Chapter 67 Respiratory Tract Infections
PATHOPHYSIOLOGY
Respiratory tract infections are those caused by either a virus or bacterium in either the upper or lower respiratory tract. Viral infections are most common. Upper respiratory tract infection affects the trachea and larynx and is known as croup or laryngotracheobronchitis. It is caused by parainfluenza virus types I, II, or III, adenovirus, respiratory syncytial virus (RSV), or influenza virus types A or B. This infection leads to inflammation and edema of the laryngeal mucosa, followed by epithelial necrosis and shedding. Narrowing of the subglottic regions results in a characteristic barky cough, harsh voice, stridor, and retractions of the chest wall. Children are more susceptible to upper airway obstruction because the diameters of their supraglottic, glottic, and subglottic regions are small. Edema in these areas can lead to asynchronous chest and abdominal movement, fatigue, hypoxia, hypercapnia, and respiratory failure. Fever is usually present. Symptoms are almost always worse at night and show improvement during the day and, for 60% of children, resolve within 48 hours.
Lower respiratory tract infections are commonly known as bronchiolitis. This illness may be caused by RSV, parainfluenza, adenoviruses, rhinoviruses, enteroviruses, or human metapneumovirus. It is characterized by cough, nasal secretions, tachypnea, expiratory wheezing, and retractions because of inflammation of the small bronchi and smaller bronchioles. Edema of the mucous membranes lining the walls of the bronchioles along with cellular infiltrates and increased mucus production result in obstruction of the bronchioles. This causes hyperinflation of the affected areas, since expired air is trapped distally, resulting in hypoxemia. The obstructions do not occur uniformly throughout the lung. In addition, resistance to airflow increases. This leads to dyspnea, tachypnea, and lower tidal volumes, which may result in hypercarbia in severely affected individuals. Symptoms are more severe in infants because the diameter of the lumina of their bronchioles is smaller.
These viral illnesses are transmitted by respiratory secretions through close contact with infected individuals or contaminated surfaces or objects. The viruses can remain on surfaces for several hours and longer than 30 minutes on hands. Good handwashing is critical in preventing transmission.
INCIDENCE
1. Incidence is seasonal: infections of the upper respiratory tract are most common in late fall and early winter. Infections of the lower respiratory tract are most common in winter and spring.
2. RSV is present in 80% of respiratory tract infections.
3. Age range of occurrence is 3 months to 3 years; peak age of onset is 2 years of age.
4. Boys are more affected than girls.
5. Lower respiratory tract infections are the leading cause of hospitalization in infants under 1 year of age.
6. Mortality is increased with children who have other underlying pulmonary or cardiac conditions.
CLINICAL MANIFESTATIONS
1. Rhinorrhea and/or nasal congestion
2. Persistent barking cough that worsens
4. Stridor (progression of stridor is an indicator of the severity of the disease)
6. Retractions at rest, nasal flaring
8. Signs of respiratory failure (agitation, restlessness, listlessness, decrease in stridor and retractions without clinical improvement, and cyanosis)
