DEFINITION

Pneumonia is an infection of the pulmonary parenchyma. Despite being the cause of significant morbidity and mortality, pneumonia is often misdiagnosed, mistreated, and underestimated. In the past, pneumonia was typically classified as community-acquired, hospital-acquired, or ventilator-associated. Over the last decade or two, however, patients presenting to the hospital have often been found to be infected with multidrug-resistant (MDR) pathogens previously associated with hospital-acquired pneumonia. Factors responsible for this phenomenon include the development and widespread use of potent oral antibiotics, earlier transfer of patients out of acute-care hospitals to their homes or various lower-acuity facilities, increased use of outpatient IV antibiotic therapy, general aging of the population, and more extensive immunomodulatory therapies. The potential involvement of these MDR pathogens has led to a revised classification system in which infection is categorized as either community-acquired pneumonia (CAP) or health care–associated pneumonia (HCAP), with subcategories of HCAP including hospital-acquired pneumonia (HAP) and ventilator-associated pneumonia (VAP). The conditions associated with HCAP and the likely pathogens are listed in Table 251-1.

Table 251-1 Clinical Conditions Associated with and Likely Pathogens in Health Care–Associated Pneumonia

	Pathogen
Condition	MRSA	Pseudomonas aeruginosa	Acinetobacter spp.	MDR Enterobacteriaceae
Hospitalization for ≥48 h	X	X	X	X
Hospitalization for ≥2 days in prior 3 months	X	X	X	X
Nursing home or extended-care facility residence	X	X	X	X
Antibiotic therapy in preceding 3 months		X		X
Chronic dialysis	X
Home infusion therapy	X
Home wound care	X
Family member with MDR infection	X			X

Note: MDR, multidrug-resistant; MRSA, methicillin-resistant Staphylococcus aureus.

Although the new classification system has been helpful in designing empirical antibiotic strategies, it is not without disadvantages. For instance, not all MDR pathogens are associated with all risk factors (Table 251-1). Therefore, this system represents a distillation of multiple risk factors, and each patient must be considered individually. For example, the risk of infection with MDR pathogens for a nursing home resident with dementia who can independently dress, ambulate, and eat is quite different from the risk for a patient who is in a chronic vegetative state with a tracheostomy and a percutaneous feeding tube in place. In addition, risk factors for MDR infection do not preclude the development of pneumonia caused by the usual CAP pathogens.

This chapter deals with pneumonia in patients who are not considered to be immunocompromised. Pneumonia in immunocompromised patients is discussed in other chapters, including Chaps. 82, 126, and 182.

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