Epidemiology and microbiology of hospital-acquired pneumonia

Hospital-acquired pneumonia (HAP) is the second most common nosocomial infection in the critically ill patient and is associated with the greatest mortality and increased morbidity and cost of care. The major risk factor for the development of HAP in intensive care is the occurrence of intubation and mechanical ventilation, giving rise to the term ventilator-associated pneumonia (VAP). Incidence of VAP varies in different populations of critically ill patients and generally ranges from 9 to 20%, with an overall rate of 10 to 15 cases per 1,000 ventilator days. The cumulative risk of developing VAP is similar to1% per day of mechanical ventilation (MV). The crude mortality rate of VAP is 60% and the estimates of attributable risk range from 27 to 43%. Mortality from VAP is influenced by host factors, the virulence of the pathogens, and the adequacy of initial antimicrobial therapy. The etiologic agents for VAP differ according to the population studied, duration of hospital stay, time after intubation, and prior antimicrobial therapy. Risk factors include nonmodifiable factors like age, chronic obstructive pulmonary disease, severe head trauma, and multiple trauma, and modifiable factors like large volume gastric aspiration, duration of MV, elevated gastric pH, histamine type 2 blocker therapy, ventilator circuit change frequency, self-extubation, and reintubation. The impact that diagnosis using invasive diagnostic techniques may have on the epidemiological characteristics of VAP are unknown, but may potentially reduce problems resulting from misclassification of this entity.