Definition and Epidemiology

Pneumonia remains one of the leading causes of morbidity and mortality in the United States, especially in older adults and in those with underlying chronic disease. In 2010, pneumonia and influenza combined were ranked as the ninth leading cause of death in the United States, with the majority of deaths attributable to pneumonia.¹ Worldwide, in children younger than 5 years, pneumonia remains the primary cause of death.² In 2011 in the United States, pneumonia was the primary cause of hospitalization death in adults, with costs exceeding 10 billion dollars.³ Pneumonia and influenza together represented a cost to the U.S. economy in 2005 of approximately 40.2 billion dollars.⁴

The Infectious Diseases Society of America (IDSA) defines community-acquired pneumonia (CAP) as an acute infection of the pulmonary parenchyma that is frequently associated with at least two symptoms of active infection, occurring in individuals who have not been hospitalized or resided in a long-term care facility for 14 days before the onset of symptoms.⁵ Hospital-acquired pneumonia (HAP; nosocomial pneumonia) is defined as pneumonia that occurs 48 hours or more after admission that did not appear to be incubating at the time of admission. Ventilator-associated pneumonia (VAP) is defined as a type of HAP that develops more than 48 hours after endotracheal intubation. Health care–associated pneumonia (HCAP), a relatively new clinical entity, is defined as pneumonia that occurs in a nonhospitalized patient with extensive health care contact, as defined by one or more of the following: intravenous therapy, wound care, or intravenous chemotherapy during the prior 30 days; residence in a nursing home or other long-term care facility; hospitalization in an acute care hospital for 2 or more days during the prior 90 days; or attendance at a hospital or hemodialysis clinic during the prior 30 days.⁶

Pathophysiology

The lungs are usually a sterile environment maintained by a host of natural defenses. The airways act as a filtration and humidification system for inspired air. Epithelial cells line the entire respiratory tract and contain cilia that constantly beat upward toward the pharynx. This action is a physical means of elimination of foreign material. Also, an intact gag reflex prevents the entry of particles, mucus, and food debris. Finally, the immune system is responsible for defense mechanisms, such as the action of phagocytes, macrophages, neutrophils, complement, and immunoglobulins, which retard advancement of pathogenic organisms that do gain access to this normally sterile environment.

In the healthy adult, these host mechanisms prevent disease much of the time. However, a number of mechanisms allow pathogens to gain entry into the lungs; these mechanisms include an altered level of consciousness from stroke, seizure, anesthesia, alcohol abuse, intoxication, and the sleep state. Epiglottic closure may be compromised in these situations and allow normal oral flora to gain entry. Certain other conditions may predispose an individual to recurrent pneumonia. These include compromised immune function, cystic fibrosis, esophageal abnormalities, bronchial obstruction, and bronchiectasis.

Pneumonia can be classified as typical and atypical, although the clinical presentation is often similar.⁷ Typical pneumonia is caused by organisms such as Streptococcus pneumoniae, which accounts for 60% to 70% of all bacterial CAP cases. Atypical pneumonia, so called because the organisms are not detectable on Gram stain or cultivatable on standard bacteriologic media, is caused by Mycoplasma pneumoniae, Chlamydophila pneumoniae, Legionella species, and respiratory viruses.⁵ In addition, patients are more likely to be exposed to certain types of pneumonia according to the setting. The most common causes of CAP in outpatients include S. pneumoniae, M. pneumoniae, Haemophilus influenzae, C. pneumoniae, and respiratory viruses.⁵ Common pathogens in inpatient, non–intensive care unit (ICU) settings include S. pneumoniae, M. pneumoniae, C. pneumoniae, H. influenzae, Legionella species, and respiratory viruses. Common causes in inpatient ICU settings include S. pneumoniae, Staphylococcus aureus, Legionella species, gram-negative bacilli, and H. influenzae.

Comorbidities also influence susceptibility to different types of pneumonia. Moraxella catarrhalis and Klebsiella pneumoniae infections are more commonly diagnosed when there is coexistent alcoholism. S. aureus and H. influenzae infections often occur after a primary influenza infection. M. catarrhalis, a gram-negative organism not thought to be pathogenic, is most commonly found in those with chronic lung conditions such as chronic obstructive pulmonary disease (COPD). It is also found in patients with other underlying chronic lung conditions such as malignant disease, with steroid use, and with diabetes.⁸

Another organism responsible for pneumonia is Legionella pneumophila. This organism was first implicated in 1976 after 182 people became ill in Philadelphia while attending an American Legion convention. The organism is a gram-negative bacillus that survives in water and soil. Contamination with the organism is acquired through inhalation of aerosolized droplets, thus making air-conditioning ventilating systems an obvious reservoir.

Clues to the specific cause of the pneumonia can be found in the patient’s history. The CAPs include the organisms found in Table 111-1.

