Atelectasis describes the loss of lung volume due to the collapse of lung tissue. It can be classified according to the pathophysiologic mechanism (eg, compressive atelectasis), the amount of lung involved (eg, lobar, segmental, or subsegmental atelectasis), or the location (ie, specific lobe or segment location).
The classification of atelectasis according to the pathophysiologic mechanism is reviewed here, as are the mechanisms of each type of atelectasis. Radiologic manifestations of atelectasis are described separately. (See "Radiologic patterns of lobar atelectasis".)
TYPES OF ATELECTASIS
Atelectasis can be divided pathophysiologically into obstructive and nonobstructive atelectasis.
Obstructive atelectasis — Obstructive (ie, resorptive) atelectasis is a consequence of blockage of an airway . Air retained distal to the occlusion is resorbed from nonventilated alveoli, causing the affected regions to become totally airless and then collapse. The rate at which this occurs depends upon several factors, particularly the amount of collateral ventilation and the composition of inspired gas.
Obstruction of a segmental bronchus is less likely to result in segmental atelectasis than obstruction of a lobar bronchus is to produce lobar atelectasis. This difference is the consequence of collateral ventilation between segments within a lobe. Collateral ventilation occurs via three distinct channels (the pores of Kohn, canals of Lambert, and fenestrations of Boren) and all of these channels must be obliterated for obstructive atelectasis to occur. Such collateral ventilation does not occur between lobes, unless the interlobar fissures are incomplete (which is the case in ≥50 percent of normal persons) . Of note, collateral ventilation is age-dependent: