Chronic obstructive pulmonary disease or COPD is a progressive, often irreversible condition of the distal airways (bronchi and bronchioles) and lung tissue. The two main conditions that make up COPD are chronic bronchitis and emphysema and since most patients have both conditions concurrently, COPD is a more concise description.
As the name suggests, the condition is chronic (long term) and while most cases are irreversible, there may be a reversible component in a minority of COPD patients. There are other obstructive pulmonary diseases, like asthma, but since the bronchospasm in this condition is reversible, it does not fall under COPD. Despite this differentiation, the clinical presentation of asthma, chronic bronchitis and emphysema may overlap to a large degree. COPD is the main type of lung disease seen in cigarette smokers, whereas asthma may affect any person of any age with underlying hypersensitivity.
COPD is characterized by breathlessness that is more pronounced in the early stages of emphysema but just as significant in the late stages of chronic bronchitis. Poor oxygenation of the lung with a concurrent accumulation of carbon dioxide in the blood stream is present in both conditions, although the emphysema patient may be well oxygenated in the early stages. Small airway disease, which is a type of chronic bronchiolitis, is present in both emphysema and chronic bronchitis, further contributing to narrowing of the airways and restricted airflow.
Definitions in Chronic Obstructive Pulmonary Disease
Although the common term smoker’s lung is used to describe both conditions due to the similarities, there are distinct differences in both chronic bronchitis.
Chronic bronchitis is the persistent inflammation of the bronchi and bronchioles resulting in a cough and sputum production that persists for at least 3 months in two consecutive years.
Emphysema is an abnormal enlargement of the air spaces with destruction of the alveoli distal to the terminal bronchioles that is irreversible.
Although asthma is not part of COPD, it is worth clearly defining it to differentiate between these conditions. Asthma is a reversible bronchoconstriction as a result of hypersensitivity triggered by exposure to various allergens.
Causes of COPD
The most common contributing factor in the development of COPD is tobacco smoking. While it is known to cause COPD, not every smoker will develop these diseases. Other causes includes exposure to industrial dusts like in cotton factories and frequent exposure to gases like sulfur dioxide. Second hand smoke (passive smoking) is an equally significant cause of COPD. Less commonly, a congenital deficiency of alpha-1 antitrypsin (AAT), may contribute to COPD as discussed below.
In chronic bronchitis, tobacco smoke irritates the lining of the airways and inflames the bronchi and bronchioles. Excess mucus production in the bronchi is a result of the irritation and further exacerbated by enlargement of the submucosal glands and goblet cells which produce mucus. Narrowing of the airways as a result of smooth muscle hypertrophy and mucus accumulation restrict the inflow of air.
With emphysema, the structural changes to the airways also cause it to collapse during expiration and therefore ‘trap’ air within the lung. The overstretching of the air sacs of the lungs, coupled with the proteases secreted by the neutrophils/macrophages in the lung tissue in response to free radicals in cigarette smoke damages the alveoli. AAT (alpha-1 antitrypsin) usually blocks the action of these proteases but in COPD, there may be a congenital deficiency or functional deficiency of AAT. The elastic recoil of the lung is lost over time further compounding hyperinflation of the lungs.
Both chronic bronchitis and emphysema are often present simultaneously although one condition may be predominant. Therefore the terms chronic bronchitis-predominant COPD and emphseyma-predominant COPD better describe the individual presentation of COPD.