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).