A vicious circle hypothesis operating during infective exacerbations of chronic bronchitis.

Respiratory tract infections are common. This is partly explained by the lung's function as an organ of gas exchange, which means that it is constantly exposed to inhaled matter, including potential pathogens. The bacteria that cause bronchial infections are relatively nonvirulent, and often form part of the normal upper respiratory tract flora. Impaired lung defences are usually required for these bacteria to colonize the respiratory mucosa, but they then produce factors which facilitate their persistence and contiguous spread through the bronchial tree. This stimulates a host inflammatory response, which becomes chronic if it fails to eliminate the infection. Proteinase enzymes and oxidants spilled by neutrophils, cause mucosal damage, which further compromises the host defences. This makes repeated or chronic infection more likely. The chronic inflammation engendered may contribute to progressive deterioration in respiratory function. Antibiotic management should aim to eradicate the infection, so removing the stimulus to further inflammation. An infection free interval allows the mucosal defences to recover. The past medical history of the patient, the severity of the exacerbation, the likely pathogens and their antibiotic sensitivity, antibiotic penetration into the respiratory mucosa and mucus, and the antibiotic side-effect profile are factors which influence choice of antibiotic, the length of the course and the dosage prescribed.