Chronic bronchitis may be defined as a disease characterised by cough and sputum for at least 3 consecutive months in a year for more than 2 successive years (Medical Research Council).
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 whieh 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 pneymonae 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.
Simple chronic bronchitis
Here sputum is mucoid.
Chronic recurrent mucopurulent bronchitis
There is mucopurulent sputum present in absence of localized suppurative disease.
Chronic obstructive bronchitis
Airway obstruction is dominant.
Chronic asthmatic bronchitis
There is long continued cough and sputum with late onset of wheeze.
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.
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 inereased.
Increased mucus predisposes to infection by various organisms, e.g., viruses and bacteria. The main bacteria are H. influenza and Strep. pneumoniae. This leads to severe inflammation of the bronchial tree resulting in mucopurulent sputum, further airway obstruction and constitutional reaction. H. influenza may persist in the sputum and may cause fibrosis and scarring of the distant alveoli or at times emphysema.
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 FEV1 are diminished and the FEV /FVC ratio falls below 75 per cent. However, this does not correlate well.
In about 50% cases of chronic bronchitis, emphysema develops. This is due to repeated infection and air trapping. Centriacinar or panacinar emphysema may develop.
Due to uneven distribution of the inspired air, there may be diminished diffusing capacity. Airway obstruction gives rise to ventilation perfusion inequality-resulting in increased PaCO2 and reduction in PaO2. With severe ventilatory failure there is falling pH together with compensatory decrease in plasma bicarbonate and respiratory acidosis.
Pulmonary hypertension and Chronic cor pulmonale
As a result of low PaO2 pulmonary vasoconstriction takes place leading to pulmonary hypertension. There are also other undetermined factors. It occurs mostly during infection. Ultimately patient develops right ventricular failure.
Onset is indisidious, usually in the fifth or sixth decade of life.
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