The investigators aim to assess the effects of positive end-expiratory pressure (PEEP) on diaphragmatic activity evaluated through ultrasound in patients admitted to intensive care unit (ICU) for acute respiratory failure (ARF) assisted via invasive mechanical ventilation in assisted mode.

All adult critically ill patients admitted to intensive care unit (ICU) and undergoing invasive mechanical ventilation (IMV) for more than 24 hours with diagnosis of acute respiratory failure (ARF) will be screened.

The exclusion criteria will be : pregnancy, neuromuscular diseases, wounds or incision at the probe placement site, contraindications to diaphragmatic and abdominal wall unltrasound, contraindications to the placement of a specific nasogastric feeding tube (Electrical activity of the diaphragm catheter).

In all enrolled patients ventilated in assisted mode, diaphragmatic activity will be evaluated through ultrasound and electrical activity of the diaphragm (primary end point) at stepwise decreasing of PEEP level. Also, the activity of remaining respiratory muscles will be assessed through sonography at the same levels of PEEP (secondary end point) Across all study phases, patients will be sedated to assure a Richmond sedation Agitation Scale (RASS) score between 0 and -1.

The following data will be recorded for each positive end-expiratory pressure explored: dosage of sedative drugs, peripheral oxygen saturation (SpO2), inspired oxygen fraction (FiO2), inpiratory and expiratory tidal volume, respiratory rate, electrical activity of the diaphragm (Eadi) (optional data), lung aeration through ultrasound or electrical impedance tomography (optional), thickness of diaphragmatic, parasternal intercostal, external oblique, internal oblique and transversus abdominis, combined with the lung ultrasound score. Thickness is measured at both end-expiration and end-inspiration for each respiratory muscles, as indirect estimation of respiratory muscles effort, and calculated according to standard formula as follows: Thickening fraction (%) = (inspiratory thickness - expiratory thickness) / expiratory thickness * 100. All ultrasonographic measurements will be performed bedside with patients in semi-recument position, only on the right side , using a portable ultrasound machine equipped with a linear (7.5 - 12.0 MHz) or convex (2.0 - 4.0 MHz) probe. The same protocol will be carried out in case of non invasive ventilation application after extubation, reducing PEEP and inspiratory support

Inclusion Criteria:

• age > 18 years
• invasive mechanical ventilation > 48 hours

Exclusion Criteria:

• pregnancy
• neuromuscular disease
• wounds or incision at the probe placement site.

All adult patient admitted in intensive care unit for hypoxemic acute respiratory failure, undergoing invasive mechanical ventilation for more than 48 hours and, after this time, ventilated in Assisted Ventilation, i.e., pressure support ventilation (PSV)

single patients data on motivated request to principal investigator