The investigators recently evaluated 4 different oximeters among the most commonly used with arterial catheter in place and compared SpO2 with SaO2 obtained on arterial gas. Correlations between SaO2 and SpO2 were poor for all oximeters, as previously known, and SpO2-SaO2 bias were different between oximeters. Some oximeters (Masimo, Nellcor) had lower biases but they detected less well hypoxemia. Some oximeters underestimated SaO2 (Nonin) but detected very well hypoxemia, and some overestimated SaO2 (Philips). The investigators concluded that oximeters provide different informations to clinicians, and oxygenation targets should take into account for these differences.

The assumption is that the SpO2 target AND oximeter used will both have an impact on oxygen flows and that these effects will add up. With a high SpO2 target, oxygen flows will be significantly greater and with the Nonin oximeter, the required flows will be greater than with the Philips oximeter.

NB: the results obtained were in a population with light skin pigmentation (96% of the patients were Fitzpatrick 1-2, reflecting the local hospitalized population).

The oximeters to be evaluated will be the Nonin (Plymouth, MN), and the Philips (Eindhoven, Netherlands). The sensors used will be digital reusable sensors for all oximeters. Because of the impact of the fingers used for SpO2 measurement, each oximeter will be randomly assigned a different finger to each patient (digit 2 and 3 on the hand opposite to the arterial catheter).

Two SpO2 targets will be assessed: 90% and 94%. A total of four 10-minute periods will be performed in randomized order: Nonin 90, Nonin 94, Philips 90, and Philips 94.

At the end of each 10-minute period, arterial gas will be collected through the arterial catheter.

The adjustment of the oxygen flow rates will be performed manually. The average oxygen flow of the last 2 minutes of the period will be considered.

Inclusion Criteria:

• ≥ 18 years of age
• Patients admitted to postoperative cardiac surgery intensive care unit
• Presence of an arterial catheter Specific for the extubate patient
• Ongoing on conventional oxygen therapy at moderate flow (up to a maximum of 5L/min) with SpO2 between 88 and 100% with the usual oximeter and nasal cannula.
• SpO2 < 92% ambient air Specific for the intubate patient FiO2 <= 0.60 with SpO2 between 88 and 100% with usual oximeter SpO2 < 92% with FiO2 0.21

Exclusion Criteria:

• No SpO2 signal with oximeter in use
• False nails or nail polish
• Methemoglobinemia >0.015 on last available arterial gas
• Patient in isolation (multi-resistant bacteria, C-Difficile, SARS-CoV-2 ...) Specific for the extubate patient
• Expected to use another respiratory support within one hour of inclusion (NIV or nasal high flow oxygen) Planned extubation within one hour of inclusion