Thoracic epidural analgesia and thoracic paravertebral analgesia are currently the standard strategies for thoracic surgery but the difficult of performing them in all patients and their potential complications are all factors that limit their use. Systemic administration of opioids is the simplest and common strategy to provide analgesia but it may be associated with several undesirable effects, such as respiratory depression, sedation, nausea, constipation and vomiting.