The blood contains carbon dioxide, a "waste product" of cellular metabolism that is exhaled by the lungs at the same time that oxygen is taken in. The normal concentration (partial pressure) of carbon dioxide in the blood is 40 mm Hg. Hypercapnia is when
the carbon dioxide level goes above 45 mm Hg. (though it
usually needs to be higher than this for symptoms to occur).
Hypercapnia commonly occurs in severe Emphysema. Oxygen given to patients with Emphysema may reduce their ability to breathe, resulting in Hypercapnia. This is why exact doses of oxygen are usually figured out for those with Emphysema so that they receive neither too little nor too much oxygen. Note: if a patient is actively short of breath, never withhold oxygen from him.