If you decrease the air-to-oxygen ratio, what happens to the delivered FiO2?

If you decrease the air-to-oxygen ratio, what happens to the delivered FiO2?

• A. FiO2 increases
• B. FiO2 decreases
• C. FiO2 stays the same
• D. FiO2 becomes unpredictable

Answer: (A)

FiO2 is determined by how much oxygen is in the gas you breathe, i.e., the mix of oxygen with air. Air is about 21% O2, while pure oxygen is 100%. If you decrease the air portion relative to oxygen, you dilute oxygen less, so the overall oxygen fraction in the delivered gas rises. In other words, lowering the air-to-oxygen ratio makes the mixture richer in O2, increasing FiO2. If you cut air flow toward zero, FiO2 approaches 100%; adding more air lowers FiO2 toward about 21%.

FiO2 is determined by how much oxygen is in the gas you breathe, i.e., the mix of oxygen with air. Air is about 21% O2, while pure oxygen is 100%. If you decrease the air portion relative to oxygen, you dilute oxygen less, so the overall oxygen fraction in the delivered gas rises. In other words, lowering the air-to-oxygen ratio makes the mixture richer in O2, increasing FiO2. If you cut air flow toward zero, FiO2 approaches 100%; adding more air lowers FiO2 toward about 21%.