Hospital Settings

CAP is defined as an acute infection of the lung parenchyma occurring in patients residing outside of a hospital or in patients who have been living in a long- term care facility for up to 2 weeks. In the United States, the annual incidence of CAP is estimated at 12 to 18 cases per 1000 population, resulting in 0.6 million to 1.0 million hospital admissions and an estimated 40 000 to 60 000 deaths per year. Up to 80% of patients with CAP are treated in the ambulatory outpatient setting.2 Depending on age and concurrent illnesses, the mortality rate associated with CAP ranges from less than 1% to 30%. The overall economic impact of CAP in the United
States was estimated at US$8 billion in 1998, with approximately 60% (US$4.8 billion) attributed to elderly patients (older than 65 years), who are the most vulnerable. In addition, the length of hospital stay was longer for older patients, 7.8 versus 5.8 days for elderly and younger patients, respectively. In a recent study of a US claims database, Colice and others estimated the cost of CAP, including direct and indirect costs, at US$12.2 billion. Similar data are not available for Canada, but given the relative populations of the 2 countries, the number of hospital admissions, the number of deaths, and the associated costs in Canada may be 10% of the US values.

Risk Factors

As described previously, many conditions increase the risk of aspiration of oropharyngeal secretions or impair pulmonary host defences. The independent risk factors for pneumococcal infections identified in one study included dementia, seizure disorders, cigarette smoking, congestive heart failure, cerebrovascular disease, living in an institutional setting, and COPD. In a study of 4175 elderly patients, over 57% of those with pneumonia had one or more of the following factors: heart disease, lung disease, asthma, immunosuppressive therapy, or alcoholism or were living in an institution.
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Several independent risk factors for mortality, similar to those predisposing patients to CAP, have been identified, including age (greater than 65 years), immunosuppression, malignancy, congestive heart failure, diabetes mellitus, alcohol consumption, neurologic disorders, and laboratory abnormalities (such as hyponatremia, hyperglycemia, azotemia, hypoalbuminemia, hypoxemia, and abnormalities on liver function testing). The presence of dyspnea, chills, altered mental status, hypothermia or fever, tachypnea, and hypotension have been associated with a higher mortality rate. Radio­graphic findings (e.g., pleural effusion or pulmonary infiltrate) have also been associated with increased risk of mortality.

Table 1. Causes of Community-Acquired Pneumonia



Streptococcus pneumoniae


Haemophilus influenzae


Staphylococcus aureus


Gram-negative bacilli




Atypical organisms



Chlamydia pneumoniae


Mycoplasma pneumoniae







The cause of CAP is unknown in the majority of cases, and a pathogen is recovered in only 40% to 60% of cases. Factors such as alcoholism, COPD, site of care, animal exposure, travel history, aspiration, viral infections, and comorbidities may influence the causative pathogen. The most common bacteria causing CAP include Streptococcus pneumoniae, H. influenzae, Staphylococcus aureus, and atypical pathogens such as Legionella spp., Chlamydia pneumoniae, and Mycoplasma spp. (see Table 1). In the ambulatory setting, M. pneumoniae appears to be the most common pathogen. Among patients with CAP who require hospital admission, S. pneumoniae is the most common cause, followed by C. pneumoniae, H. influenzae, and Legionella pneumophila. In 2% to 5% of cases of CAP, multiple pathogens have been identified. Less common causes of CAP include S. aureus and gram-negative organisms such as Klebsiella spp. and Pseudomonas aeruginosa. Nonbacterial causes of CAP include viruses (e.g., respiratory syncytial virus, influenza virus) and fungi. Secondary bacterial infections following respiratory viral infection are relatively common. Recent guidelines have stratified the likely causes of CAP on the basis of patient age, site of acquisition (community or nursing home), site of care (ambulatory, hospital ward, or ICU), and comorbidities.
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