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Epidemiology of nosocomial pneumonia in intensive care unit patients.

Pneumonia is the most commonly reported nosocomial infection in ICU patients, occurring predominantly in patients whose lungs are ventilated, at a rate of 1% to 3% per day of mechanical ventilation. Substantially increased costs and mortality have been attributed to nosocomial pneumonia. Our understanding of the epidemiology of nosocomial pneumonia in ICU populations has been limited by the reliance of most published studies on clinical diagnostic criteria, which are nonspecific. In addition to mechanical ventilation and tracheal intubation, other suspected risk factors of importance include chronic lung disease, age, severity of illness, upper abdominal or thoracic surgery, head trauma or depressed level of consciousness, and gastric acid inhibition. Aspiration appears to be the primary mode of inoculation of microorganisms into the distal lung; however, the relative importance of different sites as reservoirs for aspiration is controversial. It is hoped that studies based on improved diagnostic techniques, such as quantitative cultures of protected brush or bronchoalveolar lavage specimens, will provide the basis for an improved understanding of the epidemiology and prevention of this important infection in critically ill patients.