Air trapping is an abnormal finding in the lungs, signifying that air is retained in the tiny air sacs after a person exhales. This condition is a sign of underlying pulmonary disease, often involving the smaller airways. Computed Tomography (CT) scanning has become the gold standard diagnostic tool for visualizing this phenomenon in detail. Understanding what air trapping looks like on a CT scan helps medical professionals identify and categorize various lung disorders.
The Physiological Mechanism of Air Trapping
Air trapping occurs when the flow of air out of the lungs is obstructed or significantly restricted, particularly during the expiratory phase of breathing. The problem originates within the small airways, which are the tiny tubes less than two millimeters in diameter that lead to the air sacs. These small airways can become narrowed due to inflammation, mucus plugging, or structural changes like remodeling.
During a normal breath, the airways widen slightly on inhalation and narrow slightly on exhalation, but air can still pass freely. When an obstruction is present, air can still be forced past the narrowed point during inhalation, acting like a one-way valve. However, the positive pressure generated during exhalation causes the compromised airway to collapse or narrow further. This collapse effectively seals the air in the distal lung tissue, preventing it from being fully expelled.
The continued retention of air leads to hyperinflation in the affected lung regions. This abnormal retention of gas increases the overall volume of air remaining in the lungs after a full breath out. Over time, this process contributes to an increased work of breathing and is the cause of the shortness of breath experienced by many patients.
How CT Scans Reveal Trapped Air
A standard CT scan taken during inhalation often fails to detect air trapping because the lungs are maximally inflated, momentarily masking any obstruction. To capture this phenomenon, an expiratory CT scan is performed, which images the lungs while the patient is actively breathing out. Comparing the inspiratory and expiratory images allows a direct assessment of how effectively lung regions deflate.
Normal lung tissue increases in density as air is expelled, appearing as a brighter, whiter area on the scan. Conversely, areas of air trapping appear as regions that fail to increase in density during exhalation, maintaining a persistently dark or low-attenuation appearance. Attenuation, in this context, refers to the degree to which X-ray beams are reduced as they pass through the tissue, with less dense air causing lower attenuation, or a darker image.
The visual hallmark of significant air trapping is often described as a mosaic attenuation pattern. This pattern represents a patchwork of lung tissue where some areas deflate normally and appear bright, while adjacent, diseased areas remain dark due to retained air. The trapped air causes the affected lung segments to hold their volume, creating a stark contrast against the surrounding, properly deflated lung tissue. The extent and distribution of this mosaic pattern are then quantified to determine the severity and location of the underlying small airway dysfunction.
Clinical Significance and Related Conditions
Identifying air trapping on a CT scan is a direct indication of small airway disease and holds significant weight in guiding a clinical diagnosis. The finding itself is not a disease but rather a sign of an underlying obstructive process. Its presence helps doctors distinguish between various types of lung conditions that may present with similar symptoms.
Air trapping is a common and defining feature of Chronic Obstructive Pulmonary Disease (COPD), which includes conditions like chronic bronchitis and emphysema. In COPD, the finding helps confirm the presence of small airway inflammation or the destruction of the elastic structures that support the airways. It is also frequently seen in individuals with asthma, particularly in cases where the disease is more severe or persistent, reflecting ongoing airway inflammation and remodeling.
Beyond these common disorders, air trapping can also be the sole imaging abnormality in certain types of small airway diseases, such as constrictive bronchiolitis. This condition, which can occur after infections or lung transplantation, involves scarring and narrowing of the smallest bronchioles. In such cases, the expiratory CT scan may be the most sensitive tool available, providing evidence of obstruction even when other lung tests are inconclusive. The extent of air trapping can also be used to monitor disease progression and assess a patient’s response to prescribed medical therapies.