Inclusion Criteria:  Age >18 years Admitted to the intensive care unit (ICU); Invasive mechanical ventilation; Pharmacological sedation; Mean arterial pressure (MAP)<65 mmHg or need of fluids infusion or any vasopressor in order to keep MAP > 65 mmHg No contraindication to Semirecumbent or Trendelenburg position; Ongoing monitoring: invasive and non-invasive arterial blood pressure; central venous line with central venous pressure (CVP) monitoring Naso-gastric tube in situ Indwelling bladder catheter Trendelenburg or passive leg raising test responder Consent according to local ethical committee rules  Exclusion Criteria:  COPD gold>3, Obesity>grade 2 Severe hypoxemia (P/F <100) which can be worsened by diaphragm rise Documented or suspected increased intracranial pressure, based on medical history or actual clinical condition (es. intracranial tumor, cerebral hemorrhage, encephalitis,...); Intra-abdominal hypertension > 2 grade (>20 mmHg) Full stomach pyloric incontinence; Gastric stasis, defined as aspiration from the NG tube of fecal, bloody or green fluid >100mL; Documented or suspected increased intraocular pressure (any degree of glaucoma); No central line inserted or femoral central line only Known diaphragm lesions or hiatus hernia Ischemia of the lower limbs; Upper body fracture; Demand of specific postures after Trauma; Patients who are not able to be investigated with a leg raising test, such as the lower extremities fractures, backbone fractures or backbone pain or deformity, and those patients with large cannulas in the femoral vessels… Any device which, according to the clinician, makes it unfeasible or unsafe to put patients in the Trendelenburg position. Mechanical Circulatory Support; (CHD) (Gleen, Fontaine); Advanced right ventricular dysfunction or advanced cardiac failure, in which volume overload worsens cardiac function (plateau stage in the Frank-Starling curve); Patients planned to be pronated within 6 h; Drainage in thoracic cavity; Upper gastrointestinal bleeding. Ongoing enteral nutrition