In our hospital, the current practice to provide post operative analgesia for patients scheduled for robot-assisted mitral repair or oncological thoracic surgery is to dispense local anesthetics at the orifices of the trocars at the end of the surgery and to give a Patient Controlled Analgesia (PCA). Some of these patients are not comfortable and we observe a large percentage of chronic pain. Recently we tried to use the Serratus plane block realised before the beginning of the surgery, and it seemed that patients were more comfortable with a lower consumption of morphine. So we decided to perform a double blind, randomized study in this population of patients to compare the analgesia provided by the serratus plane block realized before the beginning of the surgery and the local infiltration of the wound at the end of the surgery.