Pseudomonas lung infections occur in patients with chronic lung disease or impaired immunity, usually in association with nosocomial factors such as endotracheal intubation, respiratory therapy, prolonged hospitalization, antibiotic use, and neutropenia. Pneumonia takes two forms: primary and bacteremic. Primary pneumonia arises in predisposed patients following nosocomial colonization and aspiration of P aeruginosa. Pneumonia is characterized by fever, tachypnea, cough with purulent sputum, shortness of breath, cyanosis, and often signs of sepsis.
The diagnosis of pneumonia caused by P aeruginosa is established by the chest x-ray findings of bilateral bronchopneumonia often with radiolucencies resembling Staphylococcus aureus pneumonia and recovery of P aeruginosa from pulmonary secretions or blood culture.
P aeruginosa frequently colonizes hospitalized patients, especially those with chronic pulmonary disease or those with endotracheal intubation and mechanical ventilation.
Colonization, especially in ventilated patients, is often difficult to differentiate from infection, and the diagnosis of infection requires the presence of signs and symptoms of infection together with recovery of P aeruginosa from expectorated sputum, blood cultures, bronchoalveolar lavage, or a protected brushing sample obtained at bronchoscopy.
P aeruginosa is particularly common in patients with ventilator-associated pneumonia. One study found it caused 27% of cases of ventilator-associated pneumonia and identified risk factors such as chronic obstructive pulmonary disease, mechanical ventilation for more than 8 days, and prior antibiotic use. In the rare instances in which community acquired pneumonia is found, it occurs primarily in patients with chronic obstructive airways disease and history of prior antibiotic use.
Bacteremic pneumonia is caused by septic embolization of the lung. Neutropenia complicating chemotherapy, underlying hematologic malignancy, or AIDS are the usual settings for this type of pneumonia. This is usually a rapidly fatal disease. Pathologically, two characteristic pulmonary lesions are encountered. One type is hemorrhagic nodules, which are primarily subpleural and surround pulmonary vessels without inflammatory infiltrates. The second type is umbilicated nodules with liquifactive necrosis and leukocyte infiltration or more frequently with coagulative necrosis. These lesions are the pulmonary form of cutaneous ecthyma gangrenosum. The radiologic appearance of bacteremic pneumonia evolves over 1 to 3 days and is initially manifested by pulmonary congestion and edema with subsequent alveolar infiltrates, pulmonary hemorrhage, and finally cavitation.