text: Okay...lets see if I can make this simple in explanation.  There is a chemo receptor in the medulla of the brain that regulates respiratory rate according to the amount of carbon dioxide in the blood.  A person with a more advanced emphysema has a difficult time breathing in enough oxygen and expelling enough co2 (carbon dioxide).  The problem that begins to happen is the retention of co2 and when the co2 gets high enough, the chemo receptor regulating respirations is knocked out of commission.  The only thing that keeps a person breathing is air/oxygen hunger.....keeping the oxygen in the body low enough that the oxygen hunger makes the person work to breath.  The challenge in oxygen therapy is to supply enough oxygen so a person can function, but not enough to supply the full oxygen needs of the body.  If the full needs of oxygen are supplied, the body will not work to breath and if the co2 is high enough to knock out the chemoreceptor....the person will no longer breath.  A loose way of calculating the percentage of oxygen with a person using a nasal cannula is this....and mind you, this is an aproximate.  Using a nasal cannula, figure 4% per liter...2 lpm=8%.  Add the percent of o2 therapy to 21% room air.  So, if a person is using o2 at 2 lpm=8%, added to 21% room air o2, you get aproximately 29% oxygen being used.