Patients' two lungs (TLV) will be mechanically ventilated with fraction of inspired oxygen (FiO2) of 0.4 in air, tidal volume (VT) of 8 mL/kg, inspiratory to expiratory (I: E) ratio of 1:2.5 and PEEP of 5 cm H2O, and respiratory rate will be adjusted to achieve a PaCO2 of 35-45 mm Hg.