The gold standard in determination of cardiac output is highly invasive and expensive in use. An accurate and reliable method, non-invasively and affordable, would improve hemodynamic management. This study is for validation of a new module in impedance cardiography to improve the accuracy in cardiac patients. Heart rate, Cardiac output/ index, and stroke volume have been determined simultaneously by thermodilution (Swan Ganz, right heart catheterisation) and impedance cardiography (VasoScreen3000 with integrated ACM module) from postoperative patients who underwent open heart surgery in intensive care unit. 34 measurements were successfully taken, on 22 patients. High agreement in heart rate and acceptable agreement with strong correlation in Cardiac output/ index and stroke volume was found. The mean difference in cardiac output was -0.44 (SD 1.06) L/min (-0.31 (SD 0.63) L/min/m2 for cardiac index), correlation after Pearson was rCO = 0.67 (p < 0.001). In this study, the appearance of mechanical ventilation affects the accuracy of impedance measurement. We concluded that impedance cardiography is a reliable and accurate method for determining cardiac performance, and is practical to use and without any discomfort to the patients.