Inclusion Criteria:  Patients aged over 18 years, admitted in an intensive care unit of the Peri-Operative Medicine Pole of the Clermont-Ferrand University Hospital, under invasive mechanical ventilation (intubation or tracheotomy), whose FiO2 required to obtain an SpO2 > 95% is ≤ 30%, equipped as part of routine care with an arterial catheter and a central venous catheter in the superior vena cava territory BMI < 30 kg.m-2 Written consent to participate in the study by the patient or by the trusted person, or the relative if he or she is present.  Exclusion Criteria:  Existence of an underlying pulmonary pathology that may alter oxygen diffusion or generate an intra-pulmonary shunt (COPD, emphysema, pneumopathy, ARDS, chronic respiratory failure...) Uncontrolled haemodynamic instability defined by an increase of more than 20% in catecholamine dosages during the last hour, despite optimisation of the blood volume according to a pre-established protocol (mini-fluid challenge) Patient already included in the study Patient's or relative's refusal to participate Guardianship or trusteeship patient No affiliation to the Social Security system