Chronic bronchitis causes
increased airflow resistance in large airways.
Chronic bronchitis is a
functional disorder, defined clinically as 'cough productive of sputum on most
days for 3 months of the year
for at least 2 successive years'.
Airways obstruction in
chronic bronchitis is due to luminal narrowing and mucus plugging.
This leads to
alveolar hypoventilation, hypoxaemia and hypercapnia (type II respiratory
failure).
Individuals with chronic bronchitis and type II respiratory failure
are typically cyanosed, but do not usually have distressing dyspnoea.
Hypoxic
pulmonary vasoconstriction may cause secondary pulmonary hypertension and, with
time, lead to right-heart failure (cor pulmonale).
In chronic bronchitis the
main abnormality is secretion of abnormal amounts of mucus, causing plugging of
the airway lumen.
Hypersecretion is associated with hypertrophy and hyperplasia
of bronchial mucus-secreting glands.
This can be appreciated by the Reid index,
the ratio of gland wall thickness in the bronchus, which is increased in cases
of chronic bronchitis.
Inflammation in chronic bronchitis is typically not present, although
individuals with excessive mucus production frequently
develop coincidental respiratory tract infections, leading to secondary
inflammation.
Squamous metaplasia is common in patients who have persistent or recurrent
superimposed infections.
