Epidemiology

Pneumonia is one of the most common infectious diseases encountered in the emergency department (ED), with more than 1.5 million visits for this infection annually.¹ When combined with influenza, pneumonia ranks as the sixth leading cause of death in the United States and the most common cause of infection-related mortality.²^–⁵

Pneumonia is typically defined as an infection of the lung parenchyma with associated symptoms of cough, fever, and abnormal breath sounds on physical examination. Usually an infiltrate is seen on chest radiographs.

Because of the many potential pathogens causing this disease, a thorough history and physical examination are very important in guiding selection of the appropriate empiric antibiotic coverage in patients with pneumonia.

Pathophysiology

The lungs are constantly exposed to potential pathogens by both organisms in inspired air and those living in the oropharynx and upper respiratory tract. As a result of multiple layers of defense, the lower respiratory tract usually remains sterile. Many protective mechanisms—the cough and gag reflexes, upper airway particle filtration, proper mucociliary clearance, and humoral and cellular immunologic defenses at the alveolar level—play an important role in guarding the lungs against infection.

Pneumonia typically occurs when the protective defenses just described are breached and the lungs are exposed to a heavy inoculation of organisms or to very virulent organisms. The lung’s defenses can be impaired at multiple levels. Aspiration can result when altered levels of consciousness secondary to a neurologic insult or alcohol or drug intoxication impair the gag reflex. The upper airway defenses are often bypassed by endotracheal tubes or tracheostomy. Smoking and chronic lung disease can impair mucociliary function. Bronchial obstruction secondary to tumor or lymphadenopathy can lead to obstruction and pneumonia. Immunologic impairment as a result of infection with human immunodeficiency virus (HIV), chemotherapy, splenectomy, or advanced age also predisposes to pneumonia. The elderly are particularly vulnerable because of impairments at many of these levels and a higher incidence of comorbid conditions ⁵ (Fig. 50.1).

Fig. 50.1 Host defenses against infection.

CVA, Cerebrovascular accident; HIV, human immunodeficiency virus; IgA, immunoglobulin A.

(Modified from Donowitz G, Mandell G. Acute pneumonia. In: Mandell GL, Bennett JE, Doline R, editors. Mandell, Douglas, and Bennett’s principles and practice of infectious disease. 6th ed. Philadelphia: Saunders; 2005. pp. 819-41.)

Presenting Signs and Symptoms

The classic symptoms of pneumonia are fever, cough, purulent sputum, and shortness of breath. Patients may also have several nonrespiratory symptoms such as sweats, chills, confusion, headache, fatigue, abdominal pain, nausea, and myalgias. Infants may have decreased oral intake, lethargy, and apnea. Patients of advanced age are less likely to have typical symptoms, instead often exhibiting only weakness or altered mental status.

A goal-directed, comprehensive history is very important in the evaluation of a patient with pneumonia. Historical clues such as risk for aspiration, recent travel, animal or environmental exposure, HIV status or risk, alcoholism, and comorbid illnesses can point toward specific causes and guide the proper choice of initial empiric therapy (Table 50.1).^6,⁷ Special care should be taken to identify patients at risk for health care–associated pneumonia (HCAP), such as recent health care and antibiotic exposure, which could predispose them to multidrug-resistant pathogens and alter treatment choices.⁸

Table 50.1 Epidemiologic Conditions Related to Specific Pathogens in Patients with Selected Community-Acquired Pneumonia

CONDITION	COMMONLY ENCOUNTERED PATHOGENS
Alcoholism	Streptococcus pneumoniae, oral anaerobes
Chronic obstructive pulmonary disease and/or smoking	S. pneumoniae, Haemophilus influenzae, Moraxella catarrhalis, Legionella spp., Chlamydia pneumoniae
Poor dental hygiene	Oral anaerobes
Aspiration/lung abscess	Oral anaerobes
Exposure to bats or to soil enriched with bird droppings	Histoplasma capsulatum
Exposure to birds	Chlamydia psittaci, avian influenza (poultry exposure)
Exposure to rabbits	Francisella tularensis
Exposure to farm animals or parturient cats	Coxiella burnetii (Q fever)
Human immunodeficiency virus infection:
Early	S. pneumoniae, H. influenzae, Mycobacterium tuberculosis
Late	Above plus Pneumocystis jiroveci (carinii), Cryptococcus, Histoplasma
Travel to or residence in the southwestern United States	Coccidioides spp.
Travel to or residence in Asia	Burkholderia pseudomallei, severe acute respiratory syndrome
Influenza active in the community (“flu season”)	Influenza, S. pneumoniae, Staphylococcus aureus, H. influenzae
Structural lung disease (e.g., bronchiectasis)	Pseudomonas aeruginosa, Burkholderia cepacia, S. aureus
Injection drug use	S. aureus, skin anaerobes, M. tuberculosis, S. pneumoniae
Endobronchial obstruction	Anaerobes, S. pneumoniae, H. influenzae, S. aureus
Recent hospitalization or nursing home residence	Drug-resistant S. pneumoniae, gram-negative bacilli, S. aureus
In the context of bioterrorism	Bacillus anthracis (anthrax), Yersinia pestis (plague), F. tularensis (tularemia)

