Sevoflurane-Remifentanil Interaction: Multiple Response Surfaces, Validation of Calibration Stimuli, Validation of the Intraoperative Isobole Concept and Investigating Remifentanil Induced Opioid Tolerance

Inclusion Criteria: * American Society of Anesthesia class I and II patients * Aged 18-60 years * Scheduled for surgery under general anesthesia Exclusion Criteria: * Weight less than 70% or more than 130% of ideal body weight * Neurological disorder * Diseases involving the cardiovascular system (hypertension, coronary artery disease, prior acute myocardial infarction (AMI), any valvular and/or muscular disease involving decrease of the ejection fraction, arrhythmias, which are either symptomatic or require continuous medication/pacemaker/automatic implantable cardioverter defibrillator \[AICD\]) * Pulmonary disease including asthma, severe chronic obstructive pulmonary disease (COPD), etc. * Gastric diseases including reflux, endocrinological diseases * Recent use of psycho-active medication, including alcohol.