The ORI™ or Oxygen Reserve Index (Masimo, Irvine, CA, USA) is a non-invasive monitoring system for measuring oxygen reserve. It is a digital sensor (almost identical to the SpO2 sensor) which allows an analysis of the absorption of arterial, venous and capillary components. The measured index, unitless, ranges from 0.00 to 1.00 for moderate hyperoxia levels: from 100 to 200 mmHg. It can alert the clinician to a drop in oxygen stores via the drop in SvO2 before a drop in SpO2 is observed. We propose to carry out a study to elucidate correlation between ORI™ and PaO2.

Pulse oximetry or SpO2 is the standard and mandatory monitoring of oxygenation during mechanical ventilation of intensive care and operating theatre patients. Its use is based on the difference in infrared and red light absorptions of oxyhaemoglobin and reduced haemoglobin. The calculation of the absorption percentage takes into account the pulsatility of the signal, reflecting the arterial component, while eliminating the continuous signal, reflecting the venous component. It is then considered that SpO2 is a reflection of SaO2, or arterial oxygen saturation. Oxygen exists in two forms in the arterial circulation: dissolved (PaO2) or bound to haemoglobin (SaO2).

Because of the sigmoidal shape of the oxyhaemoglobin dissociation curve, SpO2 is a late marker of arterial hypoxaemia. Indeed, SpO2 only starts to decrease after a marked drop in PaO2.

Hypoxaemia is a frequent situation, both in the operating theatre during the period of securing the airway, intra-operatively, or post-operatively, after extubation, on episodes of alveolar hypoventilation, such as atelectasis. In intensive care units, it can occur in injured lungs with various aetiologies (infectious, inflammatory, cardiac, etc). It is an independent predictive factor of mortality.

Exposure of patients to high hyperoxia (FiO2>0.7), over a long period of time, can lead to pulmonary endothelial damage (due to the formation of reactive oxygen species, ROS), denitrogenation atelectasis, and possibly a systemic inflammatory cascade. It is currently suggested that hyperoxia may also have a haemodynamic impact, with a fall in cardiac output and peripheral vasoconstriction, particularly in healthy volunteers and patients with cardiac decompensation. However, hyperoxia is a frequent situation during the perioperative period as it provides safety and a potential oxygen reserve in case of adverse events: hemodynamic degradation, cardiac arrest, extubation...

The ORI™ or Oxygen Reserve Index (Masimo, Irvine, CA, USA) is a non-invasive monitoring system for measuring oxygen reserve. It is a digital sensor (almost identical to the SpO2 sensor) which allows an analysis of the absorption of arterial, venous and capillary components. The measured index, unitless, ranges from 0.00 to 1.00 for moderate hyperoxia levels: from 100 to 200 mmHg. It can alert the clinician to a drop in oxygen stores via the drop in SvO2 before a drop in SpO2 is observed.

There is little literature on ORI™. A previous study showed a poor correlation between ORI™ (ranging from 0.24 to 0.55) and PaO2, which ranged from 100 to 150 mmHg, respectively. The company, Masimo, has recently made a change in its algorithm, allowing it to re-calibrate its sensor. However, no new correlation research has been conducted. We therefore propose to carry out a new test phase.

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

FiO2 steps will be conducted by study investigator. Participants, care providers and outcomes assessor will remain blinded of FiO2 steps.