The pathologist defines emphysema as the abnormal, permanent enlargement of air spaces distal to the terminal bronchioles, accompanied by destruction of alveolar walls. The physiologist defines emphysema as the permanent loss of elastic recoil of the lungs. The clinician defines emphysema as overexpansion of a region of the lungs that is not reversible with maximal bronchodilator therapy.

Enlargement of air spaces without destruction of their walls is termed overinflation. The term emphysema seldom is used in pediatrics. Although many children have lungs that fit the preceding descriptions, few come to lung biopsy or autopsy
for definitive diagnosis. Furthermore, the rapid increase in the number of alveoli until a child reaches the age of 8 years allows for a dramatic improvement in the clinical status of children who experience even severe emphysematous changes in the first year of life. Several clinical syndromes and common pediatric respiratory diseases have significant components of emphysema.

All airways down to the terminal bronchioles are present by 16 weeks of postconceptional age. Thus, the full complement of airways is developed in the most premature infants who are viable.

An acinus is the unit distal to the terminal bronchiole that includes the alveolar ducts and alveoli ventilated by a single terminal bronchiole. Adjacent acini are separated by fibrous septa. Alveoli develop by budding from alveolar ducts. They increase in number until the child reaches approximately 8 years of age. After that, the alveoli continue to expand until the lungs reach adult size, at approximately 17 years in girls and 20 years in boys. From then on, the alveoli gradually simplify, and the alveolar surface area decreases by approximately 4% per decade through adult life.

At the end of expiration, all respiratory muscles usually are relaxed, and the volume of the lungs is determined by the balance between elastic recoil of lung tissue and compliance of the thoracic cavity. Normally, a network of elastic fibers runs throughout the lungs in the interstitial spaces that provide the elastic recoil of the lungs. Disruption or destruction of this elastic network occurs in emphysema. Recoil decreases, diminishing the normal tendency for the lungs to shrink, and the functional residual capacity (i.e., the lung volume at the end of passive expiration) increases. This process causes numerous secondary changes. The diaphragms do not ascend to their normal position at the end of expiration, so they are at a mechanical disadvantage for developing negative pressure in the chest for inspiration. Phrenic muscle fibers are shorter, so they develop less tension. Consequently, the diaphragms pull less air into the lungs. Also, because of decreased driving pressure, expiratory flows decrease, causing a decrease in forced expiratory volume over 1 second (FEV₁) and a consequent decrease in maximum minute ventilation, which reduces exercise capacity. With severe emphysema, loss of alveolar surface area decreases the surface area available for gas exchange. It can be measured either as a low diffusion capacity for carbon monoxide or as a decrease in oxygen saturation during exercise.

Elastic recoil of the lung can be determined by measuring pulmonary compliance. Almost all pediatric patients with emphysema have regional defects, however, and other regions of the lungs may exhibit restrictive processes. Compliance measured in the pulmonary function laboratory reflects the conflicting effects of these two opposing abnormalities. Because of this difficulty, and because measurement of pulmonary compliance requires swallowing an esophageal balloon, this test seldom is performed in children.

In clinical practice, useful tests for a patient with emphysema include a forced expiratory flow volume loop that typically shows a mild decrease in normal forced vital capacity, a moderate decrease in FEV₁, and a more severe decrease in flows at low lung volumes or forced expiratory flow between 25% and 75% of vital capacity. A component of bronchoconstriction may coexist, but the patient with emphysema has significant residual abnormalities, even after the administration of potent bronchodilators.

Measurements of lung volumes show an increase in functional residual capacity and residual volume. With severe emphysema, an increase in total lung capacity also is present. A test of the diffusion capacity for carbon monoxide can help to quantitate the diffusion defect. A progressive exercise stress test, with measurement of oxygen saturation, often demonstrates a significant decrease in oxygen saturation with moderate exercise.