Emphysema is a lung disease with damaged alveoli. Alveoli is the air sac, the inner walls of the air sacs weaken and rupture, when you exhale, the air sacs can't work properly, then you feel out of breath.
Most people with emphysema also have chronic bronchitis. The two add up to COPD.
Emphysema has 4 stages:
FEV 1 Staging for Emphysema
Stage I = mild: Greater than or equal to 80 percent predicted
Stage II = moderate: Less than 80 percent, greater than 50 percent predicted
Stage III = severe: Less than 50 percent, greater than 30 percent predicted
Stage IV = very severe: Less than 30 percent predicted, or less than 50 percent in chronic respiratory failure
Stage 3 and 4 are the advaned stage.