OVERVIEW OF COMMUNITY-ACQUIRED PNEUMONIA - PROGNOSIS AND CLINICAL-FEATURES

Pneumonia is the sixth leading cause of death and the number one cause of infectious death in the United States.(42) Despite its significance, pneumonia is not a reportable disease, and most information comes from vital statistics(42) or clinical studies, which are usually university hospital based.(1, 3, 5, 6, 26, 32, 33, 41, 45) From these sources, the annual incidence of community-acquired pneumonia (CAP) in the United States is estimated to range from 2 to 4 million cases, with as many as 20% of cases requiring hospitalization.(11) The majority of CAP is treated in the outpatient setting, and mortality is low (<5%). CAP requiring hospitalization is associated with increased mortality, often approaching 25%, and accounts for a substantial amount of hospital resources. The mortality rate associated with pneumonia is highest among older individuals (>65 years) and appears to be increasing after remaining steady during the 1950s and 1960s and declining during the 1970s (Fig. 1). For the most recently reported 8-year period (1982-1990), the overall incidence of pneumonia mortality has risen from 10.9 cases to 14.0 cases per 100,000, and this increase has been especially steep among older patients. Reasons for this rise in pneumonia-related deaths are not clear, but it has occurred despite the introduction of newer broad-spectrum antimicrobial agents and vaccines and more expensive supportive care. The incidence and severity of CAP are influenced by both the ability of the patient to defend against infection and the virulence of the infecting agent. A number of studies have identified specific host factors that are associated with CAP, including age, the presence of coexisting illness, and the presence of immunosuppression. The severity of CAP at presentation is also modified by the virulence of the pathogen. Together, these factors directly affect the clinical manifestations of infection and the severity of illness, which influence decisions regarding hospitalization and the need for intensive care. Although the mortality and morbidity of CAP remain significant, both the clinical course and outcome can be improved by the rapid and appropriate institution of antimicrobial therapy. This, in turn, depends on prompt diagnosis, which is not always a simple process. Use of clinical criteria alone lacks both sensitivity and specificity, and no diagnostic test or even battery of tests is able to detectevery potential pathogen. Most patients are begun on empiric therapy pending results of diagnostic tests, and the regimen initially selected is frequently continued if no pathogen is identified. Such an approach is justifiable because of the high attributable mortality associated with untreated pneumonia. This article reviews the limitations of present strategies used in the workup of CAP and develops an empiric approach to the management of CAP based on prognostic and clinical features.(14, 25) In developing this approach, an attempt is made to address common questions in the initial management of CAP. These questions include: Which patients should be hospitalized? Which diagnostic tests should be performed at presentation? What are the most likely pathogens in specific settings? What approaches should be considered in patients who are not improving on therapy? Guidelines for specific empiric regimens, based on the concepts presented here, have been published and are not repeated here.(14, 25) It is beyond the scope of this article to deal with all aspects of CAP in all possible patient populations. For this reason, this article deals predominantly with the initial presentation of adult patients with CAP who are not recognized as having severe immunosuppression (e.g., acquired immunodeficiency syndrome [AIDS]).