Modified from File T, Niederman M. Antimicrobial therapy of community-acquired pneumonia. Infect Dis Clin North Am 2004;18:993-1016.

Red Flags

Look for key clues in the history and physical findings that would suggest pneumonia caused by an unusual or resistant organism.

Identify patients at risk for health care–associated pneumonia.

Maintain high suspicion for aspiration pneumonia in the elderly and in patients with altered mental status or a recent cerebrovascular accident.

Pathogens

Community-acquired pneumonia is often defined as pneumonia in a patient who has not been hospitalized and has not resided in a long-term care facility for more than 14 days before the appearance of symptoms.³ With the growing prevalence of mixed-organism infections, drug-resistant pathogens, and patients with comorbid illnesses, this definition has become somewhat more complicated. Guidelines from the American Thoracic Society (ATS), Centers for Disease Control and Prevention, and Infectious Diseases Society of America (IDSA) now address the treatment of patients at increased risk for pseudomonal infection, those with significant comorbid conditions, and those at risk for infection with drug-resistant Streptococcus pneumoniae.

The most common etiologic agent causing pneumonia is S. pneumoniae, which is responsible for about two thirds of all cases (Table 50.2).³ Other bacterial pathogens that are often isolated are Mycoplasma pneumoniae, Chlamydia pneumoniae, Haemophilus influenzae, and Legionella pneumophila (often called “the atypical organisms”). Community-acquired pneumonia is also caused by several viruses, including influenza, parainfluenza, and respiratory syncytial virus (RSV). Other pathogens, such as Pseudomonas aeruginosa, drug-resistant S. pneumonia, and methicillin-resistant Staphylococcus aureus (MRSA), should additionally be considered in patients who have had recent health care exposure or have recently taken broad-spectrum antibiotics. Multiple other organisms may be considered based on a history of specific exposures, travel, and lung or immunosuppressive diseases.

Table 50.2 Etiology of Community-Acquired Pneumonia

PATHOGEN	PREVALANCE (%)
Streptococcus pneumoniae	20-60
Haemophilus influenzae	3-10
Staphylococcus aureus	3-5
Gram-negative bacilli	3-10
Miscellaneous (includes Moraxella catarrhalis, group A streptococci, and Neisseria meningitidis, each accounting for 1-2% of cases)	3-5
Legionella spp.	2-8
Mycoplasma pneumoniae	1-6
Chlamydia pneumoniae	4-6
Viruses	2-15
Aspiration	6-10

From Niederman M. Review of treatment guidelines for community acquired pneumonia. Am J Med 2004;117;52S.

Special Populations

Children

Pneumonia and acute bronchiolitis are the most common lower respiratory tract infections in children. They are caused by a variety of viruses and bacteria, with the prevalence varying by age group ^9,¹⁰ (Table 50.3). As in adults, S. pneumoniae is the predominant organism causing pneumonia except in newborns, in whom group B streptococci and gram-negative bacilli dominate. H. influenzae type b remains an important bacterial pathogen causing pneumonia in the developing world. It has nearly been eliminated in the United States through immunization practices. Pneumococcal vaccines also appear to be lowering the incidence of invasive pneumococcal disease and pneumonia, but more data are needed.¹¹ Many viruses, mainly influenza, parainfluenza, and RSV, can also cause pneumonia in children. Most children with pneumonia have cough, fever, and abnormal lung findings. Signs of tachypnea and increased work of breathing are often present and may be the only signs of disease in infants.⁹

Table 50.3 Typical Pathogens in Pediatric Pneumonia and Recommended Treatment