1 Define severe community-acquired pneumonia (CAP)

Patients with severe CAP have a number of characteristics:

Unfortunately, it is challenging to prospectively identify this cohort of patients. Of particular concern are patients who are initially triaged as having nonsevere CAP but subsequently need ICU admission (up to 50% of ICU admissions fall under this category in some studies). Such patients tend to have a higher mortality than equally sick patients who have been directly admitted to an ICU. A number of severity of illness scores have been developed to help define severe CAP, a popular one being derived from the joint Infectious Diseases Society of America–American Thoracic Society guidelines for the management of CAP in adults (Box 20-1), which incorporates elements of the confusion, urea, respiratory rate, and blood pressure (CURB) score. By this definition, patients with one major criterion or three minor criteria are designated as having severe CAP. Another widely used score is the Pneumonia Severity Index (PSI). However, none of these scores has been prospectively validated for individual patients. Clinical judgment remains critical; do not blindly follow scores! In recent years other approaches have been explored to identify patients with severe CAP; some of these are discussed below (see answer 9 on recent developments in CAP).

2 Which pathogens most commonly cause severe CAP?

The most common causes of severe CAP in ICU patients are (in order of decreasing incidence):

Pathogens are identified in fewer than 50% of cases. Specific therapy seems to have no particular advantage over empiric therapy except in ICU patients, where every effort should be made to reach an etiologic diagnosis and tailor therapy accordingly. A typical work-up for an ICU patient would include sputum Gram stain in addition to sputum and blood cultures. In addition urinary antigen tests for Legionella and pneumococcus should be considered. More invasive tests, such as bronchoscopic bronchoalveolar lavage, may be considered in individual patients.

3 How is CAP diagnosed? Are the sputum Gram stain and culture diagnostically helpful for CAP?

CAP is diagnosed on the basis of the presence of a constellation of signs and symptoms (fever, cough, sputum production, and pleuritic chest pain) with radiographic evidence of lung infiltrates. Sputum Gram stain and culture can be obtained noninvasively and are inexpensive diagnostic tests. A sputum Gram stain specimen is considered satisfactory for interpretation when the neutrophil count is ≥ 25 and the epithelial cell count is ≤ 10 per low power field. Gram staining can have multiple benefits: the results can be used to broaden coverage to cover microorganisms that are typically not covered by empiric regimens, such as S. aureus and GNRs. Conversely, the absence of characteristic Gram stains and sputum culture is a strong argument for presumptively excluding S. aureus and GNRs as probable etiologies for the pneumonia. In addition, a positive Gram stain validates a subsequent sputum culture. Keep in mind the diagnostic limitations of sputum Gram stain and culture, including the inability to visualize atypical organisms, contamination by oral flora, and the difficulty encountered by some patients to provide adequate specimens.

4 What determines the selection of empiric antimicrobial therapy for patients with severe CAP?

The initial empiric antibiotic regimen for patients in the ICU with severe CAP is outlined in Box 20-2. Broadly speaking, the general principles of antibiotic therapy are as follows:

5 What risk factors would prompt broader coverage?

Risk factors that would prompt broader antimicrobial coverage can be conveniently considered by the type of organism to be covered:

6 When should antibiotics be initiated, and what is the optimal duration of treatment?

Although earlier guidelines recommended initiation of antibiotics within 4 hours of diagnosis of CAP, more recent evidence has prompted a subtle shift in the current guidelines. They still recommend prompt initiation of antibiotics but do not specify a window of greatest benefit. The guidelines do recommend that patients admitted through the emergency department (ED) should receive their first dose of antibiotics while in the ED.

Patients with CAP should be treated for a minimum of 5 days, should be afebrile for 48 to 72 hours, and should not have more than one CAP-associated sign of clinical instability (Box 20-3) before stopping treatment.