In most cases of CAP, diagnosis is made by history and physical examination; identification of the causative agent is usually not necessary. Although the list of organisms causing CAP is long and increasing, relatively few organisms are responsible for most cases of pneumonia. In primary care practice, two of the most important issues related to pneumonia are awareness of the most common infectious pathogens and their treatment and decisions about the appropriateness of outpatient treatment. The clinical presentation of pneumonia includes a history of fever, chills or rigors, malaise, and cough with or without sputum production.⁹ The patient may also report hemoptysis, dyspnea, and pleuritic chest symptoms. The provider should focus on symptoms of bacterial, viral, and atypical pneumonia syndromes. Chest auscultation may reveal rales that do not clear with a cough, which may be found in both bacterial and atypical pneumonia. Consolidation, including dullness to percussion, bronchial breath sounds, and egophony (E-to-A changes), is found more commonly in the bacterial pneumonia syndromes. Chest radiographs are highly variable and may be normal in the early course of the disease. In addition, chest radiographs of patients with viral and mycoplasmal pneumonia may show large infiltrates with minimum outward symptoms. A prodrome of headache and sore throat is often associated with atypical pneumonia. Patients aged 18 to 44 years are almost twice as likely as those older than 75 years to complain of pleuritic chest pain and to have fever.

Some patients (e.g., older adults) may show none of the classic signs of pneumonia but may have atypical complaints such as fatigue, lethargy, decreased appetite, increased falls, and mental status changes (such as confusion, stupor, or coma). In addition, older adults are more likely to be seen initially with tachypnea but less likely to have a cough or fever.

Bacterial Pneumonia Syndromes

Gram-Positive Bacteria.

S. pneumoniae is the leading cause of pneumonia in any adult age group with or without comorbid conditions.^7,8 Those at risk for S. pneumoniae infection characteristically have some chronic condition, such as diabetes, COPD, asplenia, advanced age, cigarette smoking, congestive heart failure, dementia, alcoholism, or immunosuppression. From 20% to 60% of all hospitalized patients are infected with pneumococci.⁷

The history may include an abrupt onset of high fever with shaking chills, productive cough with purulent sputum, and possibly pleuritic-type chest pains. Physical examination may reveal signs of consolidation (egophony, increased fremitus, dullness to percussion, rales, and rhonchi), and chest films reveal single or multiple lobar consolidation. Sputum analysis by Gram stain indicates gram-positive diplococci in pairs and short chains and large numbers of polymorphonuclear leukocytes.

S. aureus, although rarely a cause of CAP, must be considered, especially after a primary influenza infection, in older adults and in those with diabetes. Suppurative conditions, including empyema, lung abscess, and pneumothorax, are common complications. Seeding to distant sites, such as bones, joints, liver, endocardium, and the meninges, may also occur.

Group A streptococci rarely cause CAP but have been found in epidemics among close groups that live together, such as military units. Symptoms may be similar to those of S. pneumoniae infection, and Gram stain reveals clumped spherical cocci, similar in appearance to a bunch of grapes.

Gram-Negative Bacteria.

H. influenzae, another causative agent of CAP, is a small, gram-negative rod with a polysaccharide capsule. There are six serotypes, of which type B is the most severe and invasive (causing meningitis and sepsis). Some strains of H. influenzae are nonencapsulated and therefore cannot be typed. These are also capable of causing disease, but the disease is usually noninvasive and therefore less severe. These nontypable strains of H. influenzae are usually found in acute bronchitis. Pneumonia caused by H. influenzae is usually caused by an encapsulated strain. Older adults and those with underlying chronic lung conditions are most susceptible to this bacterium. The history usually includes an abrupt onset of fever, shaking chills, and cough with purulent sputum. The patient may describe pleuritic chest pain, and physical examination reveals signs of consolidation. A bronchopneumonia pattern is seen on the chest radiograph.

Aerobic gram-negative bacilli rarely colonize the upper airway in healthy individuals but are often found in people with an underlying disease such as alcoholism and in those who reside in health care facilities or nursing homes. Aspiration of the organisms is thought to be the mode of infection. Pseudomonas organisms, K. pneumoniae, and Escherichia coli may also become pulmonary pathogens. The mortality rate associated with gram-negative pneumonia is relatively high compared with other types of pneumonia. Therefore, a history of recent hospitalization or nursing home residency should heighten suspicion for a gram-negative pathogenesis. Polymicrobial infection is seen more often in older adults, and increased colonization of gram-negative bacilli of the upper airway is related to recent antimicrobial use, decreased activity, diabetes, and alcohol use.

M. catarrhalis is a β-lactamase–producing gram-negative aerobic diplococcus that was recently identified as a common pathogen found in individuals with COPD.^7,8 In patients with COPD, it is often the only organism isolated from the lower respiratory tract. Other chronic conditions, such as alcoholism, steroid use, diabetes, and malignant disease, increase the risk for M. catarrhalis infection. The highest incidence of this infection tends to be in the winter months.

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