Pneumonia, an infection of the lung parenchyma, is classified as communityacquired (CAP) or health care–associated (HCAP). The HCAP category is subdivided into hospital-acquired pneumonia (HAP) and ventilatorassociated pneumonia (VAP). HCAP is associated with hospitalization for ≥48 h, hospitalization for ≥2 days in the prior 3 months, residence in a nursing home or extended-care facility, antibiotic therapy in the preceding 3 months, chronic dialysis, home infusion therapy, home wound care, and contact with a family member who has a multidrug-resistant (MDR) infection.
PATHOPHYSIOLOGY
• Microorganisms gain access to the lower respiratory tract via microaspiration from the oropharynx (the most common route), inhalation of contaminated droplets, hematogenous spread, or contiguous extension from an infected pleural or mediastinal space.
• Before disease manifests, the size of the organism burden must overcome the ability of macrophages and other components of innate immunity (e.g., surfactant proteins A and D) to clear bacteria.
• Classic pneumonia (typified by that due to Streptococcus pneumoniae) presents as a lobar pattern and evolves through four phases characterized by changes in the alveoli: – Edema: Proteinaceous exudates are present in the alveoli. – Red hepatization: Erythrocytes and neutrophils are present in the intraalveolar exudate. – Gray hepatization: Neutrophils and fibrin deposition are abundant. – Resolution: Macrophages are the dominant cell type.
• In VAP, respiratory bronchiolitis can precede a radiologically apparent infiltrate.
COMMUNITY-ACQUIRED PNEUMONIA
Microbiology
Although many bacteria, viruses, fungi, and protozoa can cause CAP, most cases are caused by relatively few pathogens. In >50% of cases, a specific etiology is never determined. • Typical bacterial pathogens include S. pneumoniae, Haemophilus influenzae, Staphylococcus aureus, and gram-negative bacteria such as Klebsiella pneumoniae and Pseudomonas aeruginosa. • Atypical organisms include Mycoplasma pneumoniae, Chlamydia pneumoniae, Legionella species, and respiratory viruses (e.g., influenza viruses, adenoviruses, respiratory syncytial viruses). – A virus may be responsible for up to 18% of cases of CAP that require hospital admission. – 10–15% of CAP cases are polymicrobial and involve a combination of typical and atypical organisms. • Involvement of anaerobes, which play a significant role in CAP only when aspiration precedes presentation by days or weeks, often results in significant empyemas.
Epidemiology
CAP affects ~4 million adults each year in the United States, 80% of whom are treated on an outpatient basis. CAP causes 45,000 deaths annually and is associated with an overall yearly cost of $9–10 billion.
• Incidence rates of CAP are highest at the extremes of age (i.e., <4 and >60 years). • Risk factors for CAP include alcoholism, asthma, immunosuppression, institutionalization, and an age of ≥70 years (vs. 60–69 years). • Many factors—e.g., tobacco smoking, chronic obstructive pulmonary disease, colonization with methicillin-resistant S. aureus (MRSA), recent hospitalization or antibiotic therapy—influence the types of pathogens that should be considered in the etiologic diagnosis.
Clinical Manifestations
Pts frequently have fever, chills, sweats, cough (either nonproductive or productive of mucoid, purulent, or blood-tinged sputum), pleuritic chest pain, and dyspnea.
• Other common symptoms include nausea, vomiting, diarrhea, fatigue, headache, myalgias, and arthralgias.
• Elderly pts may present atypically, with confusion but few other manifestations.
• Physical examination often reveals tachypnea; increased or decreased tactile fremitus; dull or flat percussion reflecting consolidation and pleural fluid, respectively; crackles; bronchial breath sounds; or a pleural friction rub.
