The objective of the study is to determine how controlled mode ventilation and support mode ventilation impact ventilator-free days and diaphragmatic atrophy.

Ventilator-induced diaphragmatic dysfunction (VIDD) is characterized by diaphragmatic atrophy and weakness leading to an inability to liberate from the ventilator. Patients with VIDD have increased intensive care unit and hospital length of stay, higher reintubation rates, and need for tracheostomy. There are two commonly used modes of ventilation in the ICU - controlled and support mode ventilation. Conventional practice is to initiate mechanical ventilation with controlled mode followed by a support mode to facilitate weaning and eventual extubation. However, this approach may induce irreversible diaphragmatic atrophy. Observational studies suggest that controlled ventilation is associated with higher rates of diaphragmatic atrophy than support modes of ventilation. Diaphragmatic atrophy occurs within the first 24 hours of mechanical ventilation. To date, it remains unknown whether early initiation of support mode ventilation prevents VIDD and its associated complications in comparison to controlled mode ventilation.

The investigators hypothesize that by initiating subjects on support mode ventilation, an associated increase in ventilator-free days will be seen in comparison to subjects on controlled mode ventilation. Subjects on support mode ventilation will have less diaphragmatic atrophy and weakness than subjects on controlled mode ventilation. To investigate this hypothesis,investigators are conducting a phase III randomized trial examining the effects of volume support mode versus assist control mode ventilation on ventilator-free days and rate of diaphragm atrophy.

enrolled subjects requiring mechanical ventilation will be randomized to either volume support mode or assist control mode within 24 hours of mechanical ventilation initiation. Diaphragm thickness will be measured by ultrasound daily and subsequently diaphragm atrophy rate will be calculated in each arm. The operator acquiring ultrasound images will be blinded to the ventilator mode that the subject was randomized to. Subjects in the study will follow standard ICU sedation awakening trials and spontaneous breathing trials. The medical team in charge of the subject will determine when the subject will be liberated from the ventilator.

There will be up to a 24 hour period from initiation of standard, non-study mechanical ventilation during which the subject can be consented and enrolled. This will allow the research team time to contact the subject and/or family in order to obtain informed consent. Once randomized, all subjects will be initiated on study mode of ventilation. The medical team will direct other aspects of care.

The phase III trial will be powered to determine if there is a statistically significant difference in rate of diaphragm atrophy and ventilator-free days between volume support mode and assist control mode.

Inclusion Criteria:

subjects > 18 years of age that have been intubated and mechanically ventilated for < 36 hours at the time of screening will be eligible for enrollment

Exclusion Criteria:

1. pregnancy
2. cardiopulmonary arrest
3. history of diaphragmatic paralysis or neuromuscular disease
4. chronic obstructive pulmonary disease (COPD) or asthma exacerbation with evidence of auto-PEEPing requiring intubation
5. neuromuscular blockade
6. expectation to be liberated from ventilator in < 24 hours
7. history of mechanical ventilation in the last 6 months
8. presence of tracheostomy
9. high cervical spine injury

Investigator and outcomes assessor will be masked to randomization and ventilator settings