Inclusion Criteria:  Adults above 18 Planned for a major abdominal surgery under general anesthesia Under mechanical ventilation Presence of a central venous and arterial lines allowing transpulmonary thermodilution cardiac output measurement Patient's consent with a social insurance  Exclusion Criteria:  Do not consent to participate History of heart attack, arteriopathy or aneurysm Contraindication to use transpulmonary thermodilution to measure cardiac output : coagulopathy cardiac arrythmia presence of pace-maker or defibrillator severe valvulopathy Patients with Acute Respiratory Distress Syndrome (according to Berlin criteria) History of arterial hypertension (treated or not) History of seizure, chronic headache, asthma or heart failure Left Ventricular Ejection Fraction (LVEF) < 45% or right ventricular dysfunction History of pulmonary lobectomy or surgery History of restrictive or obstructive pulmonary disease Body Mass Index (BMI) < 15 or > 40 kg/m² Pregnancy Known allergy to vasopressin Patients under protection of justice (guardianship, curators...)