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 conditions⁵ (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

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

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

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

Special Populations

Evaluation and Diagnostic Testing

Patients with pneumonia should undergo a complete initial assessment of vital signs, including pulse oximetry and evaluation of the respiratory rate.

Chest Radiographs

Most authorities agree that chest radiographs (posteroanterior and lateral if possible) should be obtained in all patients with suspected pneumonia. Radiographs help confirm the diagnosis of pneumonia and provide possible clues to the causative agent (Table 50.4). Radiographs will also demonstrate associated conditions, such as lung abscess, bronchial obstruction, and pleural effusion (Figs. 50.2 to 50.4). Computed tomography may be necessary for further evaluation of complex infiltrates with pleural effusions, suspected masses, or abscesses. Typical radiologic patterns are expected with certain infections (see Table 50.4). It is important to remember that multiple factors, such as age, immune status, hydration status, and underlying lung disease, can alter the radiographic appearance of pneumonia.¹⁵

Table 50.4 Etiologic Agents of Pneumonia and Common Radiographic Patterns

RADIOGRAPHIC PATTERN	PATHOPHYSIOLOGY	TYPICAL PATHOGENS
Lobar or multilobar infiltrate with air bronchograms	Edema fluid/inflammatory reaction in alveoli occurring initially in the periphery of the lung Larger bronchi usually remain patent and cause air bronchograms	Bacterial pneumonias (Streptococcus pneumoniae, Klebsiella, Legionella)
Interstitial infiltrates	Edema and inflammatory cellular infiltrate in interstitial tissue surrounding small airways and vessels	Mycoplasma pneumoniae, Pneumocystis jiroveci (carinii), viral pneumonias
Bronchopneumonia	Diffuse patchy infiltrates and peribronchial thickening secondary to large and intermediate airway involvement	Staphylococcus aureus, gram-negative organisms, fungi
Abscesses	Abscess cavity, isolated or within areas of consolidation	S. aureus, Pseudomonas, anaerobes
Cavitary masses	Inflammation and necrosis of lung parenchyma	Mycobacterium tuberculosis, Nocardia, Aspergillus, Actinomyces

From Tarver R, Teague S, Heitkamp D, et al. Radiology of community acquired pneumonia. Radiol Clin North Am 2005;43:497-512.

Fig. 50.2 Chest radiograph of a 33-month-old child showing a left lower lobe infiltrate and a small effusion.

Fig. 50.3 Chest radiograph of an adult with extensive lobar infiltrate and air bronchograms.

Fig. 50.4 Chest radiograph of a child showing an interstitial infiltrate.

Treatment

After decisions regarding respiratory, hemodynamic, and ventilatory support, the most important decision in treating patients with pneumonia is selection of timely and appropriate initial antibiotic therapy. Multiple factors, such as suspected causes, severity of illness, recent antibiotic therapy, patient disposition, antibiotic allergies, and comorbid illnesses, contribute to the selection of optimal antimicrobial therapy.

As already described, the ATS and IDSA have issued detailed guidelines for the treatment of community-acquired pneumonia. Several studies have demonstrated lower mortality in hospitalized patients treated with guideline-recommended antimicrobial regimens.⁶ In a 2004 study of 420 patients with pneumonia, Mortensen et al.²² found significantly lower 30-day mortality in patients who received antimicrobial therapy in concordance with the ATS or IDSA guidelines.

The recommendations in the guidelines are summarized in Box 50.1. The guidelines group recommendations according to the site of therapy (outpatient, inpatient, or ICU), severity of disease, comorbid illnesses, and risk for resistant organisms or Pseudomonas aeruginosa.

Ideally, antibiotic therapy should be started in the ED as soon as possible after the diagnosis is made. Because pneumonia accounts for approximately 1.7 million hospitalizations per year with an estimated annual cost of more than $9 billion, improving pneumonia care has become a focus of the Joint Commission on Accreditation of Healthcare Organizations and the Centers for Medicare and Medicaid Services.²⁰ Several defined quality measures (core measures) have been indentified, including the use of evidence-based antibiotic choices, timely antibiotic administration, appropriate use of blood cultures before administration of antibiotics, and administration of pneumococcal and influenza vaccines for appropriate patients. Although the validity of some of the measures are questioned by some authorities, these data are publicly reported and adherence with the measures is often a priority for institutions.

^* Risks include antimicrobial therapy in the preceding 90 days, current hospitalization of 5 days or longer, high frequency of antibiotic resistance in the institution or community, hospitalization for 2 days or more in the preceding 90 days, residence in a nursing home or extended care facility, home infusion therapy, long-term dialysis within 30 days, home wound care, family member with multidrug-resistant pathogens, and immunosuppressive diseases or therapy.

^† If MRSA is a risk factor or has a high incidence locally.

From American Thoracic Society. Guidelines for the management of adults with hospital-acquired, ventilator-associated, and healthcare-associated pneumonia. Am J Respir Crit Care Med 2005;171;388-416.

Follow-Up, Next Steps in Care, and Patient Education

Several validated prediction tools can be used to identify patients with pneumonia who are at increased risk for complications and would benefit from inpatient or ICU treatment. These tools evaluate important patient characteristics, such as age, vital signs, radiographic and laboratory data, and comorbid illnesses, and identify patients at increased risk for 30-day mortality.

In 1997 the Pneumonia Patient Outcomes Research Team (PORT)²³ published a prediction rule to identify patients at low risk for death from pneumonia within 30 days; it is now known as the PORT criteria or the Pneumonia Severity Index. A 2005 comparison of three prediction guidelines by Aujesky et al.²⁴ found the Pneumonia Severity Index to be most accurate in identifying low-risk and high-risk patients with pneumonia. The system was developed through analysis of data from thousands of patients with community-acquired pneumonia and was validated in more than 50,000 patients. Patients are divided into five classes based on increasing risk for mortality. Class I (mortality, 0.1% to 0.4%) is assigned on the basis of an age of 50 years or younger and key information from the history and physical examination. Other classes are identified with a point system calculated from the presence of historical data and physical and laboratory findings (Box 50.2).²³

Class I and class II patients are considered to have very low risk (30-day mortality, <1%) and can probably be treated as outpatients. Many patients in class III (30-day mortality, <2.8%) might also be candidates for outpatient treatment. Patients in class IV or class V are at much higher risk (30-day mortality, 8.5% to 31.1%) and should probably be admitted to the hospital.^23,24

The British Thoracic Society has also published a prediction tool, CURB-65, to identify patients at high risk for 30-day mortality. Confusion, elevated blood urea nitrogen, elevated respiratory rate, low systolic or diastolic blood pressure, and age older than 65 years are the high-risk variables identified. A simplified version (CRB-65) allows prediction based on clinical variables alone (eliminating blood urea nitrogen) and may be useful in the outpatient setting. A score of 2 or higher predicts increased risk for mortality, and these patients should probably be admitted. ICU admission should be considered for those with a score of 3 or higher.¹

It must be emphasized that these prediction rules are only guidelines. Many other factors, such as unique comorbid conditions, psychosocial factors, ability to take oral medications, and patient and caregiver reliability, must also be considered in the decision to admit or discharge the patient. Patients demonstrating signs of shock or respiratory distress should be treated in an ICU setting.