 - Meets weaning criteria: Improvement or resolution of the underlying process that precipitated need for mechanical ventilation, PaO2 60 mm Hg on PEEP and FiO2 Requirements of 8 cm H2O and FiO2 0.50, Stable oxygenation: PEEP and FiO2 requirements not increased in the past 24 hrs, Ability to initiate an inspiratory effort and trigger the ventilator, Subject enrollment approved by the primary service attending physician 