a) spontaneous breathing at FiO2=0.21; b) 10' after double lung ventilation (TLV); c) 10'after OLV; d) 10' after OLV and capnothorax (Group CA) or OLV, capnothorax and CPAP (Group CC) e) double lung ventilation (TLV) at the end of the surgical procedure Differences of DO2, shunt fraction (QS/QT), and derived variables between the two groups will be tested by T-test.