Inclusion Criteria:  Adult patients (≥18 years) having elective major abdominal surgery with general anesthesia Expected duration of surgery at least 120 minutes Planned postoperative monitoring in intensive care unit  Exclusion Criteria:  Emergency surgery Age <18 years Body weight <50 kg Pregnancy Atrial fibrillation Planned patient positioning in other position than supine position Surgery within the last 30 days Inaccessibility of the head during surgery Peripheral artery disease stage ≥IIb Chronic vasculitis Supraglottic airway management Enterostomy repair surgery