Atelectasis

Atelectasis (collapse of the lung) is a collapsed / airless condition of the lung. It can be due to several reasons which will be explained later. It is usually associated with exchange of inadequately oxygenated blood from pulmonary arteries into veins - giving rise to hypoxia.

It may be caused by :
1. Obstruction of airways by mucous plugs
2. Hypoventilation (secondary to pain) / inadequate tidal volume*
3. Inadequate surfactant production
4. Compression of the lung by tumors, aneurysms, enlarged lymph nodes, etc.
5. Complications following abdominal / thoracic surgery
6. Accumulation of air / fluid in the pleural cavity - pneumothorax, hydrothorax

Various forms of atelectasis can be distinguished in adults on the basis of underlying mechanism or the distribution of alveolar collapse:

1. Resorption

It happens when there is an obstruction of airways which prevent the ventilation of distal part of lungs. The most frequent case of resorption atelectasis is obstruction of a bronchus by a mucous plugs. But any other kind of obstructions (aspiration of foreign bodies, blood clots during oral surgery etc.) may actually causes this type of lung's collapse.

2. Compression

Usually occurs due to compression of lung because of accumulation of fluids / air in the pleural cavity. Ascites also may cause the elevation of diaphragm that leads to lung's compression.

3. Microatelectasis (nonobstructive atelectasis)

Due to complex set of evens, the most important of which is loss of surfactant.

4. Contraction

Due to local fibrosis or general fibrosis of lung or pleura in which will lead to impairment of its elasticity.

*Tidal Volume - the volume of air inspired and expired in a normal breath.

Source : Taber's, Robin's etc.