If you are providing 2 liters per minute of oxygen to a patient what percentage of oxygen are they breathing?
Human lungs are 2 liters in volume by average. So...
Average breath rate is 12 breaths per minute.
In one minute the patient would take in 24 liters of air. You
are supplying 2 liters of pure oxygen + 4.8 liters from air (20% of
the 24 liters (Percent oxygen in dry air)) = 6.8 liters of Oxygen
per minute.
6.8 liters oxygen / 24 liters intake = 28.3% Oxygen
or you can look at the table given by the books at
http://www.fpnotebook.com/ER/Procedure/LwFlwOxygn.htm
Nasal Cannula, Flow 0 liters per minute: 21% (Room Air), Flow 2
liters per minute: 29%
But I like the mathematical method. That's just me!