From the Guidelines
Atelectasis appears opaque on imaging because collapsed lung tissue becomes denser than normal aerated lung, as a result of alveolar collapse generated by inflammatory lung edema, impairment of chest wall movement, and surfactant deficiency, as noted in studies on respiratory support in patients with acute respiratory distress syndrome 1. When a lung segment collapses, air is removed from the alveoli, causing the tissue to become more compact and concentrated in a smaller space. This increased density of lung tissue per unit area results in greater x-ray attenuation, creating a white or opaque appearance on chest radiographs. The key factors contributing to this opacity include:
- Inflammatory lung edema
- Impairment of chest wall movement
- Surfactant deficiency, all of which can lead to alveolar collapse, as discussed in the context of respiratory support in patients with acute respiratory distress syndrome 1. The opacity is proportional to the extent of collapse - complete atelectasis of a lobe appears as a dense white area, while partial atelectasis shows less intense opacification. Additionally, surrounding structures may shift toward the collapsed area due to volume loss, further contributing to the radiographic appearance. This density change reflects the fundamental principle that x-rays pass more easily through air-filled structures than through solid or fluid-filled tissues, making the air-depleted collapsed lung appear whiter than normal aerated lung tissue.
From the Research
Causes of Opacity in Atelectasis
- Atelectasis is a state of collapsed and non-aerated region of the lung parenchyma, which can occur due to various mechanisms such as airway obstruction, compression of parenchyma, and increased surface tension in alveoli and bronchioli 2
- The opacity in atelectasis can be attributed to the collapse of lung tissue, which prevents air from entering the affected area, resulting in a dense appearance on chest radiographs 2
- Different types of atelectasis, such as resorption atelectasis, passive atelectasis, compressive atelectasis, and adhesive atelectasis, can also contribute to the opacity seen in this condition 3
Diagnostic Challenges
- Differentiation from lobar consolidation can be a clinical dilemma, and chest radiographs using both anterior-posterior and lateral projections are necessary to document the presence of atelectasis 2
- Examination of air-bronchograms on a chest radiograph may be helpful to determine whether proximal or distal airway obstruction is involved 3
Treatment Implications
- The treatment of atelectasis varies depending on the duration and severity of the causal disease, and may include chest physiotherapy, postural drainage, bronchodilator and anti-inflammatory therapy, and removal of persistent mucous plugs by bronchoscopy 2
- In some cases, positive end-expiratory pressure might be a useful adjunct to treatment, particularly in passive and adhesive atelectasis 3