Chapter 13 Community-Acquired Pneumonia
Definitions
Nosocomial pneumonia: Hospital-acquired pneumonia starting at least 48 to 72 hours after admission.
Aspiration pneumonitis: Chemical injury to lung caused by inhalation of sterile gastric contents.
Epidemiology
An estimated 4 million cases of community-acquired pneumonia (CAP) occur annually in the United States, accounting for at least 600,000 hospital admissions. CAP is the sixth leading cause of death. The mortality rate for hospitalized CAP patients is approximately 1 in 10 but this rate is higher in specific populations (e.g., nursing home residents) and approaches 40% in the severely ill who require admission to the intensive care unit.
Aspiration Pneumonia
Aspiration pneumonia accounts for up to 15% of CAP cases and is common among nursing home residents. The risk of aspiration pneumonia is higher in the elderly and patients with dysphagia, stroke, or critical illness and lower in patients without teeth. Usually, the episode of aspiration is not witnessed; the diagnosis is inferred when patients at risk for aspiration present with an infiltrate in a characteristic pulmonary location (posterior upper lobe or apical lower lobe segments from recumbent aspiration, and basal lower lobe segments from upright or semi-recumbent aspiration). Patients with aspiration pneumonia have clinical features similar to those of patients with CAP but have a higher incidence of pulmonary cavitation and abscess formation.
Early studies identified anaerobic organisms as the predominant pathogens in patients with aspiration pneumonia, but this has not been confirmed in recent studies.
Aspiration Pneumonitis
Aspiration pneumonitis occurs in patients with altered consciousness (e.g., seizures, drug overdose, and anesthesia). The aspirated gastric contents are usually sterile but the acidity burns the lung, causing an intense inflammatory reaction. Bacterial infection may subsequently develop, but the prevalence of this complication is unknown. Infection likely plays some role if the gastric contents are colonized with pathogenic organisms (e.g., gastroparesis, enteral feedings, and ant-acid therapy raise the gastric pH and increase the risk of bacterial colonization).
Aspiration pneumonitis has a broad spectrum of presentation, ranging from cough or wheeze to cyanosis, shortness of breath, hypoxemia, hypotension, acute respiratory distress syndrome, and death.
Causes of Community-Acquired Pneumonia
Most cases of CAP are limited to a few key organisms (Box 13-1), although in most cases the cause of the pneumonia is not identified. S. pneumoniae (pneumococcus) accounts for approximately two thirds of all cases of bacteremic pneumonia. Other common pathogens include M. pneumoniae, C. pneumoniae, and Legionella species, which have been reported to cause “atypical” pneumonia (pneumonia that does not present with classic signs and symptoms). In the past, the presenting signs and symptoms of pneumonia were thought to predict the causative agent, but we now know this to be untrue—there is a wide spectrum of presentation for each organism that is known to cause CAP, and these pathogens cannot be distinguished based only on symptoms, clinical signs, and findings on chest x-ray.
Symptoms and Signs
Immunocompetent adults presenting with pneumonia may have fever (∼ 80%), cough (> 90%), sputum production (∼ 66%), dyspnea (∼ 66%), and pleuritic chest pain (∼ 50%). However, these symptoms may also occur in patients with bronchitis or upper respiratory tract infections. Therefore, symptoms at presentation do not reliably distinguish between CAP and other respiratory illnesses. Bronchitis and upper respiratory tract infections are usually caused by viruses and can almost always be differentiated from pneumonia using chest x-ray. One caveat is that the chest x-ray may be normal in patients with CAP who are dehydrated. In this setting, the infiltrate should become visible following adequate hydration.
Extrapulmonary symptoms, including gastrointestinal symptoms, headache, myalgias, and arthralgias, occur in up to one third of CAP patients.
Physical exam findings depend on the severity of the infection. The patient may have fever, tachypnea, hypoxemia, pulmonary crackles, bronchial breath sounds, and respiratory distress with accessory respiratory muscle use.
Laboratory Data
Initial laboratory testing for patients with CAP requiring hospital admission should include:
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