Automatic Oxygen Administration After Major Abdominal and Thoracic Surgery

Inclusion Criteria: * Preoperative anesthetic visit for major thoracic or abdominal surgery ( ARISCAT risk score ≥ 26) with general anesthesia * Patient consent Randomization Criteria: * Admission in post-anesthesia care unit after major thoracic or abdominal surgery * Randomization and device establishment within a time less than one hour after the endotracheal intubation * Availability of the prototype FreeO2 * Absence of criteria of gravity justifying immediately a different technique of ventilatory support: * Disturbance of consciousness with a Glasgow Coma Score ≤ 12 * Serious ventricular rhythm disorders * Hemodynamic instability (SBP \<80mmHg or recourse to vasopressors) * Cardiac or respiratory arrest * pH \< 7.35 and PaCO2 \> 55 mm Hg * Necessity of an oxygen flow less than 15 L / min to maintain a SpO2 higher than 92%. * Absence of necessity of a urgent surgery * Oxygen saturation measured by Spo2 sensor Exclusion Criteria: * BMI ≥ 35 kg/m2 * Obstructive sleep Apnea (with or without Mechanical therapy) * Emergency Surgery for life-threatening * Age \<18 years * Pregnant women, lactating * perturbed or non-cooperative patient