NEW APPROACHES IN THE DIAGNOSIS OF NOSOCOMIAL PNEUMONIA

Pneumonia, the nosocomial infection associated with the highest mortality rate, is estimated to occur in 300,000 patients per year.(51) The treatment of nosocomial pneumonia is remarkably simplified when the responsible pathogen is accurately defined, allowing administration of the most effective, narrowest-spectrum antibiotic therapy. Studies have shown that appropriate antimicrobial treatment of nosocomial pneumonia significantly improves outcome.(23, 121) Bacterial pneumonia is the inflammatory response of the host to the uncontrolled multiplication of organisms invading the distal airways. It is characterized histologically by accumulation of neutrophils in the distal bronchioles and alveoli, leading to fibrinosuppurative consolidation. Sputum, which represents the mucopurulent exudate of this host-parasite interaction, provides useful information on the presence and cause of pneumonia.(4) Microscopic and culture analysis of the expectorated sputum, however, has proven limitations. The oropharynx is normally colonized by a complex microbial flora containing organisms at high concentrations (10(6) CFU/mL),(9) which easily contaminate secretions expectorated from the lower respiratory tract. Contamination is particularly troublesome in hospitalized patients, who frequently have the upper airway or endotracheal tube colonized with potential pathogens, making it difficult to differentiate infecting from colonizing bacteria. Diagnostic methods have been developed for retrieving secretions from the lower respiratory tract that avoid contamination by microorganisms colonizing the oropharynx, trachea, and endotracheal tube. Two invasive procedures, transtracheal and transthoracic needle aspiration, have been extensively investigated but have fallen into disfavor because of the risk for serious complications and the evolution of bronchoscopy-based techniques. This article presents the current state-of-the-art relative to new approaches for the diagnosis of nosocomial pneumonia. We examine both invasive and noninvasive diagnostic techniques, discuss microbiologic and microscopic analyses of lower airway secretions, and consider factors influencing the results of these tests. Finally, we analyze the use of these diagnostic techniques in the management of patients with respiratory failure suspected of having nosocomial pneumonia. Preliminarily, it is important to review the medical background motivating the growth of clinical investigation in this field. Most ongoing research involves pneumonia developing in patients on mechanical ventilation, or ventilator-associated pneumonia (VAP).