This study is designed to evaluate the relationship between diaphragm activity during mechanical ventilation and the development of ventilator-induced diaphragm dysfunction (VIDD). Diaphragm structure, activity, and function are monitored longitudinally over the first 7 days of mechanical ventilation.

Multiple factors are responsible for injury to the diaphragm during mechanical ventilation and critical illness. Suppression of respiratory drive and respiratory muscle activity can contribute to diaphragm dysfunction and increase the risk of prolonged mechanical ventilation.

The objective of this study is to determine whether diaphragm inactivity or injurious loading of hte diaphragm during mechanical ventilation leads to the development of diaphragm weakness. To address this question, we are monitoring the diaphragm longitudinally over the first week of mechanical ventilation.

Diaphragm activity is monitored by measurements of transdiaphragmatic pressure and diaphragm electrical activity. Diaphragm function is assessed by maximal transdiaphragmatic pressure generation and by measurements of diaphragm neuromuscular coupling. Diaphragm structure is assessed by daily ultrasound imaging.

Study measurments commence at enrolment (within 36 hours of intubation) and continue for the first 7 days of the study (or until extubation or death, if earlier).

Inclusion Criteria:

• Adult patients (>18) with acute respiratory failure receiving invasive mechanical ventilation for less than 36 hours diagnosed with any of the following conditions: acute severe brain injury; moderate or severe ARDS; septic shock; pneumonia

Exclusion Criteria:

• Predicted probability of remaining alive and on the ventilator on ICU day 7 is less than 50%
• Liberation from mechanical ventilation is expected/planned within 24 hours
• High cervical spine injury (C5 or higher)
• Receiving mechanical ventilation for neuromuscular disease
• Acute exacerbation of an obstructive lung disease
• Known esophageal varices or any other condition for which the attending physician deems an orogastric catheter to be unsafe
• Esophageal, gastric or duodenal surgical procedures within the last 6 months
• Received mechanical ventilation for > 48 hours in the preceding 6 months

Critically ill patients who are receiving invasive mechanical ventilation for less than 36 hours.