In patients with acute hypoxemic respiratory failure (AHRF), High Flow Nasal Therapy (HFNT) improves oxygenation, tolerance, and decreases work of breathing as compared to standard oxygen therapy by facemask.

The hypothesis is that this flow challenge (ROX index variation from 30 to 60L/min) could be used as a test for assessing changes in lung aeration, analyzed by the variation in end expiratory lung volume (ΔEELV), in patients treated with HFNC. It may allow to personalize the flow settings during HFNC. In this sense, an increase in EELV will be observed with higher flows in responders and, therefore, these participants may benefit from increasing the flow. In contrast, to increase the flow in non-responders (no significant increase in EELV with higher flows) increase the risk of patient self-inflicted lung injury (P-SILI).

Inclusion Criteria:

• Adult patient admitted to the ICU requiring NHF due to AHRF that will be defined as an SpO2/FIO2 ratio <315

Exclusion Criteria:

• Patient with indication for immediate CPAP, NIV, or invasive mechanical ventilation
• Hemodynamic instability defined as a need of continuous infusion of epinephrine or norepinephrine > 1 mg/h
• Severe acidosis (pH ≤ 7.25)
• Pregnant woman
• Tracheotomised patient
• Formalized ethical decision to withhold or withdraw life support
• Patient under guardianship
• Patient deprived of liberties
• Patient already enrolled in the present study in a previous episode of acute respiratory failure
• Patient who does not consent.