Text 19 Community-Acquired Pneumonia Definition & Pathogenesis

Community-acquired pneumonia is diagnosed outside of the hospital or is diagnosed within 48 hours after admission to the hospital in a patient who has not been hospitalized in an acute care hospital for 2 or more days within 90 days of the infection; or has resided in a long-term care facility; or has received intravenous antimicrobial therapy, chemotherapy, or wound care within the 30 days prior to the current infection; or has attended a hospital or hemodialysis clinic.

Pulmonary defense mechanisms (cough reflex, mucociliary clearance system, immune responses) normally prevent the development of lower respiratory tract infections following aspiration of oropharyngeal secretions containing bacteria or inhalation of infected aerosols. Community-acquired pneumonia occurs when there is a defect in one or more of the normal host defense mechanisms or when a very large infectious inoculum or a highly virulent pathogen overwhelms the host.

Prospective studies have failed to identify the cause of community-acquired pneumonia in 40–60% of cases; two or more causes are identified in up to 5% of cases.

Bacteria are more commonly identified than viruses. The most common bacterial pathogen identified in most studies of community-acquired pneumonia is Spneumoniae, accounting for approximately two-thirds of bacterial isolates. Other common bacterial pathogens include H influenzae, Mycoplasma pneumoniae, Chlamydia pneumoniae, S aureus, Neisseria meningitidis, M catarrhalis, Klebsiella pneumoniae, other gram-negative rods, and Legionella species. Common viral causes of community-acquired pneumonia include influenza virus, respiratory syncytial virus, adenovirus, and parainfluenza virus. A detailed assessment of epidemiologic risk factors may aid in diagnosing pneumonias due to the following causes: Chlamydia psittaci (psittacosis), Coxiella burnetii (Q fever), Francisella tularensis (tularemia), endemic fungi (Blastomyces, Coccidioides, Histoplasma), and sin nombre virus (hantavirus pulmonary syndrome).

Clinical Findings

A. Symptoms and Signs

Most patients with community-acquired pneumonia experience an acute or subacute onset of fever; cough with or without sputum production, and dyspnea. Other common symptoms include rigors, sweats, chills, chest discomfort, pleurisy, hemoptysis, fatigue, myalgias, anorexia, headache, and abdominal pain.

Common physical findings include fever or hypothermia, tachypnea, tachycardia, and mild arterial oxygen desaturation. Many patients will appear acutely ill. Chest examination is often remarkable for altered breath sounds and rales. Dullness to percussion may be present if a parapneumonic pleural effusion is present.

The differential diagnosis of lower respiratory tract symptoms and signs is extensive and includes upper respiratory tract infections, reactive airway diseases, congestive heart failure, COP, lung cancer, pulmonary vasculitis, pulmonary thromboembolic disease, and atelectasis.

B. Laboratory Findings

Controversy surrounds the role of Gram stain and culture analysis of expectorated sputum in patients with community-acquired pneumonia. Most reports suggest that these tests have poor positive and negative predictive value in most patients. Some argue, however, that the tests should still be performed to try to identify etiologic organisms in the hope of reducing microbial resistance to drugs, unnecessary drug costs, and avoidable side effects of empiric antibiotic therapy. Expert panel guidelines suggest that sputum Gram stain should be attempted in all patients with community-acquired pneumonia and that sputum culture should be obtained for all patients who require hospitalization. Sputum should be obtained before antibiotics are initiated except in a case of suspected antibiotic failure. The specimen is obtained by deep cough and should be grossly purulent. Culture should be performed only if the specimen meets strict cytologic criteria, eg, more than

25 neutrophils and fewer than 10 squamous epithelial cells per low power field. These criteria do not apply to cultures of legionella or mycobacteria.

Additional testing is generally recommended for patients who require hospitalization: preantibiotic blood cultures (at least two sets with needle sticks at separate sites), arterial blood gases, complete blood count with differential, and a chemistry panel (including serum glucose, electrolytes, urea nitrogen, creatinine, bilirubin, and liver enzymes). The results of these tests help assess the severity of the disease and guide evaluation and therapy. HIV serology should be obtained from all hospitalized patients.