The goal of this study is to learn about the respiratory mechanics in patients undergoing mechanical ventilation. The investigators can achieve this through the offline analysis of data provided by the ventilator. Within the field of respiratory mechanics, the study focuses particularly on the quantification of lung instability.

What does lung instability mean? By this definition, the investigators refer to the part of lung tissue that opens during inspiration and then collapses during the subsequent expiration. The more diseased the lung (for example, in the case of viral pneumonia), the greater the quantity of this tissue.

How is lung instability measured? In the context of the study analysis, lung instability is measured through the analysis of the low flow pressure-volume loop during ventilation. This graph illustrates how the volume of air in the lungs varies in response to the pressures applied by the ventilator during slow inflation and deflation phases. This maneuver is considered quick and safe and has been an integral part of our clinical practice for several years. Through this maneuver, investigators can examine the range of pressures provided by the ventilator during tidal ventilation. To assess lung instability, hysteresis is analyzed, which represents a distinctive characteristic of the pressure-volume loop. Greater hysteresis indicates a higher degree of lung instability.

During the study, investigators will record not only hysteresis but also classical respiratory mechanics parameters (for example, elastance of the respiratory system, i.e., how stiff the lung is), parameters regarding gas exchange (blood oxygen and carbon dioxide levels), biometric data (for example, height and weight), and imaging (CT scans, lung ultrasound, and electrical impedance tomography) to relate them to the degree of lung instability.

Inclusion Criteria:

• patients undergoing passive mechanical ventilation

Exclusion Criteria:

• hemodynamic instability
• confirmed intracranial hypertension

patients admitted to the reference intensive care unit requiring passive mechanical ventilation