oxygen concentration fluctuation

[SBURNSTEVEN]

Can someone pleas help me understand this....I came across this in a CCRN book I'm using as a study guide...

For a low-flow oxygen delivery system (assuming nasal cannula); FiO2 is dependent on rate and depth of ventilation and fit of device. If minute ventilation increases, oxygen concentration decreases because the amount of room air (diluent) increases in relation to the amount of oxygen via the oxygen delivery system.

[bmxRRT]

It's all about the ratio of air to oxygen. If you set the nasal cannula to 2Lpm, it stays at 2Lpm, it's constant. If the patient increases the amount of air they inhale, that means more of the 21% fio2 room air will be mixed in with the constant 2Lpm that is going into the patient, thus diluting and reducing the total fio2 the patient receives. Think of it as watering down the fio2. The oxygen coming out of the nasal cannula is 100% oxygen, the room air is 21%. At 2Lpm the fio2 is calculated to be around 28% assuming normal resting minute ventilation.

If the patient huffs and puffs, they are pulling in more of the 21% room air thus "watering down" the end mixture with the 2Lpm coming in, equating to a lower total fio2 than 28%. This is why high flow devices exist, to be able to set a particular fio2 and ensure the patient receives that at all times regardless of minute ventilation.

-BSRRT