Pneumonia
An acute infection of the lung parenchyma that often impairs gas exchange, pneumonia can be classified in several ways. Based on microbiological etiology, it may be viral, bacterial, fungal, protozoal, mycobacterial, mycoplasmal, or rickettsial in origin.
Based on location, pneumonia may be classified as bronchopneumonia, lobular pneumonia, or lobar pneumonia. Bronchopneumonia involves distal airways and alveoli; lobular pneumonia, part of a lobe; and lobar pneumonia, an entire lobe.
Infection is also classified as one of three types—primary, secondary, or aspiration pneumonia. Primary pneumonia results directly from inhalation or aspiration of a pathogen, such as bacteria or a virus; it includes pneumococcal and viral pneumonia. Secondary pneumonia may follow initial lung damage from a noxious chemical or other insult (superinfection) or may result from hematogenous spread of bacteria from a distant area. Aspiration pneumonia results from inhalation of foreign matter, such as vomitus or food particles, into the bronchi. It’s more likely to occur in elderly or debilitated patients, those receiving nasogastric (NG) tube feedings, and those with an impaired gag reflex, impaired swallowing, poor oral hygiene, or a decreased level of consciousness. (See Understanding types of pneumonia.)
Pneumonia occurs in both sexes and at all ages. More than 3 million cases of pneumonia occur annually in the United States. The infection carries a good prognosis for patients with normal lungs and adequate immune systems. In debilitated patients, however, bacterial pneumonia ranks as the leading cause of death. Pneumonia is also the leading cause of death from infectious disease in the United States.
Causes
In bacterial pneumonia, which can occur in any part of the lungs, an infection initially triggers alveolar inflammation and edema. Capillaries become engorged with blood, causing stasis. As the alveolocapillary membrane breaks down, alveoli fill with blood and exudate, resulting in atelectasis. In severe bacterial infections, the lungs assume a heavy, liverlike appearance, as in acute respiratory distress syndrome (ARDS.)
Viral infection, which typically causes diffuse pneumonia, first attacks bronchiolar epithelial cells, causing interstitial inflammation and desquamation. It then spreads to the alveoli, which fill with blood and fluid. In advanced infection, a hyaline membrane may form. As with bacterial infection, severe viral infection may clinically resemble ARDS.
Understanding types of pneumonia
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