Air entrapment can be considered abnormal if it affects lung volume over a segment of the lung and is not confined to the upper segment of the lower lobe or the apex of the tongue. Some air pockets are common in people with normal lungs. Air entrapment in normal subjects usually affects a small portion of the lung (<25% of the cross-sectional area of ​​the lung in one scan plane), usually the upper segment of the lower lobe, the anterior middle lobe, or the tongue, or involve dependent regions of individual lung lobules, particularly the lower lobe. Areas of reduced density and vascularity, and air entrapment, are abnormal if they affect lung volumes equal to or greater than portions of the lung and if they are not confined to the upper portion of the lower lobe or the vertex of the tongue. Intrathoracic air trapping results in a change in radiopacity and increased lung transparency. Air pockets are a relatively common condition. It can be localized or systemic, like chronic obstructive pulmonary disease. Air trapping can be difficult to detect on a routine chest x-ray, which is traditionally taken at full inspiration. Air pockets can only be seen on an expiratory chest x-ray. The affected lung or part of the lung becomes more transparent and the lung volume is altered. The overall size of the air-trapped lung may be the same, smaller, or larger than the contralateral normal lung on an inspiratory x-ray, but on exhalation, the area of ​​trapped air may be larger than its size. This leads to signs of hyperinflation of the part of the lung that traps air. Signs include displacement of the mediastinum away from the affected side and failure to elevate the ipsilateral hemidiaphragm. The pinched area may appear relatively translucent compared to the more normal lung. Air trapping can affect whole lungs, lobes, or segments. Alternatively, it can be patchy and occur at the leaflet level. Causes of air entrapment include obstruction of the central bronchi by intra- or extra-bronchial lesions such as tumors or foreign bodies. The airways or bronchi are usually large enough in diameter to allow air to enter during inspiration. However, air entrapment occurs when the bronchial caliber decreases during expiration. Air entrapment can occur distally to the point of complete airway obstruction. This allows collateral airflow to pass through intra-alveolar connections (such as foramen cones) and intra-acinary connections (such as Lambertian tubes) to allow air to move from normal parts of the alveoli to the distal lung. It's for Leave those congested areas inflated to fully congested airways. If the crack is imperfect, entangled air drift will occur. Regions of trapped air may appear more transparent on a standard inspiratory chest x-ray (i.e. total lung volume). This is because the area is relatively anemic and pulmonary blood flow is reduced due to alveolar hypoxia. Blood flow is diverted to more normal areas of the lungs. Areas with trapped air should be distinguished from other causes of radiolucent pulmonary or skin flaps. Other causes are technical factors, chest wall abnormalities, and pulmonary vascular disease. Technical factors that can lead to unilateral hyperpermeability include grid cutoff, x-ray beam misalignment, and anode healing.