High frequency nasal cannula (HFNC) provides an inspiration O2 fraction of 21-1% and it has a minimal dead space and can be well tolerated due to humidified air. Group I patients will be applied HFNC before and after bariatric surgery, Group II patients will be applied a constant O2 concentration via face mask. Oxygen pressure, oxygen saturation, carbon dioxide, respiration frequency, forced vital capacity will be measured before and after surgery.

Morbid obese patients are prone to atelectasis and hypoxemia after bariatric surgery. High frequency nasal cannula (HFNC) provides an inspiration O2 fraction of 21-1% and it has a minimal dead space and can be well tolerated due to humidified air. It provides 5-7 cm H2O positive end expiratory pressure to the airways and may prevent atelectasis. In the preoperative room, forced vital capacity, peripheral oxygen saturation (SpO2), respiratory frequency, heart rate (HR), blood pressure (BP), carbon dioxide, oxygen pressure and carbon dioxide partial pressure values in blood gas analysis will be recorded. We will apply HFNC in the preoperative and postoperative period to Group I patients, while applying a constant O2 concentration to Group II patients. After appliying HFNC for two hours in Group I patients and O2 via face mask in Group II patients measurements will be repeated. During surgery, similar mechanical ventilation parameters will be applied to the patients. After the operation Group I patients will be applied HFNC, while Group II patients will be applied O2 via face mask in the recovery room for two hours. Measurements will be repeated. Primary end point of the study is the association between oxygen pressure, carbon dioxide partial pressure values among preoperative before and after HFNC and O2 and postoperative times.

Inclusion Criteria:

• Patients with a body mass index of 40-55 kg/m2
• American Society of Anesthesiologists score II-III
• Patients undergoing laparoscopic sleeve gastrectomy or one anastomosis gastric bypass

Exclusion Criteria:

• Obstructive sleep apnea syndrome
• Using a non invasive mechanical ventilation device preoperatively
• Patients whoo will predicted to postanesthesia care unit admission postoperatively

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