At enrolment. Before the Sigh test, the investigators will anonymously collect patients' demographic information (e.g., age, sex, height, weight), past (e.g., hypertension, chronic medications) and recent (e.g., etiology of the acute respiratory failure, days since intubation) medical history, severity of lung injury (e.g., ventilation setting, arterial blood gases, respiratory system compliance, diagnosis of ARDS) and of systemic diseases (e.g., presence of shock, number of organs failure), ventilation settings (e.g., PEEP, FiO2, PSV level).