- start of prone-positioning cycles before being enrolled in the study - spo2/fio2 <200 - need of high-flow oxygen therapy (hfnc) or need of non-invasive ventilation and/or cpap or indications for intubation and mechanical ventilation - relative or absolute contraindications for prone positioning (eg facial fractures, advanced pregnancy, mental status alterations, spinal instability...)

- start of prone-positioning cycles before being enrolled in the study - spo2/fio2 <200 - need of high-flow oxygen therapy (hfnc) or need of non-invasive ventilation and/or cpap or indications for intubation and mechanical ventilation - relative or absolute contraindications for prone positioning (eg facial fractures, advanced pregnancy, mental status alterations, spinal instability...)