Effect of Thoracic Epidural Analgesia on Intravenous Closed-Loop Anesthesia

The association of general anesthesia and of thoracic epidural anesthesia is used in thoracic surgery with a demonstrated benefit on the risk of postoperative respiratory complications. The objective of the study is to analyze the influence of thoracic epidural administration of chirocaine (5 mg/ml) on the administration of anesthetic agents (propofol and remifentanil), these drugs being administrated using a closed-loop system with the Bispectral Index as the controller. Two groups of patients are compared: * one group will receive general anesthesia and thoracic epidural administration of saline. Chirocaine will be administered before patient's awakening, * one group will receive general anesthesia and thoracic epidural administration of chirocaine (5 mg/ml). In both groups, the closed-loop system ensures that le depth of anesthesia is similar in both groups.