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Respiratory tract infections are a major cause of morbidity and mortality in the United States and represent a common reason for which patients seek medical care. The most common infections involving the lower respiratory tract are bronchitis, bronchiolitis, and pneumonia. Lower respiratory tract infections (LRTIs) in children and adults commonly result from viral or bacterial invasion of lung parenchyma. Viral infections are diagnosed by the recognition of characteristic constellation of clinical signs and symptoms. Treatment of viral LRTI consists largely of supportive care measures. Bacterial LRTIs (in particular bacterial pneumonia) requires expedient, effective, and specific antimicrobial therapy. An appropriate treatment regimen for the patient with a LRTI can be established with the aid of a thorough medical history, physical examination, chest radiograph, knowledge of common bacterial pathogens, and results of properly collected cultures. This chapter will focus on pneumonia; see the special considerations section of this chapter for information on bronchitis and bronchiolitis.
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Pneumonia is inflammation of the lung with consolidation and is the most common infectious cause of death in the United States.1 Approximately three million cases are diagnosed annually at a cost of more than $20 billion to the health care system. Pneumonia occurs throughout the year and in persons of all ages; however the most severe pneumonia cases are in the very young, the elderly, and the chronically ill. The main classifications of pneumonia are: community-acquired, hospital-acquired (including aspiration), and health care-associated (the term health care-associated pneumonia, as used in this chapter, includes hospital-acquired pneumonia [HAP], ventilator-associated pneumonia [VAP] and other health care-associated pneumonia [HCAP]).
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The etiology of bacterial pneumonia varies in accordance with the type of pneumonia. Table 21-1 lists common bacterial pathogens associated with the various classifications of pneumonia.2-4 Viruses are a common cause of community-acquired pneumonia in adults (often coinfecting with bacteria), and in children (65%). Viral pneumonia in children is frequently caused by respiratory syncytial, influenza or parainfluenza virus.
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Microorganisms gain access to the lower respiratory tract by three routes.5 Organisms may enter the lung via:
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