The Endotracheal Cardiac Output Monitor (ECOM) is a system which records the voltages produced by a current (2 ma 100 Khz) delivered to the tracheal mucosa by electrodes on an endotracheal tube. The proximity of the ascending aorta and trachea allows the design of a device that can optimize the current delivery and signal recording from impedance changes in the ascending aorta. This study will test the accuracy and efficacy of the ECOM system in anesthetized patients.