The Validity of Maximal Diaphragm Thickening Fraction to Measure Diaphragm Function in Mechanically Ventilated Patients

This study is designed to determine whether maximal diaphragm thickening fraction measured by ultrasound during volitional maximal inspiratory efforts is a valid measure of diaphragm function in mechanically ventilated patients.

Inclusion Criteria: * Adult patients (≥ 18 years old) with acute respiratory failure receiving invasive mechanical ventilation via endotracheal tube or tracheostomy * Patients who meet readiness-to-wean criteria (triggering the ventilator; FiO2 ≤ 50%; PEEP ≤ 8 cm H20; hemodynamic stability) Exclusion Criteria: * Contraindication to phrenic nerve stimulation (chest tube; cardiac pacemaker or implanted defibrillator; cervical implants; use of neuromuscular blocking agents within the 24 hours preceding the first diaphragm assessment (with the exception of succinylcholine used during rapid-sequence induction of anaesthesia for intubation); cervical spine injury; known pregnancy) * Patients experiencing an acute exacerbation of an obstructive lung disease (chronic obstructive pulmonary disease (COPD), asthma, bronchiectasis)