Passive atelectasis, also known as relaxation atelectasis, refers to a form of lung atelectasis due to loss of the negative pressure state in the pleural space. With a loss of the negative intrapleural pressure, the lung is no longer held against the chest wall and is said to relax back to its normal volume. There may not be a significant increase in density from the atelectasis as perfusion of the affected lung also decreases.
The clinical presentation would depend on the extent of atelectasis and how quickly it develops. Small and gradually developing areas of atelectasis may be asymptomatic or present as a non-productive cough. Larger areas that develop more quickly can present with features of hypoxia and respiratory failure.
Physical examination of the lung may reveal diminished movement of the affected area, dullness on percussion, decreased or absent breath sounds, and deviation of the trachea to the affected site.
Causes of passive atelectasis may be grouped into three main categories: