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The role of trunk inclination on respiratory function has been explored in patients with "typical" Acute Respiratory Distress Syndrome (ARDS) (1-3). Data regarding patients with COVID-19-associated ARDS (C-ARDS) are currently lacking.Aim of our study was to assess the effects of changes in trunk inclination on lung mechanics and gas exchange in mechanically ventilated patients with C-ARDS. MethodsThis single-center physiological cross-over study (ethical committee approval #70-11022021) was conducted on adult patients admitted to our COVID-ICU between March 3 and May 4, 2021. Diagnosis of C-ARDS, deep sedation, paralysis, and volume-controlled mechanical ventilation, were the inclusion criteria. Contraindications to mobilization (e.g., intracranial hypertension, spinal cord injury, tracheal lesions) and pregnancy constituted exclusion criteria. Patients were enrolled according to study personnel availability. A 5-Fr esophageal balloon (CooperSurgical, Trumbull, Connecticut) was inserted. The balloon was inflated with 1 ml of air and the correct position/function was verified before each measurement (4).Mechanical ventilation parameters, kept constant throughout the study, were set by the attending physician. Usually, PEEP is set according to the best respiratory system compliance (C RS ) assessed with a recruitment maneuver followed by a decremental PEEP trial. Of note, trunk inclination during PEEP selection is not standardized.Patients underwent three 15-minute steps in which trunk inclination was changed from 40° (semi-recumbent, baseline) to 0° (supine-flat), and back to 40° during the last step.At the end of each step, partitioned respiratory mechanics, arterial/central venous blood gas analysis and basic hemodynamics were recorded. Ventilatory ratio was calculated.

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