Community-Acquired Pneumonia
Jake M. Chanin
Carlos A. Q. Santos
General Principles
Community-acquired pneumonia (CAP) is a significant cause of morbidity and mortality in the United States.
CAP ranks highly among all causes of death. The exact mortality rate varies widely depending on the treatment setting (outpatient vs. inpatient vs. intensive care unit [ICU]), the presence or absence of associated comorbidities, and the age of the patient.
It causes over 1 million hospitalizations annually.
Administration of appropriate antimicrobials and management for severe pneumonia have a significant benefit on patient survival.
Nearly half of all cases do not have an identified etiologic agent.
Early empiric treatment is essential, and bacterial resistance must be considered.
The most widely recognized guidelines for the treatment of CAP include those of the American Thoracic Society, the Infectious Diseases Society of America, and the Canadian Infectious Disease Society and Canadian Thoracic Society.
Definition
CAP is a primary infection of lung parenchyma. Bacterial or viral invasion causes inflammation and alveolar infiltration that result in focal consolidation.
CAP is distinctive from health care–associated pneumonia (HCAP) and hospital-acquired pneumonia (HAP) in that the infection is acquired in the community.
Classification
Typical bacterial: Streptococcus pneumoniae, Staphylococcus aureus, Haemophilus influenzae, Group A streptococci, Moraxella catarrhalis, mixed anaerobes (aspiration), and aerobic gram-negative organisms.
Atypical bacterial: Legionella pneumophila, Mycoplasma pneumoniae, Chlamydophila pneumoniae.
Viral: Influenza A and B, respiratory syncytial virus (RSV), adenovirus, rhinoviruses, rubeola, varicella.
Epidemiology
Pneumonia and influenza combine to be the eighth leading cause of death in the United States. In 2008, there were over 56,000 deaths due to pneumonia and influenza.1 The rate of deaths due to pneumonia is increasing. Pneumonia is more common in the winter months and elderly. Men and African-Americans are slightly more affected than women and Caucasians.
Etiology
The most common etiology for CAP is S. pneumoniae.
Frequently, pneumonia is preceded by an upper airway infection or viral illness.
Pathophysiology
Lobar pneumonia is characterized by consolidation of a large portion of lung. Consolidation of airspaces is caused by host inflammatory infiltration in response to bacterial infection of lung tissue.
Bronchopneumonia similarly involves acute inflammation, but the consolidated areas are patchy and often multilobar or bilateral. This pattern is more common to atypical viral pneumonias or mycoplasmal pneumonias.
Risk Factors
Predisposing comorbid conditions: chronic obstructive pulmonary disease (COPD), heart failure, chronic renal disease, and chronic bronchitis.
Host factors: advanced age, tobacco use, prior history of pneumonia, recent viral respiratory infection.
Immunosuppressed states: HIV infection, chemotherapy, solid organ and stem cell transplant recipients.
Mechanical: dysphagia, lung cancer, mechanical obstruction of bronchus, hiatal hernia, radiation esophagitis.
Aspiration risk factors: alcoholism and drug intoxication, altered mental status, seizure disorder, stroke, procedural sedation, and anesthesia.
Mucus clearance: cystic fibrosis, Kartagener syndrome, immotile cilia syndrome, Young syndrome.
Prevention
Prevention should include major risk factor modifications such as smoking cessation and vaccination against influenza and pneumococcus.
Diagnosis
The gold standard for diagnosis of pneumonia is a posteroanterior and lateral CXR demonstrating new pulmonary infiltration. Clinical signs and symptoms should correlate with active infection and pulmonic consolidation.
Clinical Presentation
Patients will typically complain of fever, chills, productive cough, shortness of breath, and chest pain.
Physical examination findings include fever, tachypnea, tachycardia, abnormal breath sounds including rhonchi or crackles, increased tactile fremitus, dullness to percussion, and reduced chest movement.
Differential Diagnosis
The differential diagnosis for pneumonia includes pathology that causes radiographic consolidations that can mimic pneumonia. This includes acute heart failure exacerbation and other causes of pulmonary edema, malignancy, pulmonary embolism (PE), septic embolism, and foreign body.
Multiple different bacteria, viruses, and fungi can cause acute pneumonia.
Diagnostic Testing
The primary objective is to identify the causative agent. Other laboratory testing should be undertaken to assess the severity of illness. Typical studies include basic laboratory chemistries, blood cell counts, and cultures.
Laboratories
Initial inpatient and outpatient studies should include blood cultures, complete blood count (CBC) with differential, basic metabolic profile (BMP), and liver function tests (LFT).
Blood cultures are positive in 5–18% of hospitalized patients.2,3 Ideally blood cultures should be obtained before antibiotics are given, but this should not delay administration of early empiric treatment.
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