Atelectasis areas in the positive end-expiratory pressure (PEEP) group and the zero positive end-expiratory pressure (ZEEP) group as expressed by percentages of total lung area. The data are presented as the median, interquartile range (box), and range (whiskers), except for one observation that was considered an outlier (subject no. 3 in the PEEP group), P = 0.002. Computed tomography scans to study atelectasis sizes were performed 5 to 10 mm above the right diaphragm dome at the end of surgery.