Atelectasis is also called collapse or volume loss.  An entire lung, a lobe, a segment or a subsegment may collapse, often showing characteristic radiographic findings, including abnormal lines or increased density in the involved part of the lung.  When collapse of an entire lung is present there is displacement of the mediastinum and diaphragm towards the involved lung unless either or both of these structures are fixed in position.

With lobar atelectasis the mediastinal and diaphragmatic shift is less marked, but characteristic displacement of fissures and the hilum is seen.  Careful observation will frequently show increased lucency of the remaining overexpanded lung on the involved side, with spreading of the vasculature.

Causes of atelectasis include

1. for total lung, lobar or segmental atelectasis:  neoplasm, foreign body, misplaced endotracheal tube, secretions, mucous plugs, extrinsically compressing lymph nodes, scarring, blood clots, broncholiths.
2. Subsegmental (discoid, platelike): secretions, splinting for any reason, particularly chest or abdominal pain, pulmonary embolism.

When the lung fields are symmetric, a frequent problem is the inability to decide whether the underexpanded lung is abnormal or whether the contralateral overexpanded lung is abnormal (emphysematous).  The decision can most easily be made by obtaining a film in full expiration.  The mediastinum will swing towards the normal lung regardless of the position of the mediastinum on the inspiratory film.  Such a swing is explained by the fact that the normal lung dispels the most air during expiration regardless of which lung is abnormally ventilated.

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