Cardiac output can be measured using invasive techniques such as: Fick principle, continuous mixed venous oximetry, Bradley method, indicator dilution principle as dye dilution and thermodilution techniques, systemic arterial pressure pulse contour analysis. Most of these techniques require insertion of pulmonary artery catheter (PAC), which is unfortunately associated with serious complications and requires special experience and skills. Recently, non-invasive methods for cardiac output monitoring arise independent of PAC with decreased risks and complications. Among these non-invasive methods : Ballistocardiography, this technique has not become popular ; Roentgenographic methods, By using X-Ray pictures ; Radionuclide imaging ventriculography, By using radioactive isotopes ; Ultrasound-based methods : Transesophageal echocardiography (TEE), transcutaneous suprasternal Doppler and Transtracheal Doppler ; Thoracic Electrical Bioimpedance (TEB): Impedance Plethysmography and impedance based endotracheal cardiac output monitoring. Nowadays, a new non-invasive method for cardiac output monitoring was created using the partial CO2 rebreathing fick principle, it is totally non-invasive and is so easy to perform just by connecting the monitor to the endotracheal tube.