Standard general anesthesia
All the patients were routinely fasted and did not take any preoperative medication. Noninvasive blood pressure, heart rate, electrocardiography, pulse oxygen saturation, and the Bispectral index (BIS) were monitored after arrival in the operating theater. The general anesthetic induction was conducted with propofol 2 mg/kg, sufentanil 0.4 µg/kg, and cisatracurium 0.15 mg/kg and double-lumen tracheal intubation was initiated to accomplish lung isolation when the patient had lost consciousness and the BIS index value had decreased to below 40. After intubation, the mechanical ventilation was set with 100% oxygen at a tidal volume of 6 to 8 ml/kg, an inspiratory-to-expiratory ratio of 1:2, and a respiratory frequency of 12–16 breaths/min. The ventilation was switched to one-lung ventilation at the time of skin incision. A normal end-tidal carbon dioxide (CO