2) Time: The study will be performed during surgery and the ICU stay. It should not delay surgery more than 5 minutes. Placement of the electrode patches is rapid. The endotracheal electrodes are attached to the endotracheal tube and placement of the tube is routine. Recording of cardiac outputs and impedance data will not delay surgery. Placement and removal of the aortic flow probe is rapid but may delay surgery for a few minutes. Data collected in the ICU will only be correlations between thermodilution and impedance data, since no transit time data can be collected outside the operating room. ICU data will not delay ICU or hospital discharge.