The Clinical Pharmacology Unit will perform a permuted blocks randomization scheme for each of the two hospitals participating in the trial. The result of this randomization will be kept in this unit and will not be accessible to investigators. Investigators will receive a series of opaque sealed envelopes with a number corresponding to the order of entry of the patients into the trial. When the patient is in the operating room, investigators must open the envelope which contains the group of treatment assigned to that patient. The patient will not know in which intervention group is included. Investigator who performs the spinal puncture and investigator who collects the data and performs the evaluation will no be the same person. This will allow a blind evaluation.