Atelectasis in the perioperative patient. 
To report the impact of atelectasis on perioperative outcomes.  Atelectasis occurs in the dependent parts of the lungs of most patients who are anesthetized.  Development of atelectasis is associated with decreased lung compliance, impairment of oxygenation, increased pulmonary vascular resistance and development of lung injury.  Here, we examine the etiology, contributing factors, consequences, diagnosis and treatment of atelectasis.  Atelectasis describes the state of absent air in alveoli attributable to collapse, but recent findings suggest that alveoli are filled with foam and fluid.  It is now known that atelectasis plays an important role beyond abnormal gas exchange and that prevention or reversal of atelectasis in some populations of postoperative patients may improve outcome.  Atelectasis in the presence of preexisting lung disease or limited cardiopulmonary reserve may have significant consequences.  Increasing understanding of the underlying nature of atelectasis and its contribution to acute lung injury will improve our approach to the prevention and management of atelectasis.