You are performing mouth-to-mask ventilations with oxygen connected and set at a flow rate of 15 L/min. What percentage of oxygen is your patient receiving?

• A. 75%
• B. 65%
• C. 55%
• D. 45%

Answer: (C)

Oxygen concentration during assisted ventilation depends on how well the mask seals and how much room air is entrained during inhalation. In a mouth-to-mask setup with oxygen connected at 15 L/min, the gas you deliver mixes with ambient air, so you don’t reach the pure 100% O2. The result is an approximate FiO2 in the mid-50s, around 55%. That’s why 55% is the best match among the options. Higher percentages would require a device that seals more completely (like a nonrebreather with a tight seal), while a much lower percentage isn’t expected with this flow. Keep in mind this is an approximate value and can vary with mask fit and the patient’s breathing pattern.