Inclusion Criteria:  The patient is scheduled for elective unilateral total knee arthroplasty (TKA) under spinal anesthesia Fluent skills in finnish language (to understand the given information and to be able to give informed consent and communicate with the study personnel) Age between 35 and 80 years Weight between 50 and 100 kg American Society of Anesthesiologists status 1-3 Written informed consent from the patient  Exclusion Criteria:  A previous history of intolerance to the study drug or related compounds and additives Disease or condition affecting patient's ability to give written informed consent Existing or recent disease possible affecting absorption, distribution, metabolism, excretion or response to the study drug History of severe cardiac disease (valvular insufficiency, severe left ventricular dysfunction) or abnormal ECG rhythm (bradycardia < 50/min, 2nd or 3rd degree atrioventricular-block, pacemaker) Preoperative systolic blood pressure <110 mmHg Chronic use of strong opioids, pregabalin, gabapentin, amitriptyline or duloxetine Participation in any other study concomitantly or within one month prior to the entry into this study Clinically significant abnormal findings in physical examination or laboratory screening Pregnancy or breastfeeding Use of any drugs known to cause enzyme induction or inhibition for a period of 30 days prior to the study, use of any natural products (including grapefruit products) for at least 14 days prior to the study and caffeine containing products for at least 24 hours prior to the study. The use of regular doses of paracetamol is allowed.