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Respiratory tract infections remain the major cause of morbidity from acute illness in the United States, and most likely, they represent the single most common reason patients seek medical attention.(1) The respiratory tract has an elaborate system of host defenses, that, when functioning properly, are very effective against invasion, and removing potentially infectious substances from the lungs.
Lower respiratory tract infections generally occur only when lung defense mechanisms are impaired. Examples of impaired defenses would include dysgammaglobulinemia or decreased ciliary function caused by the chronic inflammation that accompanies cigarette smoking or continued exposure to other irritants. Defenses may be additionally compromised when a particularly virulent microorganism or large inoculum invades the lung parenchyma. The majority of lower respiratory tract infections follow a colonization of the upper respiratory tract, that, after achieving sufficiently high concentrations, gain access to the lungs through aspiration of oropharyngeal secretions. Microbes can also enter the lung through the inhalation of infected aerosolized particles, or via the blood from an extrapulmonary source; however, this occurs less frequently.
The most common lower respiratory tract infections are bronchitis, pneumonia, and bronchiolitis. Bronchitis, both acute and chronic, occurs most often in the winter months. Cold, damp climates and the presence of high concentrations of irritating substances also seem to precipitate attacks. An appropriate treatment regimen for the patient with an uncomplicated lower respiratory tract infection can usually be established by a history, physical examination, chest radiograph, and properly collected sputum cultures, interpreted knowing the most common lung pathogens and their antibiotic susceptibility patterns within one's community.(1)
Chronic bronchitis is a condition with continuous or recurrent excess mucus secretions into the bronchial tree. There is a cough that occurs most days during a period of at least three months of the year for two consecutive years in a patient where other causes of chronic cough have been excluded.(2) The excess mucus is produced as a result of continued bronchial irritation caused by one or a combination of factors. Cigarette smoking has been identified as the most prominent bronchial irritant; however, exposure to occupational dusts, fumes, environmental pollution, and bacterial (and possibly viral) infections must also be included.
As mucus-producing glands are continually stimulated, a hypertrophy and hyperplasia result. Goblet cells are generally absent from the smaller bronchi of normal subjects, yet the number and size is markedly increased in both the larger and small bronchi of the chronic bronchitis patient. Dilation of the mucus gland ducts is also observed. As a result of these changes, those with chronic bronchitis have increased mucus in their peripheral airways further compromising lung defenses. Additional morphologic changes occur in the bronchi, including increased smooth muscle, cartilage atrophy, inflammation characterized by neutrophil and lymphocytic infiltration, and loss of cilia. These bronchial changes do not contribute significantly to obstruction.(3)
Although the majority of chronic bronchitis sufferers have a positive history of cigarette smoking, as many as 10 percent have no smoking history. For these patients, other airway irritants, alone, or more likely in combination, are responsible for the pathogenesis. The influence of recurrent respiratory tract infection in childhood or young adult life to the later development of chronic bronchitis remains obscure.
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