Chronic bronchitis may be defined as a disease characterized by cough and sputum for at least 3 consecutive months in a year for more than 2 successive years. in this condition there is chronic obstruction to the alveolar inflow of air either due to chronic bronchitis or bronchial asthma.
Smoking: Smoking causes bronchoconstriction, sluggish ciliary movement, increases airway resistance, hypertrophy of the mucous glands, increased number of goblet cells and hypersecretion of mucus. Though smoking is said to be the most important cause, yet only 10%-15% of the smokers develop COPD. Usually a cigarette smoking history of greater than 20 pack Years is associated with the disease. (1 pack year is equivalent to smoking 20 cigarettes a day for 1 year.)
Atmospheric pollution: Industrial and domestic smoke as well as sulphur dioxide are main causes of air pollution which are responsible for chronic bronchial irritation and increased resistance to the airflow.
Infection: The role of infection is uncertain, but it appears that once it develops chronic irritation is maintained and progresses to emphysema. The main organisms are Haemophilus influenzae and Streptococcus pneumoniae. Mycoplasma pneumonae may also be involved.
Occupation: Coal-miners and industrial workers are often exposed to dust and fumes which may irritate the bronchial tree.
Familial and genetic abnormalities associated with Alpha-antiprotease deficiency may also be present.
Types of chronic bronchitis
• Simple chronic bronchitis. Here sputum is mucoid.
• Chronic recurrent mucopurulent bronchitis. There is mucopurulent sputum present in absence of localised suppurative disease.
• Chronic obstructive bronchitis. Airway obstruction is dominant.
• Chronic asthmatic bronchitis. There is long continued cough and sputum with late onset of wheeze.
Pathogenesis, Pathology and Functional abnormality Mucous glands Due to chronic irritation, mucous glands undergo hypertrophy which is the main pathological finding in chronic bronchitis. The ratio between the thickness of gland and thickness of bronchial wall is called Reid Index. This is normally 0.26 and in chronic bronchitis it becomes 0.59. This index is the diagnostic criterion of chronic bronchitis. Goblet cells In the bronchioles Goblet cells proliferate and are overdistended with mucus.
Mucus secretion is enormously increased due to hypertrophy of mucous glands and proliferation of Goblet cells. This is the cause of chronic cough and sputum. Secretion of mucous glands mainly contributes to the sputum volume, while that of Goblet cell is responsible for airway obstruction. Thus there are wheeze, rhonchi and breathlessness. This mucus is chemically altered as its fucose and sialic acid concentration is increased.
Increased mucus predisposes to infection by various organisms, e.g., viruses and bacteria. The main bacteria are H. influenzae and Strep. pneumoniae. This leads to severe inflammation of the bronchial tree resulting in mucopurulent sputum, further airway obstruction and constitutional reaction. H. influenzae may persist in the sputum and may cause fibrosis and scarring of the distant alveoli or at times emphysema. Airway obstruction This is the most important functional abnormality and is caused by numbers of factors, e.g., overproduction of mucus, inflammatory swelling and oedema, spasm of smooth muscle, fibrosis, air trapping at bronchioles and emphysema. In the earlier part of the disease intermittent and later on permanent obstruction develops. With severe airway obstruction PEF and FEVI are diminished and the FEVI/FVC ratio falls below -5 per cent. However, this does not correlate well.