Epidemiology

Pneumonia is a common infection in the community. The estimated incidence in developed countries has been reported to be between two and four cases per 1000 adult population. The rate of admission to hospital for community-acquired pneumonia appears to be more variable, with figures ranging from 22 to near 50 per cent. The need for admission to an intensive care unit (ICU) has been estimated to be between 3 and 5 per cent, although some studies report higher percentages. In general, elderly patients or those suffering chronic debilitating diseases are more prone to require hospital and, eventually, ICU admission. However, approximately one-third of patients admitted to hospital and the ICU are previously healthy individuals.

Community-acquired pneumonia is a relevant cause of mortality. Although the incidence of fatal outcome decreased dramatically after the introduction of antibiotics, the mortality in developed countries has remained essentially stable during the last 40 years, with a reported incidence of approximately 30 deaths per 100 000 population. Mortality is directly related to the severity of the episode. The mortality in cases treated in an ambulatory setting has been evaluated to be below 1 per cent, whereas in those cases needing hospital admission it is between 11.5 and 25 per cent, and several studies report a mortality of almost 50 per cent in ICU-admitted cases.

Various demographic and clinical factors have been related to the probability of a fatal outcome. Age has been shown to be independently associated with mortality, although its relative importance appears to be lower than other prognostic factors. Male gender may also be related to death probability, but to a low degree. Chronic debilitating diseases such as diabetes mellitus, chronic obstructive pulmonary disease, alcoholism, hepatic cirrhosis, chronic heart failure, renal insufficiency, neuralgic or mental disease, malignancy, and immunosuppression have been shown to be clearly related to mortality. As shown in TabJeJ.., different clinical features at presentation, and also some laboratory and radiographic findings, have been associated with pneumonia outcome in univariate and multivariate studies. Finally, some etiologies, particularly Staphylococcus aureus, Gram-negative enteric rods, and Legionella pneumophila, present a higher probability of a fatal outcome when compared with other common etiological agents such as Streptococcus pneumoniae, Mycoplasma pneumoniae, or Hemophilus influenzae.

Table 1 Clinical features associated with unfavorable outcome of community-acquired pneumonia

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