Air trapping is a common phenomenon in respiratory health, characterized by the incomplete emptying of air from the lungs during exhalation. This condition occurs when air is retained in the furthest reaches of the lung structure, known as the distal airspaces, after a person attempts to breathe out fully. While a small amount of air naturally remains in the lungs, air trapping signifies an abnormal excess that can have significant effects on breathing mechanics. The presence of retained air is often a sign of underlying changes in the airways or the surrounding lung tissue.
Defining Air Trapping
Air trapping is clinically defined as the abnormal retention of gas in the lungs, specifically in the tiny air sacs called alveoli, following a maximal or forced expiration. This retention is a direct result of an obstruction or narrowing that limits airflow out of the lungs more severely than it limits airflow in. The trapped air causes the lungs to remain overinflated, a state known as pulmonary hyperinflation.
This overinflation is measured by looking at specific lung volumes. The volume of air that remains in the lungs after the most forceful exhalation possible is called the Residual Volume (RV). When air trapping occurs, the RV increases significantly beyond normal physiological levels. This increase in residual volume also results in an elevated Functional Residual Capacity (FRC), which is the amount of air left in the lungs after a normal, relaxed breath out. Operating at this higher resting volume makes it more difficult to take a full, deep breath and contributes to feelings of breathlessness.
The Physiological Mechanism
The physical process that causes air trapping involves a breakdown of the normal forces that govern exhalation. Expiration is typically a passive process driven by the natural elastic recoil of the lung tissue, but in disease states, this mechanism is disrupted. The problem lies in the small airways, which lack the rigid support of cartilage found in the larger bronchi.
During forced exhalation, the pressure surrounding the lungs, known as intrapleural pressure, increases significantly as the chest wall and diaphragm contract. If the small airways are already narrowed or poorly supported, this increased external pressure can cause them to collapse prematurely. This premature collapse creates a functional one-way valve.
Air can pass the obstruction during inhalation when the airways are pulled open, but the positive pressure of expiration seals the airways before the air can fully escape. The air becomes trapped behind this obstruction in the distal alveoli. In conditions where the tissue’s elastic recoil is lost, the airways also lose the radial traction—the outward pull of the surrounding tissue—that normally helps keep them open. This makes them more susceptible to dynamic compression and collapse during breathing out.
Common Conditions that Cause Air Trapping
Air trapping is a hallmark feature of various obstructive lung diseases, where the primary issue is resistance to airflow out of the lungs. Chronic Obstructive Pulmonary Disease (COPD) is the most common cause, encompassing conditions like emphysema and chronic bronchitis.
In emphysema, the destruction of the alveolar walls leads to a loss of the lung’s natural elastic recoil. This loss means there is less driving pressure to push air out, and the small airways, having lost structural support, collapse easily, leading to severe air trapping. Chronic bronchitis contributes to air trapping through persistent inflammation, swelling, and excessive mucus production within the airways. This physically narrows the airway lumen, increasing resistance and promoting dynamic compression during exhalation.
Other conditions that can lead to air trapping include bronchiolitis obliterans, which causes fibrosis and obstruction of the small airways, and cystic fibrosis, where thick, sticky mucus blocks the smaller bronchioles. Asthma, particularly severe or poorly controlled asthma, also causes air trapping due to the temporary constriction of the smooth muscle surrounding the airways (bronchospasm). In each case, the underlying mechanism is a physical or functional obstruction that allows air to enter the alveoli but prevents its complete removal.
Identifying Air Trapping
Physicians identify and quantify air trapping primarily through specialized diagnostic tests that measure lung volumes and visualize the pulmonary structure. Pulmonary Function Tests (PFTs) are the standard for numerical assessment, using techniques like body plethysmography or gas dilution to accurately measure the absolute lung volumes.
The key finding is an elevated Residual Volume (RV) and an increased ratio of the Residual Volume to the Total Lung Capacity (RV/TLC). A ratio significantly above the expected range indicates that a disproportionately large amount of air is being retained in the lungs after exhalation. This measurement confirms the presence and severity of air trapping, linking it directly to the mechanical obstruction in the small airways.
Imaging techniques, particularly High-Resolution Computed Tomography (HRCT) scans, provide a visual confirmation. To properly assess the condition, the CT scan must be performed during both full inspiration and full expiration. On the expiratory scan, areas of air trapping appear as regions of decreased attenuation, or darker spots, compared to the surrounding healthy lung tissue. This occurs because the trapped air prevents the affected lung segments from deflating and becoming denser.