Ultrasound to Evaluate Failure of Liberation From Mechanical Ventilation

This study will evaluate the heart, lungs and diaphragm of patients who fail a trial of spontaneous breathing to determine the physiological mechanism of weaning failure.

Inclusion Criteria: * patients over 18 years of age receiving invasive mechanical ventilation for at least 24 hours * patients who fulfill standard criteria for readiness for a trial of spontaneous breathing (no or minimal sedation, patient awake and interactive; minimal vasopressor/inotrope requirements (Norepinephrine \< or = 0.1, Dopamine \< 10, Vasopressin/Epinephrine off); fraction of inspired oxygen \< or = 50%; spontaneous inspiratory efforts as indicated by patient-triggered breaths; positive end-expiratory pressure \< or = 10 cm H20) * patients who fail a trial of spontaneous breathing (transient reduction of ventilator support for any duration up to 2 hours as per local ICU practice - typically PS 5 with PEEP 5 OR CPAP 5 OR T-piece) Exclusion Criteria: * patients with a coagulopathy (INR \> 2.5, platelet count \< 30x10\^6/L) or previously diagnosed bleeding diathesis or are receiving anticoagulant drugs at therapeutic doses (ie. excluding venous thromboembolic prophylaxis) * patients with a contraindication to nasogastric tube insertion (esophageal varices, upper GI tract surgery, facial trauma, etc.) * patients receiving extra-corporeal life support