Hypertonic Saline (HS) aerosols have proven efficacious mucolytics in patient with cystic fibrosis and are well tolerated in that population. Safety concerns in mechanically ventilated patients with ARDS center primarily on HS's effects on lung water distribution (intra vs. extra alveolar) and on airway reactivity. For that reason we plan a small feasibility trial with narrowly focused physiologic endpoints, namely to measure the effects of a single 5ml dose of 7% Saline aerosol on blood gas tensions, hemodynamics and the static and dynamic properties of the relaxed respiratory system. This narrowly scoped study is to lay the foundation for a larger multicenter intervention trial.