The aim of this study is to find out relationship between PaO2/FiO2 and atelectasis finding on intraoperative lung ultrasound.

Patients aged 20~65 with ASA class (American Society of Anesthesiologists physical status classification) 1 or 2 who are scheduled to undergo general anesthesia by using endotracheal intubation and arterial line cannulation will participate in this study. The patients with abnormal chest x-ray finding, pregnancy, arrhythmia, ischemic heart disease will be excluded from this study as well as the patients who are scheduled to undergo lung or heart surgery.

After 5 minutes preoxygenation, patients will be monitored with pulse oxymetry, noninvasive blood pressure. General anesthesia will be proceeded with typical method and endotracheal intubation and arterial cannulation will be proceeded. After ventilator adjustment (tidal volume as 8 times of the patient's ideal body weight), airway pressures(peak and plateau) will be recorded. Subsequently, arterial blood sampling will be proceeded while examination of the atelectasis findings on lung ultrasound before surgery. Lung ultrasound examination will be proceeded on 6 sections of patient's basal segments of the lung, including anterior, lateral, posterior segment. Juxtapleural consolidation, loss of A lines, presence of B lines, air bronchogram, loss of sliding sign and presence of pulse sign on lung ultrasound finding will be investigated and recorded.

Same ultrasound examination and arterial blood sampling will be followed after surgery ends.

The results of PaO2/FiO2 on arterial blood samplings will be compared with findings on lung ultrasound before and after surgery.

Inclusion Criteria:

• Patients aged 20~65
• with ASA class 1 or 2
• who are scheduled to undergo general anesthesia by using endotracheal intubation and arterial line cannulation

Exclusion Criteria:

• The patients with abnormal chest x-ray finding,
• pregnancy,
• arrhythmia,
• ischemic heart disease