Patients scheduled for cardiac surgery using extracorporeal circulation and median sternotomy will be consented. As part of their routine care for cardiac surgery, these patients will be intubated and pulmonary artery catheters placed. The measurements proposed in this study will therefore add little additional risk. Study patients will be intubated with an endotracheal tube which will allow endotracheal electrode measurements (ECOM 6-3D). Thermodilution cardiac output is routinely measured which will allow correlation between ECOM and thermodilution cardiac output measurements at minimal additional risk.