Currently, measurement of transdiaphragmatic pressure (Pdi) using oesophageal and gastric balloons is the gold standard for the assessment of diaphragmatic effort. This technique is relatively invasive and its interpretation may be complex. The diaphragmatic longitudinal strain (LSdi) and strain rate (LSRdi) might provide additional information in the assessment of diaphragmatic effort and movement during SBT, allowing early detection of diaphragmatic dysfunction. Patients will be monitored during a 30-120 minutes SBT consisting of no assistance on the ventilator using CPAP with a pressure level of 0 cmH2O. Parameters to evaluate diaphragm function will include diaphragmatic strain (LSdi and LSRdi), diaphragmatic thickening fraction (TFdi), and airway occlusion pressure (ΔP0.1 and ΔPocc). These parameters will be measured immediately before ('baseline') the SBT, as well as 2 minutes ('early' assessment), 15 ('intermediate' assessment) and 30 minutes ('late' assessment) after the beginning of the SBT.

Inclusion Criteria:

• Consecutive adult patients (≥18 years old) undergoing mechanical ventilation for at least 24h (regardless of the reason of intubation)
• patients fulfilling criteria for an SBT
• patients will have to be able to trigger ventilator breaths with a reasonable level of assistance,
• patients with lack of severe impairment in gas exchange,
• patients that not require significant hemodynamic support.
• Patients will be recruited by the study coordinator following completion of informed consent from the patient or legally authorized substitute decision maker.

Exclusion Criteria:

• Use of pressure support during the SBT.
• Severe hemodynamic instability (>30% increase in vasopressors in the last 6 hours or norepinephrine > 0.5 µg/kg/min).
• Severe or very severe chronic obstructive pulmonary disease (COPD) according to the GOLD criteria (stage III: FEV1 30-50% predicted; stage IV: FEV1 < 30% predicted).
• Known or suspected severe or progressive neuromuscular disorder likely to result in prolonged or chronic ventilator dependence
• Clinical judgement of the attending physician.

Consecutive adult patients (≥18 years old) undergoing mechanical ventilation for at least 24h (regardless of the reason of intubation) and fulfilling criteria for an SBT will be included in the study. Briefly, to be eligible for an SBT, patients will have to (i) be able to trigger ventilator breaths with a reasonable level of assistance, (ii) lack of severe impairment in gas exchange, and (iii) not require significant hemodynamic support. Patients will be recruited by the study coordinator following completion of informed consent from the patient or legally authorized substitute decision maker.