Inclusion Criteria:  Patients treated by high-flow nasal canula oxygen Positive COVID-19 real-time reverse transcriptase-polymerase chain reaction assay in nasal swabs  Persistent severe hypoxemia after awake prone positioning defined by one or more of the following criteria:  SpO2 < 96% with FiO2 > 80% PaO2/FiO2 ratio < 100 with FiO2 > 80% Patients treated by Almitrine  Exclusion Criteria:  Age <18 years old and pregnant women Patients under legal protection Patients with a do not intubate or do not resuscitate decision Patients requiring immediate invasive mechanical ventilation Contraindication to Almitrine Poor echogenicity