Introduction: Cardiac output is a key physiological parameter. Unfortunately, it is often difficult to measure without using invasive techniques with associated risk. The pulmonary artery catheter has, since its introduction, been considered the gold standard for the measurement of cardiac output in man. Unfortunately, pulmonary artery catheters have been associated with serious complications.1,2 Thoracic electrical bio-impedance (TEB) has been suggested as a possible non-invasive technique for the measurement of cardiac output. The accuracy and reliability of TEB has been evaluated multiple times with some studies demonstrating good correlation with thermodilution3-7 and others with poor correlation.6,8-15. TEB is not reliable in patients after cardiopulmonary bypass8,9, kidney transplants11, congestive heart failure16, pulmonary edema14, sepsis12, pregnancy15, abdominal surgery11,17, or critical illness13,14.