Diagnosis
Both confirmation of the diagnosis and assessment of the likely etiology are required. Although no data have demonstrated that treatment directed at a specific pathogen is superior to empirical treatment, an etiologic diagnosis allows narrowing of the empirical regimen, identification of organisms with public safety implications (e.g., Mycobacterium tuberculosis, influenza virus), and monitoring of antibiotic susceptibility trends.
• Chest radiography is often required to differentiate CAP from other conditions, particularly since the sensitivity and specificity of physical exam findings for CAP are only 58% and 67%, respectively. – CT of the chest may be helpful for pts with suspected postobstructive pneumonia. – Some radiographic patterns suggest an etiology; e.g., pneumatoceles suggest S. aureus.
• Sputum samples must have >25 WBCs and <10 squamous epithelial cells per high-power field to be appropriate for culture. The sensitivity of sputum cultures is highly variable; in cases of proven bacteremic pneumococcal pneumonia, the yield of positive cultures from sputum samples is ≤50%.
• Blood cultures are positive in 5–14% of cases, most commonly yielding S. pneumoniae. Blood cultures are optional for most CAP pts, but should be performed for high-risk pts (e.g., pts with chronic liver disease or asplenia).
• Urine antigen tests for S. pneumoniae and Legionella pneumophila type 1 can be helpful.
• Serology: A fourfold rise in titer of specific IgM antibody can assist in the diagnosis of pneumonia due to some pathogens; however, the time required to obtain a final result makes serology of limited clinical utility.
TREATMENT
Community-Acquired Pneumonia
DECIDING WHETHER TO HOSPITALIZE PTS
• Two sets of criteria identify pts who will benefit from hospital care. It is not clear which set is superior, and application of each tool should be tempered by a consideration of factors relevant to the individual pt. – Pneumonia Severity Index (PSI): Points are given for 20 variables, including age, coexisting illness, and abnormal physical and laboratory findings. On this basis, pts are assigned to one of five classes of mortality risk.
– CURB-65: Five variables are included: confusion (C); urea >7 mmol/L (U); respiratory rate ≥30/min (R); blood pressure, systolic ≤90 mmHg or diastolic ≤60 mmHg (B); and age ≥65 years (65). Pts with a score of 0 can be treated at home, pts with a score of 2 should be hospitalized, and pts with a score of ≥3 may require management in the intensive care unit (ICU).
ANTIBIOTIC THERAPY
• U.S. guidelines always target S. pneumoniae and atypical pathogens. Retrospective data suggest that this approach lowers the mortality rate.
• Pts initially treated with IV antibiotics can be switched to oral agents when they can ingest and absorb drugs, are hemodynamically stable, and are improving clinically.
• CAP has historically been treated for 10–14 days, but a 5-day course of a fluoroquinolone is sufficient for cases of uncomplicated CAP. A longer course is required for pts with bacteremia, metastatic infection, or infection with a particularly virulent pathogen and in most cases of severe CAP.
• Fever and leukocytosis usually resolve within 2–4 days. Pts who have not responded to therapy by day 3 should be reevaluated, with consideration of alternative diagnoses, antibiotic resistance in the pathogen, and the possibility that the wrong drug is being given.
Complications
Common complications of severe CAP include respiratory failure, shock and multiorgan failure, coagulopathy, and exacerbation of comorbid disease. Metastatic infection (e.g., brain abscess, endocarditis) occurs rarely and requires immediate attention.
• Lung abscess may occur in association with aspiration or infection caused by single CAP pathogens [e.g., community-acquired MRSA (CA-MRSA) or P. aeruginosa]. Drainage should be established and proper antibiotics administered.
• Any significant pleural effusion should be tapped for diagnostic and therapeutic purposes. If the fluid has a pH <7, a glucose level <2.2 mmol/L, and a lactate dehydrogenase content >1000 U or if bacteria are seen or cultured, fluid should be drained; a chest tube is usually required.
Follow-Up Chest x-ray abnormalities may require 4–12 weeks to clear. Pts should receive influenza and pneumococcal vaccines, as appropriate.
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