At a flow rate of 6 L/min, a nasal cannula can deliver an approximate oxygen concentration of up to:

• A. 24%.
• B. 35%.
• C. 44%.
• D. 52%.

Answer: (C)

The idea here is that how much oxygen a nasal cannula can deliver depends on flow and how much room air is entrained during inhalation. Oxygen from the cannula mixes with ambient air in the nose, so the inspired fraction isn’t the same as the gas coming from the tube. As you raise the flow, you push more room air out of the mix, but you never reach the full concentration of the delivered oxygen.

At a flow of 6 L/min, a nasal cannula typically delivers about 44% FiO2. That’s a commonly used rough value in practice. The other numbers would correspond to much lower flows (giving around the lower percentages) or would require a different delivery method capable of delivering higher concentrations. So 44% is the best, most accurate approximate answer for a nasal cannula at 6 L/min.