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.

Classically, assessing diaphragm function requires direct measurements of transdaphragmatic pressure generation. A critical requirement for valid measurements is a maximal volitional patient effort. An alternative is to standardize the stimulus to the diaphragm using magnetic twitch stimulation fo the phrenic nerve. This is the gold standard technique for diaphragm function measurement in the respiratory physiology laboratory.

Twitch measurements require technical expertise and expensive equipment; this technique is therefore basically limited to the laboratory. Bedside ultrasonography has been proposed as a method for measuring diaphragm function by assessing the thickening of the muscle during a maximal inspiratory effort. In order to achieve a maximal volitional effort in mechanically ventilated patients, several methods may be employed: coached maximal efforts, coached sniffing, and transient airway occlusion to stimulate respiratory drive (Marini maneuver).

The investigators are evaluating the validity of diaphragm functional assessment using bedside ultrasound in combination with coached efforts/sniffing/Marini maneuver against the gold standard technique: twitch transdiaphragmatic pressure.

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)

Critically ill patients with acute respiratory failure requiring invasive mechanical ventilation who meet readiness-to-wean criteria.

Currently, there is no plan to make data available to other researchers.