir bag full and minimizing room-air entrainment. A well-sealed nonrebreathing mask uses a reservoir bag and one‑way valves so that during inspiration the patient draws from the bag’s oxygen, not from ambient air. Flowing about 15 L/min is commonly used because it is enough to keep the bag inflated and flush out exhaled gas, producing roughly 90% inspired oxygen when the mask fits well.

If the flow is too low, like around 10 L/min, the bag can collapse on inspiration, allowing room air to dilute the gas and lowering the FiO2. Higher flow rates, such as 25 or 40 L/min, don’t significantly raise FiO2 beyond about 90% in typical use and can be unnecessarily uncomfortable or wasteful.