De novo acute hypoxemic respiratory failure (AHRF) is associated with high overall mortality, which increases significantly with the use of orotracheal intubation. High flow nasal canula (HFNC) has turned to be the first line non-invasive oxygenation strategy aiming to avoid intubation. One of the main factors worsening lung injury and increasing mortality in invasively ventilated patients is a too high tidal volume (TV) delivered by the ventilator. Consistent data suggest that such an aggravation of respiratory lesions may occur during spontaneous ventilation if TV is too large. This phenomenon is called Patient self-inflicted lung injury (P-SILI). The effect of TV on the outcome of patients with de novo AHRF under HFNC has never been evaluated since TV is not easily accessible in patients under HFNC. Investigators hypothesized that a large TV during HFNC has an impact on the outcome. TV will be measured using chest Electrical Impedance Tomography (EIT). To calibrate the EIT data, i.e. to be able to convert changes in thoracic impedance into TV, thoracic impedance signal, flow and volume will be collected during a 4 cmH2O continuous positive airway pressure (CPAP) test, using a pneumotachograph inserted on the ventilator circuit between the mask and the Y-piece. Such a level of CPAP is supposed to reproduce the majority of the physiological effects of HFNC. Thus, EIT signal can be used to calculate TV during HFNC since it remains reliable even when the positive expiratory pressure changes.

A secondary objective is to quantify a respiratory distress index. This quantification will be recorded by respiratory inductance plethysmography (RIP), obtained using two elastic bands equipped with a sensor sensitive to their stretching, one positioned at the level of the thorax, the other at the level of the abdomen. The stretching changes of the two bands during the respiratory cycle allow evaluating their possible asynchronism by calculating the phase angle Investigators want to be able to evaluate up to 6 predictors of HFNC failure in this research with an effect size of 0.15, α risk of 0.05, and a power of 0.8. A number of 55 participants is required. Investigators plan to include 60 patients due to potential withdrawal of consent and/or unusable data.

Inclusion Criteria:

• Age ≥ 18 years old
• HFNC initiated as part of the care
• Respiratory rate > 25 / minute
• PaO2/FiO2 ratio < 300 mm Hg
• PaCO2 < 45 mm Hg
• Affiliated with a social security system
• Informed consent signed by the patient, trusted person or family member if the patient is unable to consent

Exclusion Criteria:

• Acute cardiogenic pulmonary edema
• Underlying chronic respiratory disease
• Asthma exacerbation
• Chronic obstructive pulmonary disease Exacerbation
• Hemodynamic instability, defined as systolic arterial blood pressure < 90 mm Hg or mean arterial blood pressure < 65 mm Hg or the use of vasopressors
• Glasgow Coma Score <= 12
• Contraindication to CPAP (maxillofacial surgery, facial trauma)
• Refusal of the patient to perform the CPAP test
• Need for emergency intubation according to the clinician in charge of the patient
• Patient protected by law
• Pregnancy or breastfeeding woman

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