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.