Severe community-acquired pneumonia: Determinants of severity and approach to therapy

The severity of community-acquired pneumonia (CAP) is primarily a function-of impaired host defenses and/or decreased cardiopulmonary function. Humoral immunity is the primary defense mechanism that protects against and modulates the response to infection caused by the usual bacterial pulmonary pathogens, Streptococcus pneumoniae, Haemophilus influenzae, and Moraxella catarrhalis. Intracellular immunity protects against intracellular atypical pathogens, such as Legionella. The selection of appropriate empiric therapy for severe CAP should include coverage against both typical and atypical pathogens and should be effective against strains, of penicillin-resistant S pneumoniae, ampicillin-resistant H. influenzae, and beta-lactamase-producing strains of M catarrhalis. The therapy for severe and nonsevere CAP is the same, but severe disease requires a greater duration of therapy Antimicrobial coverage is pathogen-directed, and if it covers both typical and atypical pathogens, there is no basis for adding or changing antibiotics.