Inclusion Criteria:

• Patient over 18 years of age;
• Patient hospitalized in intensive care for less than 24 hours;
• Patient with severe hypoxemic acute respiratory failure characterized by a PaO2/FiO2 ratio < 300 mmHg and > 150 mmHg under high-flow oxygen therapy at 60 L/min;
• Patient able to cooperate with the spirometry test;
• Patient able to follow the protocol and having given their informed written consent to participate in the study;
• Patient affiliated with the social security system or entitled to benefits;

Exclusion Criteria:

• Patients with an indication for intubation;
• Patients with a contraindication for or who refuse NIV;
• Patients with hemodynamic instability requiring vasopressor support greater than 0.25 μg/kg/min of norepinephrine;
• Patients with a Glasgow score < 15;
• Patients with hypercapnia (PaCO2 > 45 mmHg);
• Patients with a PaO2/FiO2 ratio < 150 mmHg;
• Patients with cardiogenic pulmonary edema;
• Patients who have already undergone a session of NIV in intensive care during this stay;
• Patients with chronic respiratory failure (home oxygen therapy or home non-invasive ventilation (excluding indications solely for sleep apnea syndrome));
• Patients who have recently had a pneumothorax;
• Patients who have recently undergone eye surgery (e.g., cataract surgery) or throat surgery;
• Patients with significant arterial disease: recent heart attack, pulmonary embolism, poorly controlled hypertension despite treatment, etc.
• Patients already included in the study;
• Pregnant women, women in labor, or women who are breastfeeding;
• Patients under guardianship, conservatorship, or deprived of their liberty;
• Patients under an activated future protection mandate;
• Patients under family authorization;
• Patients under judicial protection;