An esophageal heat exchange tube will be inserted in anesthetized surgical patients. Each will have 30 minutes of cooling (circulating fluid at 7 degrees C) and 30 minutes of warming (42 degrees C) with 15 -minute break in between; the order of the cooling and warming will be randomized. The primary outcome will be heat transfer, determined from inflow and outflow fluid temperatures and fluid flow rate assessed for both cooling and warming modes.