Critically ill patients who (1) are not able to maintain their airway, (2) cannot breathe on their own, or (3) both, are ones who often require tracheal intubation and support from a breathing machine (mechanical ventilator). When the patient is ready to be liberated from the mechanical ventilator because the initial insult for intubation has been resolved, the patient is screened using the readiness to wean test in preparation for extubation. As the patient passes this screening, a spontaneous breathing test (SBT) is initiated. Currently, there are many debates surrounding which SBT technique is most favorable. At Toronto General Hospital, the clinical team uses a zero-end expiratory pressure (ZEEP) trial. Once the patient successfully passes their SBT they are then extubated. The patient will undergo a spontaneous breathing trial of continuous positive airway pressure (CPAP) of 5 cmH2O and ZEEP, in which time the investigators will be using a new technology called electrical impedance tomography (EIT), to study and compare the end expiratory lung volume (EELV); investigators will use an esophageal catheter to measure and monitor pressures in the lung, and also assess the patient's work of breathing. This will be repeated once the patient has been extubated safely.