Chronic bronchitis can be defined as a chronic productive cough lasting more than 3 months occurring within a span of 2 years. There is a strong causal association with smoking and is very often secondary to chronic obstructive pulmonary disease (COPD). 
There are many known causes of chronic bronchitis, but the most important causative factor is exposure to cigarette smoke either due to active smoking or passive inhalation. Many inhaled irritants to the respiratory tract such as smog, industrial pollutants, and toxic chemicals can cause chronic bronchitis. Although bacterial and viral infections usually cause acute bronchitis repeated exposure to infections can cause chronic bronchitis. The predominant viruses that are causative are Influenza type A and B, and the dominant bacterial agents are Staphylococcus, Streptococcus, and Mycoplasma pneumonia. People who have an associated background in respiratory diseases such as asthma, cystic fibrosis, or bronchiectasis have a higher predisposition to develop chronic bronchitis. People who have repeated exposure to environmental pollutants such as dust or airborne chemicals such as ammonia and sulfur dioxide have a higher risk of developing chronic bronchitis. Chronic gastroesophageal reflux is a well documented but less frequent cause of chronic bronchitis. 
The occurrence of Chronic bronchitis in the general population has been documented to vary between 3% to 7% of healthy adults. However, it is estimated to be as high as 74% among those diagnosed to have COPD. Many among those in the general population experiencing symptoms of chronic bronchitis may not have a definitive respiratory diagnosis. It has been documented that subjects under the age of 50 years who are otherwise healthy and have chronic bronchitis are at a higher risk of morbidity and mortality when compared to healthy subjects.The increasing prevalence of chronic bronchitis is thought to be associated with increasing age, tobacco smoking, occupational exposure, and socioeconomic status.
Chronic bronchitis is thought to be caused by overproduction and hypersecretion of mucus by goblet cells. Epithelial cells lining the airway response to toxic, infectious stimuli by releasing inflammatory mediators such as interleukin 8, colony stimulating factor and other pro-inflammatory cytokines.There is also an associated decrease in the release of regulatory substances such as angiotensin-converting enzyme and neutral endopeptidase.The alveolar epithelium is both the target as well as the initiator of the inflammatory process in chronic bronchitis.During an acute exacerbation of chronic bronchitis, the bronchial mucous membrane becomes hyperemic and edematous with diminished bronchial mucociliary function.This, in turn, leads to airflow impediment because of luminal obstruction to small airways. The airways become clogged by debris and this further increases the irritation.The characteristic cough of bronchitis is caused by the copious secretion of mucus in chronic bronchitis.
Gross pathology reveals boggy mucosa with excessive mucinous secretion, pus, prominence of bronchial pits overlying the orifice of bronchial mucous glands.
Early changes in chronic bronchitis on microscopic histology shows hypersecretion of mucus in large airways with hypertrophy of submucosal glands in the trachea and bronchus.Later an increase in goblet cells in small airways contributes to airway obstruction due to excessive mucous. Submucosal mucous glands occupy an increasing proportion of bronchial wall. This is measured by Reid's index which is the ratio of the thickness of the mucous gland layer to the thickness of the wall between epithelium and cartilage. The standard Reid's index is 0.4. In chronic bronchitis, Reid's index is increased. Chronic bronchitis is also associated with a varying degree of dysplasia, squamous metaplasia.
The most common symptom of patients with chronic bronchitis is a cough. The history of a cough typical of chronic bronchitis is characterized to be present for most days in a month lasting for 3 months with at least 2 such episodes occurring for 2 years in a row. A productive cough with sputum is present in about 50% of patients. The sputum color may vary from clear, yellow, green or at times blood tinged. The color of sputum may be dependent on the presence of secondary bacterial infection. Very often changes in sputum color can be due to peroxidase released by leucocytes in the sputum. Therefore, color alone is not a definite indication of bacterial infection.
It is of prime importance to elicit a complete history from the patient including information regarding possible exposure to inhaled irritants or chemicals as well as full details regarding smoking habits. Fever is uncommon in chronic bronchitis and when present can be suggestive of associated influenza or pneumonia. Generalised malaise is a commonly associated symptom. Rarely patients may complain of chest pain or abdominal muscle pain caused by continuous forceful coughing. When there is inflammation of the airway, there can be an associated wheeze.
Uncomplicated chronic bronchitis presents with a cough, and there is no evidence of airway obstruction physiologically. When patients have chronic asthmatic bronchitis, there is usually a wheeze present due to a hyperactive airway leading to intermittent bronchospasm. When there is obstructive bronchitis which is the more severe end of the spectrum of the disease, there is small airway disease which at times results in emphysema.
The most critical factor in the diagnosis of chronic bronchitis is a typical history to exclude other possible diseases of the lower respiratory tract.
The investigations which assist in confirming the diagnosis of chronic bronchitis are a complete blood count with differential. A serum procalcitonin level can help in distinguishing associated bacterial from non-bacterial infection. A chest x-ray in the elderly and when physical findings suggest pneumonia is important. A culture of the sputum when a bacterial infection is suspected is indicated. The additional investigations which are a helpful measurement of oxygen saturation, and pulmonary function test.
The primary aim of treatment for chronic bronchitis is to relieve symptoms, prevent complication and slow the progression of the disease. The primary goals of therapy are aimed at reducing the overproduction of mucus, controlling inflammation and lowering cough. These are achieved by pharmacological as well as nonpharmacological interventions.
The mainstay of pharmacological interventions are the following:
Non Pharmacological Measures
The most critical nonpharmacological intervention is smoking cessation. Smoking cessation improves mucociliary function and decreases goblet cell hyperplasia. Smoking cessation has also been shown to reduce airway injury resulting in lower levels of exfoliated mucus in tracheobronchial cells.
Pulmonary rehabilitation is an important part of treatment for chronic bronchitis is pulmonary rehabilitation which consists of education, lifestyle modification, regular physical activity and avoidance of exposure to known pollutants either at work or living environment.
The presence of chronic bronchitis is known to cause of worsening airflow obstruction and a deterioration in lung function. Large epidemiological studies have shown that chronic mucus hypersecretion was associated with a decline in FEV1. Patients with chronic bronchitis symptoms had a threefold increased risk of developing new COPD compared to the asymptomatic population.Chronic bronchitis increases mortality. Those suffering from chronic bronchitis were found to have higher levels of serum IL8 and C reactive protein which indicates that systemic inflammatory response could increase the risk of cardiac disease and other comorbidities. Chronic Bronchitis also leads to a lessened quality of life.
Chronic bronchitis has a significant impact on morbidity and quality of life. Smoking is a major contributor hence smoking cessation needs to form an important strategy in prevention. Alteration in lifestyle and regular vaccination for influenza as well as pneumonia can reduce the morbidity in those who have chronic bronchitis.