The main objective of this study is to show that "diaphragmatic excursion measures upon emergency admission" (CDA values) on patients with acute respiratory failure are predictive of the need to use mechanical ventilation (invasive or non-) in the first four hours.

The secondary objectives of this study are:

A-determine a prognostic threshold for diaphragmatic excursion measures able to predict the use of mechanical ventilation (invasive or not) in the first four hours.

B-to study the association between CDA values and persistence of respiratory distress during the first 4 hours of care among patients who, at 4 hours, are not mechanically ventilated.

C-to study the association between CDA values and respiratory re-exacerbation or a continuing need for mechanical ventilation during the 24 first hours after admission to the emergency room D-to study the association between diaphragmatic excursion measures made after initial emergency care (at 4 hours; =CDH4), and the persistence of respiratory distress at 4 hours, in patients who at 4 hours are not mechanically ventilated.

E-to study the association between CDH4 values and respiratory re-exacerbation during the 24 first hours after admission to the emergency room among patients who, at 4 hours, are not mechanically ventilated F-to study the association between changes in values of diaphragmatic excursion measure before and after initial emergency care (CDA → CDH4) and respiratory re-exacerbation during the 24 first hours after admission to the emergency room among patients who, at 4 hours, are not mechanically ventilated.

G-analyze patient outcomes by subgroup according to diagnosis (decompensated COPD, OAP, lung disease, asthma, pleural disease).

Inclusion Criteria:

• The patient has given informed consent (and signed the consent form) or, in case of an emergency situation, an investigator has committed to obtaining the consent of the patient as soon as his/her condition permits
• Patient affiliated with or beneficiary of a health insurance plan
• Acute Respiratory Distress (DRA) defined by: (1) respiratory rate > 25 and/or signs of struggle and hypoxia AND (2) SpO2 values < 90% and/or [pH < 7.35 and pCO2 > 6 kPa (45 mm Hg)]
• Breathing spontaneously (no ventilation)

Exclusion Criteria:

• Patient currently participating in or having participated in another interventional study in the previous three months, or patient in an exclusion period determined by a previous study
• Patient under judicial protection or any kind of guardianship
• Refusal to sign the consent
• Patient pregnant, parturient, or lactating
• Neurological or neuromuscular disease modifying the operation of the diaphragm without any decompensation (Polyneuropathy, Lou Gehrig's disease, Myasthenia ...)
• Patient who received mechanical ventilation at home
• Patient who received treatment by mechanical ventilation during the pre-hospitalization care phase
• Patient admitted with respiratory failure requiring immediate installation of a mechanical ventilation and thus not allowing the performance of two ultrasound measurements
• Patient suffering from a pneumothorax