Patients' two lungs will be ventilated, with a fraction of inspired oxygen (FiO2) of 0.5 in air, tidal volume (VT) of 8 mL/kg predicted body weight (PBW), inspiratory to expiratory [I: E] ratio of 1:2.5 and positive end expiratory pressure (PEEP) of 5 cm H2O, and respiratory rate adjusted to achieve an arterial carbon dioxide tension of 35-45 mm Hg. Peak airway pressure will be limited to 35 cm H2O and a fresh gas flow (FGF) < 2 L/min will be used.