In this study two warming methods will be compared in knee arthroplasty surgery. Barrier EasyWarm will be used in the study group and BairHugger upper body warming blanket in the control group. Both groups will be prewarmed 30 minutes before spinal anaesthesia. In the operating room warming will be continued with the same warming method. Primary end point is core temperature after arriving to post anaesthesia care unit. Hypothesis is that Barrier EasyWarm is not inferior to BairHugger in preventing inadvertent intraoperative hypothermia.

Few methods exist to measure the core temperature non-invasively. Zero-heat-flux technique is used in this study. During the study we will test the accuracy of the 3M BairHugger Temperature Monitoring System by placing two sensors onto the patients (n = 30) fore head. After that the accuracy of the Dräger Tcore is examined by comparing it with the 3M BairHugger Temperature Monitoring System. So the patients (n=30) have both these different core temperature monitoring systems on their foreheads. This observational monitoring study is performed from the patient number 80 until the end.

Inclusion Criteria:

• BMI 25-40
• ASA class I-III
• elective primary unilateral kneearthroplasty
• spinal anaesthesia

Exclusion Criteria:

• ASA class > III
• unstable coronary artery disease
• revision surgery, bilateral arthroplasty
• general anaesthesia
• decreased mental status