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Lower Respiratory Tract Infections
CHAPTER CONTENTS
INTRODUCTION
Lower respiratory tract infections are an important cause of morbidity and mortality worldwide in children and in adults. Community-acquired pneumonia, for example, is the most deadly infectious disease in the United States. This chapter takes an anatomic approach to lower respiratory tract infections, moving from the large bronchi (bronchitis) down to the very small bronchioles (bronchiolitis) and then into the alveoli where pneumonia occurs.
BRONCHITIS
Definition
Bronchitis is a self-limited inflammation of the bronchi. Acute bronchitis must be distinguished from chronic bronchitis where patients have a cough for more than 3 months. The information in this chapter refers to acute bronchitis.
Pathophysiology
The coughing so characteristic of bronchitis is an attempt to clear the mucus produced by the inflammatory response to viral infection. Bronchitis occurs more often in the winter months than in the summer. Smoking predisposes to bronchitis (and pneumonia) by damaging the cilia in the bronchi, leading to an inability to clear mucus from the respiratory tract.
Clinical Manifestations
Cough is the most prominent symptom of bronchitis. Initially, bronchitis presents with the symptoms of an upper respiratory infection, namely, nasal congestion, scratchy sore throat, and perhaps a low-grade fever. Physical examination typically reveals expiratory wheezes. However, if cough persists for more than 5 days and pneumonia has been ruled out, acute bronchitis should be suspected. Bronchitis is self-limited and usually resolves in 1 to 2 weeks. However, cough may persist for several more weeks due to airway hyperreactivity.
Pathogens
Respiratory viruses are the most common pathogens (influenza A and B, parainfluenza virus, coronavirus, rhinovirus, respiratory syncytial virus [RSV], and human metapneumovirus). Bacterial pathogens are not thought to play a significant role in acute bronchitis.
Diagnosis
The diagnosis is primarily made clinically. Cough, with or without sputum production, which may persist for more than 5 days, is the typical presentation. Patients are usually afebrile but may have a low-grade fever. Sputum cultures are typically not done. In patients with chronic cardiorespiratory disease, a rapid antigen test for influenza virus may be useful because oseltamivir (Tamiflu) can shorten the duration and intensity of symptoms.
Because treatment of both upper respiratory infections and acute bronchitis is largely supportive, these distinctions may have less clinical significance. What may be more important clinically is to distinguish acute bronchitis (usually viral) from pneumonia (mainly bacterial; see section on Pneumonia), which does require antimicrobial therapy. A chest radiograph may be performed to determine whether pneumonia is present.
Treatment
Treatment involves reassurance and symptom relief with agents such as nonsteroidal anti-inflammatory drugs and/or a bronchodilator such as ipratropium. If influenza is diagnosed, oseltamivir (Tamiflu) may reduce the length and severity of symptoms. Antibiotics should be used only in those for whom a bacterial etiology has been clearly demonstrated.
Prevention
Influenza vaccine can prevent bronchitis and pneumonia caused by influenza A and B viruses. The neuraminidase inhibitor oseltamivir (Tamiflu) should be given to unimmunized individuals with chronic cardiorespiratory disease. Handwashing is recommended to reduce the carriage of respiratory viruses.
BRONCHIOLITIS
Definition
Bronchiolitis is inflammation of the bronchioles—the small airways less than 2 mm in diameter. The focus in this section will be on bronchiolitis among infants and young children where the etiology is primarily infectious.
Pathophysiology
Particularly among children under 2 years of age, viruses can directly damage the epithelial cells of the terminal bronchioles, causing inflammation and obstruction of the small airways. Prematurity is an important predisposing factor.
Clinical Manifestations
Usually children initially have symptoms consistent with an upper respiratory tract infection and then are noticed to have increased respiratory distress. Children under 2 years old in particular may have tachypnea, wheezing, nasal flaring, and chest retractions. In severe cases, hypoxia, apnea, and respiratory failure may ensue. In most cases, recovery occurs in 1 to 2 weeks.
Pathogens
RSV is the most common pathogen. Other etiologies include influenza virus, parainfluenza virus, adenovirus, coronavirus, rhinovirus, and human metapneumovirus. In children, viruses are the main etiology of bronchiolitis. Bacteria are not thought to be involved. In adults, the causes are more varied and range from viruses, to inhaled toxic chemicals in the workplace, to idiopathic causes. Bronchiolitis caused by RSV occurs primarily in the winter months.
Diagnosis
The diagnosis is primarily clinical. Upper respiratory tract infection symptoms followed by lower respiratory tract symptoms and signs (e.g., nasal flaring, wheezing) in a young child during the fall and winter would be very suggestive of bronchiolitis. Chest radiograph typically shows hyperinflation of the lungs. An enzyme immunoassay (EIA) for RSV antigen in respiratory secretions is available for diagnosis in hospitalized patients. A polymerase chain reaction (PCR) assay that detects the RNA of RSV is also available.
Treatment
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