Dose-response of Ketamine in Patient Controlled Analgesia in Orthopaedic Surgery Patients

Inclusion Criteria: * 20 - 75 years of age * Scheduled for elective posterolateral lumbar spine fusion with bilateral transpedicular screw instrumentation under general anaesthesia. * Written informed consent from the participating patient Exclusion Criteria: * A previous history of intolerance to the study drug or related compounds and additives * Concomitant drug therapy with opioids or strong CYP3A4 or CYP2B6 inductor(s) or inhibitor(s) 2 weeks prior to study. * Patients younger than 20 years and older than 75 years. * BMI \> 35, sleep apnoea, any other sleep disorder or condition that requires treatment with continuous positive airway pressure or automatic positive airway pressure device. * Existing significant liver or kidney disease * History of ischemic heart disease or conduction disturbance * History of alcoholism, drug abuse, psychological or other emotional problems that are likely to invalidate informed consent * Donation of blood for 4 weeks prior and during the study.