Air Trapping - Causes, Symptoms, Diagnosis, and Treatment

Introduction

Air trapping is a common condition that refers to the retention of air in all or a part of the lung during expiration, as complete expiration becomes difficult. Air trapping is also referred to as gas trapping. This occurs due to complete or partial airway obstruction or abnormalities in pulmonary compliance. It can also occur in normal individuals.

What Is Air Trapping?

Air that is breathed in during inhalation enters the lungs, and oxygen moves to the blood. The carbon dioxide from the blood enters the lungs and is breathed out during exhalation. This is the process of respiration. Air trapping or gas trapping is the air retention in the lungs during exhalation because complete exhalation of air becomes difficult. Air trapping is the inability to completely exhale the air during exhalation. This is a common condition that is observed during radiographic examinations. It can be localized or generalized air trapping.

What Are the Causes of Air Trapping?

Various factors can cause air trapping. Small airway diseases usually cause it. Air trapping can occur alone or in association with interstitial lung disease, bronchiectasis, or tree-in-bud opacity. The following causes are associated with air trapping:

Air trapping occurs in isolation in the following conditions:

• Chronic bronchitis.

• Obliterative bronchiolitis.

• Asthma.

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Air trapping occurs in association with bronchiectasis in the following conditions:

• Non-tuberculous mycobacterial infection.

• Cystic fibrosis.

• Idiopathic bronchiectasis.

Air trapping occurs in association with interstitial lung disease in the following conditions:

• Sarcoidosis.

• Hypersensitivity pneumonitis.

Other causes associated with air trapping include the following:

• High BMI (body mass index) or obesity causes air trapping due to reduced chest wall compliance which causes a rapid, shallow breathing pattern.

• Pulmonary tumor is seen in the neuroendocrine cell proliferation spectrum and diffuse idiopathic pulmonary cell hyperplasia (DIPNECH).

• Severe scoliosis.

• Vasculitides conditions like granulomatosis with polyangiitis and eosinophilic granulomatosis with polyangiitis.

How Does Air Trapping Occur?

Air trapping occurs as a result of partial or complete airway obstruction or due to abnormalities in pulmonary compliance. The lungs are composed of millions of air sacs that are called alveoli. The alveoli are the common site of airflow obstruction. These air sacs are elastic in nature and stretchable. The air sacs expand like balloons during inhalation. The alveoli later spring back to their original size during exhalation. The walls of the air sacs are damaged, and their stretchability is lost in airway diseases, and they do not completely empty the air during breathing. Airway diseases cause inflammation, narrowing of the airways, and production of inflammatory exudates in the small airways. A combination of airway inflammation, airway narrowing, and mucus plugging block a part or all of the lungs and lead to air trapping. The loss of elastic recoil of the lungs and destruction of alveolar support in airway disease causes airflow obstruction. The airflow obstruction causes progressive trapping of air during exhalation, as breathing out becomes difficult.

What Are the Symptoms Associated With Air Trapping?

Air trapping usually does not cause any symptoms. If present, the following symptoms are associated with air trapping:

• Shortness of breath.

• Sudden sharp, stabbing chest pain.

• Tightness in the chest.

• Fatigue.

What Are the Complications Associated With Air Trapping?

• Air trapping causes hyperinflation of the lungs due to the retention of air in the lungs. This limits the inhalation of air and reduces oxygen availability. Air trapping results in hyperinflation at rest and dynamic hyperinflation during exercise. Hyperinflation increases the carbon dioxide levels and also flattens the diaphragm muscle.

• Air trapping decreases the stretching effect of the alveoli, compresses the capillaries and the arterioles, and decreases pulmonary blood flow. It also causes lung collapse (atelectasis) as the elastic recoil of the lung tissues is lost.

• Air trapping indicates poorly aerated lungs. It causes complications in smokers who dive. The lung volume collapses and forces air into the poorly aerated regions of the lungs during diving. The air-trapped regions of the lungs expand when the individual arises from a deep depth. This puts pressure on the lung tissues resulting in the rupture of the lung tissues.

How Is Air Trapping Diagnosed?

Air trapping is a sign that indicates illness, and it is not a diagnosis. It helps to arrive at a differential diagnosis. The following tests are performed to diagnose air trapping:

• X-Ray - A chest X-ray is taken to view if the lungs are filled with air. Hyperinflation causes the diaphragm muscle to flatten. This can be identified on a chest X-ray.

• CT (Computed Tomography) scan - A CT scan is performed to visualize a detailed image. Air trapping is an imaging sign that is observed as a mosaic attenuation. It shows the presence of decreased attenuation of pulmonary parenchyma, and it is manifested as less than normally increased attenuation during expiratory acquisition. It shows the presence of a hyperlucent appearance on exhalation because the diseased portion of the lung has lost connective tissue recoil. The healthy portions appear dense due to lung collapse. The hyperlucent appearance of retained gas helps in the diagnosis of air trapping.

• Pulmonary Function Tests - Spirometry test is performed to evaluate the exhaled volume of air. It shows the presence of elevated residual volume, and forced expiratory volume is also measured to evaluate the volume of the retained air.

How Is Air Trapping Treated?

Air trapping is managed with medications like inhaled corticosteroids to decrease air trapping. Bronchodilators are given to open the airways and reverse hyperinflation to allow the trapped air to escape. Large-diameter endotracheal tubes, bronchodilators, long expiratory times, short inspiratory times, sedatives, and lower respiratory rates are used in mechanically ventilated patients to avoid dynamic hyperinflation caused by air trapping.

Conclusion

Air trapping is an imaging sign that is usually diagnosed on a CT scan as an incidental finding. It indicates a poorly aerated lung but is a clinically benign condition on its own. It can occur in isolation, or it can be associated with airway diseases. It usually affects the alveoli of the lungs. It is caused by airway obstruction or changes in pulmonary compliance. It usually does not present any symptoms. However, it is important to consult the doctor on experiencing symptoms like shortness of breath or chest pain.