Chapter 120

The respiratory system is comprised of lung parenchyma and compliant airways. Flow limitation and elevated airway resistance induce flutter of the airway wall that generates the high-pitched sounds known as wheezing. Wheezing implies obstructive airway disease when diffuse, and focal obstruction when localized. However, severe flow limitation may exist without wheezing. Because intrathoracic airway lumen size normally increases during inspiration and decreases during expiration, wheezing is generally more prominent during expiration. Bronchiolitis is the most frequent cause of wheezing in infants, and asthma is the most frequent cause in children and adolescents.

### Epidemiology

Approximately 30% of children will have an illness associated with wheezing by 3 years of age and 50% by 6 years of age.1 Most children have benign, transient wheezing episodes that do not persist beyond 6 years of age. These “early wheezers” may have congenitally smaller airways that predispose them to wheezing with viral illnesses during infancy and early childhood. Maternal smoking is a risk factor for both transient and persistent wheezing. However, the approximately 14% of children whose wheezing appears before 3 years of age and persists at 6 years have additional risk factors for asthma, including eczema, maternal asthma, and elevated immunoglobulin E (IgE) levels during infancy.

### Pathophysiology

This section includes a discussion of pulmonary anatomy, mechanics, and physiology relevant to clinical assessment and management of the pediatric patient with asthma, bronchiolitis, or other obstructive airway diseases that cause wheezing.

The lungs consist of a supporting network of connective tissue and a series of compliant tubes that become more numerous and more narrow as they progressively branch peripherally. The cross-sectional surface area of an airway is proportionate to the square of the lumen radius. A decrease in airway radius exponentially diminishes the lumen available for airflow. However, because the net cross-sectional area of the tracheobronchial tree increases as these airways branch, airway resistance progressively diminishes peripherally. The nasal passages account for 50% of total airway resistance. Nasal resistance may increase substantially in the presence of nasal mucus or edema, a clinically important event, especially in the infant with bronchiolitis. The conducting airways extend from the trachea to the terminal bronchioles and do not participate in gas exchange. More distally, the transitional and respiratory zones have increasing numbers of alveoli and comprise the gas-exchanging units.

Lung tissue has elastic properties, the tendency to resist deformation or stretch with an opposing force that attempts to return the structure to its former state. Any collapse or decompression of alveoli or airways deforms adjacent tissue, which leads to elastic forces that serve to reestablish and maintain airway patency. This relationship, termed mechanical interdependence, promotes heterogeneous lung emptying by maintaining airway patency until end expiration.

Forces of stretch and recoil are active on each lung as a unit and on the chest wall. The resting state of these forces is ...

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