Partially assisted mechanical ventilation is ideally titrated to maintain adequate inspiratory muscle activity (i.e. preventing disuse atrophy) while avoiding excessive (i.e. fatiguing) inspiratory loads. The advent of ventilator modes enabling flow and pressure assistance in proportion to patient inspiratory effort (i.e. Proportional Assist Ventilation (PAV), Neurally-Adjusted Ventilator Assist (NAVA)) permit careful titration of ventilatory support to achieve desired levels of patient inspiratory effort. However, data on the acceptable range of values for inspiratory effort and respiratory muscle load-capacity balance during mechanical ventilation are currently very limited. Such data are critical to inform a physiologically sound evidence-based approach to titrating partially-assisted ventilatory support. This study is designed to ascertain a physiologically acceptable range of inspiratory effort in mechanically ventilated patients. The study protocol includes measurements of the respiratory system during spontaneous breathing as well as evaluating the diaphragm in real-time with the use of ultrasonography. The experimental design outlined in the present study is predicated on the assumption that the range of values for inspiratory effort and load-capacity balance observed in patients who are successfully weaned from mechanical ventilation represent the safe and appropriate range of target values.

Inclusion Criteria:

- Adult patients (>17) receiving invasive mechanical ventilation following elective cardiac surgery who will undergo a trial of spontaneous breathing

Exclusion Criteria:

• Pregnant women
• Decompensated cardiac insufficiency or acute coronary syndrome
• Severe COPD
• Contraindications to esophageal catheter placement, namely known esophageal varices

Adult patients (age >17) receiving invasive mechanical ventilation following elective cardiac surgery who will undergo a trial of spontaneous breathing