Which device delivers the highest concentration of oxygen with a reservoir bag?

• A. Nasal cannula
• B. Nonrebreathing mask
• C. Mouth-to-mouth device
• D. Venturi mask

Answer: (B)

The main idea is how the device delivers oxygen and how much room air gets mixed in during inhalation. A nonrebreathing mask has a reservoir bag and two one-way valves that prevent inhaled air from mixing with exhaled gas and keep room air out of the breathing circuit. Oxygen flows into the bag continuously, and during inhalation the patient draws a large portion of gas from that reservoir, which can be very rich in oxygen. If the flow is high enough (about 10–15 L/min), the bag stays inflated and the inhaled gas is nearly all oxygen, giving a very high FiO2, approaching the highest possible for a simple face mask.

Nasal cannulas deliver oxygen without a reservoir, and they mix with room air as you breathe, so the FiO2 tops out around 24–44% depending on flow. A mouth-to-mouth device isn’t designed to deliver controlled or high concentrations of oxygen. A Venturi mask can deliver a precise, adjustable FiO2 by mixing oxygen with ambient air, but it does not use a reservoir bag, so the maximum FiO2 is typically lower than what a nonrebreathing mask can achieve.