4. Cardiac output will be measured by iced-thermodilution and impedance cardiography every ten minutes during the operation. Samples will be obtained in with a ten cc volume of iced saline. A five hour operation would have a total volume of 800 cc of fluid. We routinely give a 1000 cc of saline to patients for cardiac cases so we will just reduce routine fluid administration to account for this extra volume from cardiac output measurements. Iced-injectate has no effect on patient temperature since active warming of the patient continues throughout the study.