Usually, severe COPD often refers to the last stage of COPD. This is usually where the patient becomes severely short of breath, or dysneic, with even minimal exertion. Usually it starts out as dyspnea with walking, and then it becomes dyspnea (feeling of breathlessness) with doing simple things like brushing your teeth or shaving.
The reason the patient gets winded so easily is because the lungs are so damaged that even minimal exertion causes them to become obstructed even further.
Likewise, the heart is starting to show signs that it is tiring from working so hard to push oxygenated blood through severely obstructed air passages (bronchioles). As the heart becomes a weaker pump, tissues of your heart and body get less oxygen, which is what causes the feeling of dyspnea.
You have to picture the heart as having two chambers. The left ventricle pumps blood through the body, and the right ventricle pumps blood through the lungs. When the lungs become diseased, as with COPD, the vessels in the lungs become rigid or stiff due to lack of oxygen, and this causes the blood pressure in the lungs to increase. This is called pulmonary hypertension.
When this occurs, the right ventricle is forced to work extra hard to pump blood through the lungs. Since the heart is a muscle, when it works extra hard it becomes enlarged. When the right heart becomes enlarged secondary to pulmonary hypertension secondary to lung disease such as COPD, this is referred to as cor pulmonale.
Eventually the right heart will fail. When the right heart fails, blood will back into the right atria, and a cascade of events will eventually lead to left ventricular failure as well. When the left heart fails, this often leads to excess fluid in the lungs that makes breathing even more difficult. Fluid is also backed up in other parts of the body, such as the ankles.
Of course heart failure can be treated by medicines that increase the contractility of the heart, such as digoxin. Likewise, diuretics can be used to remove fluid from the ankles and, especially, the lungs. And, of course, there are other therapies too.
You can still function with severe COPD, however you will need to make major lifestyle changes. I'll discuss this next week.