If the pt lyes supine, as in VATS atrial fibrillation (AF) surgical ablation, the protective effect of gravity is lost, shunt remains high and severe hypoxemia is common. Some algorithms are used to improve oxygenation during OLV: high fraction of inspired oxygen (FiO2), positive end expiratory pressure (PEEP) on the ventilated lung, continuous positive airway pressure (CPAP) on the deflated lung, but the latter can be used with caution during VATS because of the obstruction of the surgical field. Increasing the pressure in the pleural cavity with carbon dioxyde (CO2) (capnothorax) could decrease the blood flow in the deflated lung, helping in reducing shunt but this is not yet demonstrated and this manouver could decrease cardiac output. The oxygen delivery (DO2) is cardiac output (CO) by oxygen content and during this surgery both these factors could be compromised.