Acute respiratory failure (ARF) is characterised by a discrepancy between load imposed on respiratory muscles and their capacity. Recently, diaphragmatic ultrasonography has been introduced in the clinical practice to evaluate diaphragmatic function. In particular, the investigators will focus on Diaphragmatic Displacement measured by M-mode ultrasonography. The aim of this study was to compare the diaphragmatic displacement with traditional weaning parameters in potentially ready to be extubated patients undergoing a spontaneous breathing trial (SBT).

Inclusion Criteria:

• mechanically ventilated since more than 24 hours
• clinical improvement of the underlying acute cause of the respiratory failure
• adequate cough reflex
• absence of excessive and/or purulent tracheobronchial secretion
• stable cardiovascular status
• stable metabolic status
• adequate pulmonary function
• adequate mentation

Exclusion Criteria:

• age <18 years
• pregnancy
• presence of thoracostomy, pneumothorax or pneumomediastinum
• presence of flail chest or rib fractures
• neuromuscular disease
• use of muscle-paralyzing agents within 48 hours before the study
• history or new detection of paralysis or paradoxical movement of a single hemi-diaphragm on diaphragm ultrasonography

Critically ill patients undergoing a spontaneous breathing trial