Pts in supine position; tracheal intubation with left-sided double lumen tube (DLT) and position confirmed by bronchoscopy. Lungs are initially ventilated with pressure controlled mode (PCV) at a tidal volume (TV) of 7 mL/kg, I:E ratio 1:2, respiratory rate (RR) to obtain a PaCO2 40-45 mmHg, FiO2 =1 and PEEP 5 cm H2O. During OLV inspiratory pressure is reduced to obtain a TV of 5 mL/Kg, maintaining FiO2=1, PEEP 5 cm H2O and adjusting RR to obtain a PaCO2 40-45 mmHg. Capnothorax is obtained by the insufflation of CO2 and maintained at a preset pressure of 10 mmHg. CPAP on the non ventilated lung is set at 10 cm H2O and FiO2=1.