18 patients (> 18 years) of both sexes, submitted to general anesthesia, will be prospectively studied. All patients will receive, in addition to the usual monitoring, the monitoring with the electrical impedance tomography and chest ultrasonography, after being anesthetized and under neuromuscular block, being ventilated with an inspired fraction of 50% oxygen (or greater to maintain oxygen saturation > 96% ), Tidal volume of 6 mL / kg and respiratory rate to maintain expiratory tidal CO2 between 35-45 cmH2O.