“…6 In Iran, the incidence of atelectasis has been reported to be as high as 75% and 56.7% following thoracic 7 and coronary artery bypass surgeries 8 , respectively. Atelectasis is the collapse of a group of alveoli, a small lobule or larger lung units, which can be caused by a variety of factors following surgery (specifically thoracic surgery) including external cardiopulmonary bypass, sedative medications 9,10 , anesthesia 3,10,11 , respiratory muscle dysfunction [11][12][13] , pain from the sternotomy, pain from the chest drains 10,[14][15][16] , impaired phrenic nerve function, and diaphragmatic dysfunction. 5,11,17,18 There may be no significant clinical symptoms of post-surgical atelectasis.…”