APRV mode of ventilation will result in an improved partial pressure of arterial oxygenation/ fraction of inspired oxygen (P/F ratio) on day 3 of mechanical ventilation. Sub hypotheses: APRV will be associated with a reduced amount of sedation used during the ICU stay in patients with respiratory failure. APRV will be associated with a reduction in the amount of vasoactive medication used for blood pressure support in patients with respiratory failure.

This prospective un-blinded randomized trial will follow patients with respiratory failure and ALI/ARDS who require invasive mechanical ventilation in select ICUs. Patients will be allocated to respiratory support with either APRV mode or volume control (AC) mode of mechanical ventilation. Qualifying patients will be randomized by permuted block randomization within 24 hours of admission to the ICU. Identical ventilation and oxygenation thresholds will be utilized to guide titration of each ventilator protocol. Patients will remain on the assigned mode of ventilation until they are extubated and discharged from the ICU.

Procedures for treatment evaluation include daily monitoring of the Ventilator protocol in each arm. Clinical coordinator and study respiratory therapist will perform 2 daily checks of the study patients to determine compliance with the protocol and if patient meets weaning criteria. The previously published ARDS Network continuous positive airway pressure (CPAP) weaning protocol will be used for all patients enrolled.

Inclusion Criteria:

• Hypoxemic respiratory failure requiring mechanical ventilation for >24 hours.

Exclusion Criteria:

• Age under 18
• Severe chronic obstructive lung disease
• Patients in whom the only indication for mechanical ventilation is airway protection from poor neurological status.