The aim of the study is to assess the use feasibility of the FreeO2 system so as to deliver automatically oxygen in the post anesthesia care unit in a patient population admitted for major abdominal and thoracic surgery.

The investigators' hypothesis is that FreeO2 system will provide a better control of the oxygen saturation and reduce postoperative hypoxaemia.

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