Recently, a novel posterior heating system has been developed that provides enhanced pressure relief: the Laryngeal Mask Airway (LMA) PerfecTemp. Anactodal experience with this FDA-approved system suggests that the PerfecTemp warmer is effective, even in open abdominal surgery. The investigators therefore propose to test the hypothesis that intraoperative distal esophageal (core) temperatures with PerfecTemp warming are non-inferior to upper-body forced-air warming in patients undergoing open major abdominal surgery under general anesthesia.