A lower respiratory tract infection (LRTI) is an infection below the level of the larynx and includes bronchitis, bronchiolitis, and pneumonia. LRTIs result from viral or bacterial invasion of lung parenchyma. Viral infections are diagnosed by the recognition of characteristic constellation of clinical signs and symptoms and treatment consists of supportive care measures (except for influenza, antivirals may be given). Bacterial LRTIs (in particular bacterial pneumonia) requires expedient, effective, and specific antimicrobial therapy. An appropriate treatment regimen for the patient with an LRTI can be established with the aid of a thorough medical history, physical examination, chest radiograph, knowledge of common bacterial pathogens, and results of properly collected cultures. This chapter will focus on pneumonia; see the Special Considerations section for information on bronchitis and bronchiolitis.
Pneumonia is inflammation of the lung with consolidation and is classified by the setting in which it develops (community-acquired or hospital-acquired). A subset of pneumonia associated with contact with the health care system, such as a skilled nursing facility, is not well defined but is referred to as health care–associated pneumonia (HCAP). HCAP is caused by similar organisms and treated similarly to hospital-acquired pneumonia and will therefore not be discussed separately.
The etiology of bacterial pneumonia varies in accordance with the type of pneumonia. Table 22-1 lists common bacterial pathogens associated with the various classifications of pneumonia. Viruses are a common cause of community-acquired pneumonia (CAP) in adults (often coinfecting with bacteria), and in children (65%). Viral pneumonia in children is frequently caused by respiratory syncytial, influenza, or parainfluenza virus.
TABLE 22-1Common Bacterial Pathogens by Type of Pneumonia |Favorite Table|Download (.pdf) TABLE 22-1Common Bacterial Pathogens by Type of Pneumonia
|Type of Pneumonia ||Common Bacterial Pathogens |
|Community-acquired ||Typical pathogens |
| || Streptococcus pneumoniae |
| || Haemophilus influenzae |
| || Moraxella catarrhalis |
| ||Atypical pathogens |
| || Mycoplasma pneumoniae |
| || Chlamydia pneumoniae |
| || Legionella pneumophila |
|Aspiration (community-onset)a ||Oral anaerobesb |
| ||Streptococcus species |
|Hospital-acquired ||Early onset |
| || S. pneumoniae |
| || H. influenzae |
| || M. catarrhalis |
| || Methicillin-susceptible Staphylococcus aureus |
| || Enterics (ie, Klebsiella pneumoniae, Escherichia coli, Enterobacter species, Proteus species, Serratia marcescens) |
| ||Late onset |
| || |
Other drug-resistant gram-negative rods
Methicillin-resistant Staphylococcus aureus
Microorganisms gain access to the lower respiratory tract by three routes:
Aspiration of oropharyngeal secretions. Aspiration of small volumes of saliva (microaspiration) is the major mechanism by which pulmonary pathogens gain access to the normally sterile lungs.
Inhalation of aerosolized particles.
Metastatic spread to the lungs via the bloodstream from an extrapulmonary site of infection.
When pulmonary defense mechanisms ...