The word "atelectasis" is Greek in origin; It is a combination of the Greek words atelez (ateles) and ektasiz (ektasis) meaning "imperfect" and "expansion" respectively. It results from the partial or complete, reversible collapse of the small airways leading to an impaired exchange of CO2 and O2 - i.e., intrapulmonary shunt. The incidence of atelectasis in patient's undergoing general anesthesia is 90%.